# data[].attributes.headline
CSI of the Sea- 2018 online dissection event 

# data[].attributes.subject
CSI of the Sea- 2018 online dissection event 

# data[].attributes.body
This online dissection event was conducted by a CSIP team and the ZSL Public Engagement team in 2018. It focussed on the issue of bycatch and featured the  examination of a bycaught common dolphin found stranded in Devon.

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Poster: Evidence of a failed predation event between a grey seal and a harbour porpoise. 

# data[].attributes.subject
Poster: Evidence of a failed predation event between a grey seal and a harbour porpoise. 

# data[].attributes.body
This work was presented at the 2019 World Marine Mammal Conference by CSIP team member Matt Perkins. It describes pathological findings in four harbour porpoises with evidence of prior non-fatal interactions with grey seals.

# data[].attributes.attachment_link.title
2019 Matt Perkins grey seal interaction WMMC

# data[].attributes.body
Strandings leaflet for the CSIP consortium- published in 2010.

# data[].attributes.headline
CSI of the Sea- 2017 online and live audience dissection event

# data[].attributes.subject
CSI of the Sea- 2017 online and live audience dissection event

# data[].attributes.body
This dissection event was conducted by a CSIP team from ZSL and the ZSL Public Engagement team in 2017 and was streamed online and in front of a live audience. It focussed on the issue of pollution and infectious disease and featured the  examination of a harbour porpoise found stranded in Gwynedd, Wales.

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CSI of the Sea: Porpoise Dissection 2017

# data[].attributes.headline
Poster: Lesion profiles of cetaceans with neurobrcuellosis in Scotland (1990-2019)

# data[].attributes.subject
Poster: Lesion profiles of cetaceans with neurobrcuellosis in Scotland (1990-2019)

# data[].attributes.body
This poster was presented at the 2019 World Marine Mammal  Conference by CSIP team member Nick Davison. It describes lesion profiles in cetaceans found stranded in Scotland and diagnosed with neurobrucellosis (1990-2019).

# data[].attributes.headline
Poster: First report of Brucella ceti infection in a Risso's dolphin

# data[].attributes.subject
Poster: First report of Brucella ceti infection in a Risso's dolphin

# data[].attributes.body
This work was presented at the 2017 European Cetacean Society conference by CSIP team member Nick Davison. It describes the first evidence of Brucella ceti infection in a Risso's dolphin.

# data[].attributes.attachment_link.title
2017 Nick Davison first report of Brucella in a Risso's dolphin ECS

# data[].attributes.headline
Poster: First report of Brucella ceti meningencephalitis in a Sowerby's beaked whale

# data[].attributes.subject
Poster: First report of Brucella ceti meningencephalitis in a Sowerby's beaked whale

# data[].attributes.body
This work was presented at the 2016 European Cetacean Society conference by CSIP team member Nick Davison. It describes the first evidence of Brucella ceti associated meningoencephalitis in a Sowerby's beaked whale.

# data[].attributes.attachment_link.title
2016 Nick Davison first report of Brucella ceti meningencephalitis in a Sowerby's beaked whale ECS

# data[].attributes.body
This work was presented by CSIP team member James Barnett. It describes adenocarcinoma in a grey seal.

# data[].attributes.body
This work was presented at the 2016 European Cetacean Society conference by CSIP team member James Barnett. It describes pathological findings in grey seals examined at post-mortem in Cornwall.

# data[].attributes.attachment_link.title
2016 James Barnett grey seal pathology ECS

# data[].attributes.headline
Poster: First report of the isolation of Brucella ceti in long-finned pilot whales 

# data[].attributes.subject
Poster: First report of the isolation of Brucella ceti in long-finned pilot whales 

# data[].attributes.body
This work was presented at the 2014 European Cetacean Society conference by CSIP team member Nick Davison. It describes the first report of the isolation of Brucella ceti in long-finned pilot whales.

# data[].attributes.attachment_link.title
2014 Nick Davison first report of Brucella ceti in a long-finned pilot whale ECS

# data[].attributes.headline
Poster: Volunteer strandings management- a pilot whale case study

# data[].attributes.subject
Poster: Volunteer strandings management- a pilot whale case study

# data[].attributes.body
This work was presented at the 2014 European Cetacean Society conference by CSIP team member James Barnett. It describes volunteer strandings management with respect to the case study of a long-finned pilot whale stranding.

# data[].attributes.attachment_link.title
2014 James Barnett pilot whale stranding

# data[].attributes.headline
Poster: A review of 27 years of cetacean necropsies in southwest England (1985-2011)

# data[].attributes.subject
Poster: A review of 27 years of cetacean necropsies in southwest England (1985-2011)

# data[].attributes.body
This work was presented at the 2013 European Cetacean Society conference by CSIP team member James Barnett. It describes a review of cetacean necropsies in SW England (1985-2011).

# data[].attributes.attachment_link.title
2013 James Barnett 27 years SW pathology ECS

# data[].attributes.headline
Poster: First case of meningitis and arthritis associated with Brucella ceti infection in a common dolphin

# data[].attributes.subject
Poster: First case of meningitis and arthritis associated with Brucella ceti infection in a common dolphin

# data[].attributes.body
This work was presented at the 2013 European Cetacean Society conference by CSIP team member Nick Davison. It describes the first case of meningitis and arthritis associated with Brucella ceti infection in a common dolphin.

# data[].attributes.attachment_link.title
2013 Nick Davison common dolphin brucella meningitis & arthritis Poster

# data[].attributes.headline
Poster: Helicobacter cetorum infection in dolphins from SW England

# data[].attributes.subject
Poster: Helicobacter cetorum infection in dolphins from SW England

# data[].attributes.body
This work was presented at the 2012 European Cetacean Society conference by CSIP team member Nick Davison. It describes evidence of Helicobacter cetorum infection in dolphins found stranded in SW England.

# data[].attributes.body
This work was presented at the 2011 European Cetacean Society conference by CSIP team member James Barnett. It describes the role of pathology in defining live cetacean strandings response in the UK.

# data[].attributes.attachment_link.title
2011 James Barnett live strandings pathology

# data[].attributes.attachment_caption
Mariel ten Doeschate- Scans of the Beach'

# data[].attributes.headline
Poster: International Assessment of Harbour Porpoise Strandings along the North Sea Coastline

# data[].attributes.subject
Poster: International Assessment of Harbour Porpoise Strandings along the North Sea Coastline

# data[].attributes.body
This work was first presented at the World Marine Mammal Conference in Barcelona in December 2019. It was then published in the journal of Biological Conservation, under the title "Spatiotemporal mortality and demographic trends in a small cetacean:
Strandings to inform conservation management". https://doi.org/10.1016/j.biocon.2020.108733

The need to understand and quantify the impact of anthropogenic activities on marine mammal populations is critical in order to design and assess effective mitigation and population management. Harbour porpoises are the most abundant cetacean species in the north sea. Highly mobile and elusive, they are notoriously difficult to monitor, but the coastline provides a powerful vantage from which to conduct surveillance, particularly on biological and health parameters. Information on stranded individuals are recorded on a national level, yet for this study, data from all five national stranding networks bordering the North Sea were combined.
The work demonstrates the first assessment of the North Sea population as one ecological unit. Data from 1990-2017 were used to investigate spatiotemporal trends and assess biological characteristics of the stranded population. A total of 16,247 individual strandings were recorded. Stranding numbers increased annually in all regions, with a sharp increase in the southern North Sea after 2004. A Generalised Additive Mixed Model framework revealed a region-specific seasonal pattern, enabling detection of periods of unusual mortality. There was heterogeneity in age-specific sex ratio, with particularly high presence of juvenile males in the southern North Sea, possibly suggesting a population sink. The majority of neonate strandings occurred along the northern Dutch, German, and Danish coastlines, potentially suggestive of areas of importance for calving. The study provided unique high-resolution insight into region-specific population parameters of North Sea harbour porpoises. This new knowledge comes as a valuable addition to live animal survey data. Combined, these data sources can inform planning consent decisions and mitigation of impacts of offshore anthropogenic activities. This work demonstrates the value of a transboundary approach to data analysis for deriving effective conservation and management strategies of highly mobile marine species.

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Poster: Brucella infection and levels of PCBs in bottlenose dolphins found stranded in SW England

# data[].attributes.subject
Poster: Brucella infection and levels of PCBs in bottlenose dolphins found stranded in SW England

# data[].attributes.body
This work was presented at the 2011 European Cetacean Society conference by CSIP team member Nick Davison. It describes Brucella species infection and levels of polychlorinated biphenyls (PCBs) in bottlenose dolphins found stranded in SW England.

# data[].attributes.attachment_link.title
2011 Nick Davison BND Brucella and PCBs Poster

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Poster: northern bottlenose whale strandings in the UK (1990-2008)

# data[].attributes.subject
Poster: northern bottlenose whale strandings in the UK (1990-2008)

# data[].attributes.body
This work was presented at the 2010 European Cetacean Society conference by CSIP team member Matt Perkins. It describes a review of northern bottlenose whale strandings in the UK (1990-2008).

# data[].attributes.attachment_link.title
2010 Matt Perkins NBW strandings and pathology 1990-2009

# data[].attributes.headline
Poster: Evidence of violent bottlenose dolphin interactions with other dolphin species 

# data[].attributes.subject
Poster: Evidence of violent bottlenose dolphin interactions with other dolphin species 

# data[].attributes.body
This work was presented at the 2009 European Cetacean Society conference by CSIP team member James Barnett. It describes post-mortem evidence of violent bottlenose dolphin interactions with other dolphin species in SW England.

# data[].attributes.attachment_link.title
2009 James Barnett bottlenose dolphin kills

# data[].attributes.headline
Poster: Chronic non-suppurative meningoencephalitis associated with Brucella ceti infection in a striped dolphin

# data[].attributes.subject
Poster: Chronic non-suppurative meningoencephalitis associated with Brucella ceti infection in a striped dolphin

# data[].attributes.body
This work was presented at the 2009 European Cetacean Society conference by CSIP team member Nick Davison. It describes chronic non-suppurative meningoencephalitis associated with Brucella ceti infection in a striped dolphin that live stranded in Cornwall.

# data[].attributes.body
This work was presented at the 2008 European Cetacean Society conference by CSIP team member Matt Perkins. It describes pathological findings in the investigation of the northern bottlenose whale which stranded in the Thames in 2006.

# data[].attributes.attachment_link.title
2008 Matt Perkins Thames whale pathology

# data[].attributes.headline
Poster: bycatch in UK stranded cetaceans (1990-2018)

# data[].attributes.subject
Poster: bycatch in UK stranded cetaceans (1990-2018)

# data[].attributes.body
This poster was presented at the 2019 World Marine Mammal Conference by CSIP team member Rob Deaville. It collates data on diagnosed cases of bycatch in  UK stranded cetaceans between 1990-2018.

# data[].attributes.attachment_link.title
2019 Rob Deaville Bycatch review 1990-2018

# data[].attributes.headline
Poster: PCBs and porpoise testes weights

# data[].attributes.subject
Poster: PCBs and porpoise testes weights

# data[].attributes.body
This poster was presented at the 2021 Society of Environmental Toxicology and Chemistry (SETAC) conference by CSIP team member Rosie Williams. It describes an association between polychlorinated biphenyls (PCBs) and reduced testes weights in UK stranded harbour porpoises.

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2021 Rosie Williams SETAC PCBs and testes weights

# data[].attributes.body
Final CSIP Contract Report to Defra for the period 2011-2017

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CSIP Final Contract report 2011-2017 Appendices

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CSIP Final Contract report 2011-2017 Appendices

# data[].attributes.body
Appendices to Final CSIP Contract Report 2011-2017

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CSIP Final Contract report 2011-2017 Appendices

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Website: Scottish Marine Animal Strandings Scheme

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Website: Scottish Marine Animal Strandings Scheme

# data[].attributes.body
Home page of the Scottish Marine Animal Strandings Scheme

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Report a stranding Live stranding Database Tweets by @strandings

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Cornwall Wildlife Trust Marine Strandings Network strandings flyer

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Website of the Scottish Entanglement Alliance (of which SMASS is a member)- report downloads available from this page.

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Downloads - Scottish Entanglement Alliance (SEA)

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Understanding marine animal entanglement in Scottish waters. Marine animal entanglement - what is the law?

# data[].attributes.body
Poster detailing work carried out by Cornwall Wildlife Trust Marine Stranding Network volunteers through their Bycatch Evidence Evaluation Protocol.

# data[].attributes.headline
Poster: Cornwall Wildlife Trust banana pinger

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Poster: Cornwall Wildlife Trust banana pinger

# data[].attributes.body
Poster describing acoustic assessment of the CWT banana pinger.

# data[].attributes.attachment_link.title
Cornwall Wildlife Trust banana pinger poster

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Poster: PCBs association with toxic effects in harbour porpoises

# data[].attributes.subject
Poster: PCBs association with toxic effects in harbour porpoises

# data[].attributes.body
This poster was presented by CSIP team member Rosie Williams. It describes a continued association between polychlorinated biphenyls (PCBs) and toxic effects in UK stranded harbour porpoises, despite having fallen below proposed toxicity thresholds.

# data[].attributes.attachment_link.title
Rosie Williams PCBs association with toxic effects in harbour porpoises

# data[].attributes.headline
Poster: UK strandings review (1990-2015)

# data[].attributes.subject
Poster: UK strandings review (1990-2015)

# data[].attributes.body
This poster was presented at the 25th anniversary meeting of the CSIP in 2016. It summaries investigations conducted in the UK between 1990-2015.

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Cornwall Wildlife Trust Marine Strandings Network 2020 annual report.

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Poster: UK stranded beaked whale review (1990-2011)

# data[].attributes.subject
Poster: UK stranded beaked whale review (1990-2011)

# data[].attributes.body
This work was presented at the 2013 European Cetacean Society conference by CSIP team member Matt Perkins. It presents collated data on UK stranded beaked whales between 1990-2011.

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2013 Matt Perkins Beaked whale strandings 1990-2011

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Website: Natural History Museum Strandings

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Website: Natural History Museum Strandings

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Webpage for the NHM strandings team (part of the CSIP consortium).

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Contribute to our understanding of UK marine mammals by telling us about any stranded whales, dolphins and porpoises you find.

Why we are doing the project

Whales, dolphins and porpoises (collectively known as cetaceans) are a fascinating group of mammals that lead mysterious lives below the waves.

To expose some of these mysteries, the Museum started recording cetacean strandings in 1913. We are now a partner in the UK Cetacean Strandings Investigation Programme, which has expanded to include seals, large-bodied sharks and marine turtles that strand along the UK coastline.

We rely on the public to report any incidents of these stranded animals to us so we can unravel the events leading up to the moment of death. We try to respond quickly in the hope of collecting the stranded animal for post-mortem examination.

How to take part

Turn your daily dog walk or Sunday stroll along the UK coast into a dedicated search for strandings.

If you spot a stranded cetacean on the shoreline or in the surf please do the following:

1. Take a photograph. This is invaluable for species identification and assessing the level of decomposition (which affects whether we do a post-mortem examination).

2. Estimate the animal's size in metres or feet.

3. Note the exact location and date.

4. Send these details and your photos to strandings@nhm.ac.uk or call 0800 6520 333.

We would also like reports on stranded seals, large-bodied sharks and turtles.

Results so far

We record approximately 500 strandings of marine animals every year. Post-mortem examinations reveal information about causes of death, diseases, environmental contaminant levels, reproductive patterns, diet and baseline data on UK populations.

A summary of the scientific reports and a list of peer-reviewed papers published by the programme are available on the UK Cetacean Strandings Investigation Programme website.

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UK Whale and Dolphin Strandings | Natural History Museum

# data[].attributes.attachment_link.description
Contribute to our understanding of UK marine mammals by telling us about any stranded whales, dolphins and porpoises you find. Whales, dolphins and porpoises (collectively known as cetaceans) are a fascinating group of mammals that lead mysterious lives below the waves. To expose some of these mysteries, the Museum started recording cetacean strandings in 1913.

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Historical UK cetacean strandings dataset collected by the Natural History Museum (1913-1989)

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Over 100 years ago, the Natural History Museum created a project to document the cetaceans (whales, dolphins and porpoises) stranded on British beaches. Annual reports were submitted to the UK Government from 1913, and in the introduction to the first report, the founder Sir Sidney F. Harmer wrote, "The instruction to the Receivers of Wrecks, issued by the Board of Trade in 1912, at the request of the Trustees of the British Museum to send telegraphic Reports of the strandings of specimens of Whales to the Museum has resulted in the collection of a number of records of British Cetacea." The Museum continues to be part of efforts to record stranded cetaceans to this day as one of the partners of the Cetacean Strandings Investigation Programme (CSIP).

This dataset is the historical portion of these records and covers the period from 1913 to 1989. In 1990 the CSIP partnership was created, so please contact them for data from 1990 ( http://ukstrandings.org/). The data have been cleaned as follows.

Grid references were converted to latitude and longitude

Taxonomy for scientific names was corrected to that of Reid et al 2003 (Atlas of Cetacean distribution in north-west European waters).

Dates were standardised to YYYY-MM-DD format

Typos were corrected

Grid references were converted by Louise Allan. All other modifications were carried out by Ellen J Coombs under the guidance of Richard C Sabin and Natalie Cooper.
Further information on cetacean strandings and the NHM are available here:

We thank the members of the public and HM Coastguards who took the time to report strandings to us over the last 100 years (originally by filling out postcards!). We also thank the numerous UK conservation organisations and their dedicated volunteers. Without their efforts this dataset would not exist.

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Historical UK cetacean strandings dataset (1913-1989) - Data Portal

# data[].attributes.attachment_link.description
Over 100 years ago, the Natural History Museum created a project to document the cetaceans (whales, dolphins and porpoises) stranded on British beaches. Annual reports were submitted to the UK Government from 1913, and in the introduction to the first report, the founder Sir Sidney F.

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Poster: what do cetacean stranding records tell us?

# data[].attributes.subject
Poster: what do cetacean stranding records tell us?

# data[].attributes.body
This work was presented at the British Ecological Society conference by Ellen Coombs (NHM). It presents collated data on UK and Irish cetacean strandings during the 100+ year period between 1913-2015.

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Ellen Coombs - British Ecological Society

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Poster: 100 years of cetacean strandings around the UK and Ireland

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Poster: 100 years of cetacean strandings around the UK and Ireland

# data[].attributes.body
This work was presented by Ellen Coombs (NHM). It presents figures on UK and Irish cetacean strandings during the 100+ year period between 1913-2015.

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Ellen Coombs 100 years of strandings posters

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Paper: PCB pollution continues to impact populations of orcas and other dolphins in European waters (Scientific Reports)

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Paper: PCB pollution continues to impact populations of orcas and other dolphins in European waters (Scientific Reports)

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This paper published in the journal Scientific Reports in 2016 describes a large pan-European meta-analysis of mean blubber PCB concentrations in a variety of stranded dolphin species.

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Introduction

The severe impact on top predators of the bioaccumulation of DDT and other organochlorine (OC) pesticides in food webs was first identified in the early 1960s1. Most OC pollutants were banned in developed countries in 1970s-1980s and some terrestrial wildlife populations recovered. Experimental studies have established that OC pesticides and the more persistent polychlorinated biphenyls (PCBs) have a range of species-specific and dose-dependent toxic effects such as immunosuppression and reproductive impairment in all mammalian species tested, including marine mammals4,5,6,7. However, the impact of historic and current exposure to marine pollutants on European cetacean top predators is still largely unknown.

To investigate this issue we conducted a meta-analysis of European data on summed 18-25 chlorobiphenyl congeners (∑PCB) concentrations (mg/kg lipid weight (lw)) in cetaceans that were stranded and necropsied or free-living and biopsied. Data came from 1,081 animals of four species:- harbour porpoise ( Phocoena phocoena, HP)(n = 706), striped dolphin ( Stenella coeruleoalba, SD)(n = 220), bottlenose dolphin ( Tursiops truncatus, BND) (n = 131) and killer whale ( Orcinus orca, KW)(n = 24). The ∑PCB was mainly derived from two different laboratories, but with both using internationally standardised methodology enabling full integration of data. The analysis included statistical evaluation of long-term temporal trends in ∑PCB concentrations in UK-stranded HPs (1990-2012) and Mediterranean SDs (1990-2009). We compared mean ∑PCB concentrations in all four species with a range of established PCB toxicity thresholds for marine mammals. The lowest PCB toxicity threshold was a widely used and relatively low PCB toxicity threshold for onset of physiological impacts and equivalent to 9 mg/kg lipid as ∑PCB6 in this study. We also used the very highest PCB toxicity threshold reported for marine mammals, for profound reproductive impairment in ringed seals ( Phoca hispida) in the Baltic Sea and equivalent to 41mg/kg lipid as ∑PCB in this study6.

We compared mean PCBs concentrations in UK-stranded HPs that were "healthy" and died of acute physical trauma (n = 345) (control group) with mean PCBs concentrations in HPs that died of infectious diseases (n = 252) (case group) or starvation (n = 42). Where data permitted, sex differences in mean PCB concentrations were investigated to assess maternal offloading of PCBs via lactation. Finally, we reviewed the current population and reproductive status of the extant odontocete populations in European waters using systematic local and international observational survey and other data collected over the past 15-20 years (e.g. International Union for Conservation of Nature assessments)8.

Results

Spatial and temporal trends and sex differences in ∑PCB exposure

The temporal trend in ln ∑PCB lipid concentrations in UK HPs (1990-2012) and SDs from the western Mediterranean Sea (1990-2009) is shown in Fig. 1A,B. The temporal trend in both populations was highly statistically significant (p < 0.001). In UK-stranded HPs, ln ∑PCB concentrations declined slowly from 1990 to 1998 and then remained relatively stable from 1998 to 2012 against the null hypothesis of no trend (p < 0.001, F = 11.76, residual df = 701.97, trend df = 3.03) ( Fig. 1A). In Mediterranean SDs, ln ∑PCB concentrations showed a marked decline from an initial peak in 1990, but then stabilized from 2003 to 2008, but with blubber ∑PCB concentrations still consistently exceeding all mammalian toxicity thresholds. The trend is statistically significant (p < 0.001, F = 55.45, residual df = 212.03, trend df = 6.97) against the null hypothesis of no trend ( Fig. 1B).

Temporal trends in ∑PCBs in UK-stranded harbour porpoise ( Phocoena phocoena) and striped dolphins ( Stenella coeruleoalba) in the western Mediterranean Sea.

To assess the ∑PCBs trend we have fitted a Generalised Additive Model (GAM) to the data using the R (R Development Core Team, 2013) package mgcv. We used thin plate regression splines to do the smoothing and the degree of smoothing was determined by generalised cross validation. (A) Ln ∑PCBs (sum 18-25CB) mg/kg lipid concentrations in UK harbour porpoise blubber against date for all data for 1990-2012 (n = 706). The continuous line represents the smoothed trend from a Generalized Additive Model fitted to the data. The trend is statistically significant (p < 0.001, F = 11.76, residual df = 701.97, trend df = 3.03) against the null hypothesis of no trend. The dashed lines represent the 95% bootstrapped Confidence Intervals. The yellow line represents ln ∑PCBs equivalent to 20.0 mg/kg lipid and the red line 40 mg/kg lipid. (B) Ln ∑PCBs (sum 18-25CB) lipid concentrations in biopsied striped dolphin blubber from the Mediterranean Sea against date for all data for 1990-2009 (n = 220). Figure shows the natural logs (ln) of the whole data set plotted against the date found. The continuous line represents the smoothed trend from a Generalized Additive Model fitted to the data. The trend is statistically significant (p < 0.001, F = 55.45, residual df = 212.03, trend df = 6.97) against the null hypothesis of no trend. The dashed lines represent the 95% bootstrapped Confidence Intervals. The yellow line represents ln ∑PCBs equivalent to 20.0 mg/kg lipid and the red line 40 mg/kg lipid.

Some geographical regions in this study were global PCB "hotspots" for cetaceans. Mean ∑PCB lipid concentrations in BNDs and KWs from the NE Atlantic and in BNDs and SDs from the Mediterranean were among the highest recorded in cetaceans globally9, with males and females markedly exceeding all known PCB toxicity thresholds for marine mammals ( Figs 2, 3, 4, 5). Results of kernel smoothing estimates show very high risk of PCB toxicity for KWs in NE Atlantic, BNDs off SW Iberia and in the northern Adriatic Sea and BNDs/SDs in the western Mediterranean ( Fig. 6). ∑PCB data from biopsies of 152 free-living individuals constituted 38/131 (29.0%) of all BNDs; 99/220 (45.0%) of the SDs; and 15/24 (62.5%) of the KWs in this study. All HPs in the UK were stranded. Although biases can potentially occur with opportunistically sampled stranded cetaceans, the study population used here consisted of both compromised diseased individuals as well as healthy trauma cases and no clear differences between mean ∑PCB concentrations in stranded and biopsied BNDs (1990-2012), SDs (1991-2009) and KWs (1994-2012) were observed ( Fig. 7). More specifically, mean ∑PCB concentrations in stranded male and female SDs were slightly higher than biopsied male and female SDs but biopsied male BNDs had slightly higher ∑PCB concentrations than stranded male BNDs. For KWs, biopsied males and stranded females had higher ∑PCB concentrations than stranded male and biopsied female KWs, respectively ( Fig. 7). Box and whisker plots (male v female) were generated for ∑PCB and ln ∑PCB concentrations in all stranded and biopsied HPs (1990-2012), BNDs (1990-2012), SDs (1990-2008) and KWs (1994-2012) ( Fig. 8A-D).

( A-D) Distribution map (smooth mean density kernel plots) of ∑PCBs data points in Europe - all cetacean species (all ages) from 1996-2012. (A) - HPs (n = 548); (B) - BNDs (n = 110); (C) - SDs (n = 71) and (D) - KWs (n = 21). Spatial distribution of ∑PCB lipid concentrations produced in Esri ArcMap 10.1 (www.esri.com). Maps are displayed in the WGS84 co-ordinate system. Data points are shown along with local averages. These averages were calculated by kernel smoothing using a polynomial order 5 kernel with power = 0, ridge parameter = 50 and bandwidth based on the spatial distribution of the observations for each species: bottlenose dolphin 0.75 degrees; harbour porpoise 0.5 degrees; killer whale 1.2 degrees; striped dolphin 0.5 degrees. Both the data points and the local averages are displayed in three colours: yellow (∑PCB concentration =  < 20 mg/kg); orange (∑PCB concentration = 20-40 mg/kg lw); and red (∑PCB concentration =  > 40 mg/kg lw).

( A-D) Box and whisker plots for male (M) v female (F) v unknown sex (U) were generated for ∑PCB and ln ∑PCB concentrations for ( A) all stranded HPs (1990-2012); ( B) stranded or biopsied SDs (1990-2008); ( C) stranded or biopsied BNDs (1990-2012); and ( D) stranded or biopsied KWs (1994-2012). The lower line (blue) is the lower PCB toxicity threshold (=9.0 mg/kg lipid, as ∑PCB) for onset of physiological effects in experimental marine mammal studies6. The upper line is the highest PCB toxicity threshold (=41.0 mg/kg lipid, as ∑PCB) published for marine mammals based on marked reproductive impairment in ringed seals in the Baltic Sea6.

We compared ∑PCB concentrations among regions within the NE Atlantic and Mediterranean Sea. Since our ∑PCB data is lognormal, we are able to use the sample geometric mean to estimate the population median ( Tables 1 and 2) and that this estimator is unbiased for doing this10. Mean ∑PCB concentrations and geometric means (with upper and lower 95% confidence intervals) were generated for males and females of each cetacean species and region for all available data (n = 1,009) ( Table 1) and for adults only (n = 401) ( Table 2). These tabulated data were presented along with minimum and maximum values (i.e. range) for males and females for each species and region ( Tables 1 and 2). In general, the ∑PCB sample geometric means (as estimator of population median values) were 25-65% lower than the (arithmetic) means for the same species, but still exceeded even the very highest marine mammal PCB toxicity threshold (∑PCB = 41 mg/kg lipid) for some BND, SD and KW groups in both NE Atlantic and Mediterranean Sea regions ( Tables 1 and 2).

Table 1 ∑PCBs exposure (mean; sample median; geometric mean with upper and lower 95% CI; range) (mg/kg lipid) by species, region and sex (all ages).

Table 2 ∑PCB exposure (mean; sample median; geometric mean with upper and lower 95% CI range) (mg/kg lipid) by species and region (sexually matures only).

For males and females (all ages), high mean ∑PCB concentrations (>100.0g/kg lw) were found for BNDs off Iberian Peninsula (NE Atlantic), western Mediterranean and northern Adriatic Sea; for SDs in western Mediterranean and for KWs (NE Atlantic) ( Fig. 2). Among adults, high individual female ∑PCB concentrations (>100mg/kg lw) were found in KWs from all NE Atlantic regions, in BNDs from England and Wales and in SDs from the western Mediterranean Sea ( Table 2). Mean ∑PCB concentrations in adult female KWs (176.8 mg/kg lipid) in the NE Atlantic greatly exceeded mean concentrations in adult female southern resident (55.4 mg/kg lipid) and transient (58.8 mg/kg lipid) KWs10 off British Columbia (NE Pacific). Adult female KWs in the UK and Ireland had the highest mean ∑PCB concentrations (224.8 mg/kg lipid) followed by KWs in the Strait of Gibraltar (215.4 mg/kg lipid). Mean ∑PCB concentrations in adult female BNDs (61.8-158.3 mg/kg lipid) and SDs (84.5-523.7 mg/kg lipid) in some western European populations also markedly exceeded those in southern resident and transient KWs off British Columbia10 ( Fig. 3)( Table 2). Mean ∑PCB concentrations greatly exceeded all marine mammal PCB toxicity thresholds in mature ( Fig. 3) and immature ( Fig. 4) BNDs, SDs and KWs. Individuals for which the maturity status could not be established, mean ∑PCB concentrations still exceeded all marine mammal toxicity thresholds in BNDs and SDs ( Fig. 5).

The toxicological data show that these populations greatly exceed concentrations at which severe toxic effects are known to occur. Pathological findings on necropsy consistent with immunosuppression and increased susceptibility to disease included macro-parasitic and bacterial pneumonias, high lung and gastric macro-parasite burdens and generalised bacterial infections (septicaemias). These were regularly found on necropsy in stranded HPs5, BNDs and KWs. In Mediterranean SDs distemper due to cetacean morbillivirus (CeMV) infection9 was frequently seen. Several stranded KWs also had multiple dental infections leading to large mandibular abscesses identified on necropsy12. High ∑PCB contamination can cause immunosuppression7 and may be a significant contributing factor in the death of many of the stranded individuals that had fatal infectious diseases on necropsy.

For necropsied cetaceans stranded in the UK (1990-2012), ∑PCB data was obtained for 706 HPs, 38 BNDs and 7 KWs ( Tables 1 and 2). For BNDs (UK), causes of death included trauma (n = 5 male; n = 2 female); infectious disease (n = 2 male; n = 4 female). For the UK-stranded KWs, causes of death included starvation (n = 2), infectious disease (n = 2) and live stranding (n = 2). A cause of death could not be determined for 25 HPs, 11 BNDs and 1 KWs. ∑PCB lipid concentration in UK-stranded male HPs of all ages from 1990-2012 was significantly higher (sample mean = 19.4 mg/kg lw; sample median = 11.5 mg/kg lw; n = 388) than for females of all ages (sample mean = 13.8 mg/kg lw; sample median 8.35 mg/kg lw; n = 318) (ANOVA, P < 0.001) ( Fig. 2).

∑PCB lipid concentrations in subgroups of "healthy" male UK-stranded HPs that were generally in good nutritive condition and died of acute physical trauma (sample mean = 13.8 mg/kg lw; sample median = 9.57 lw; n = 201) had significantly lower ∑PCB than male HPs that were in poorer condition and died of a range of infectious diseases (sample mean = 26.5 mg/kg lw; sample median = 18.9 mg/kg lw; n = 120) (ANOVA, P < 0.001). In female UK-stranded HPs that died of acute physical trauma, ∑PCB lipid concentrations (sample mean = 8.50 mg/kg lw; sample median = 6.41 mg/kg lw; n = 144) were significantly lower than female HPs that died due to a range of infectious diseases (sample mean = 16.8 mg/kg lw; sample median = 12.2 mg/kg lw; n = 132) from the same period (ANOVA, P < 0.001) ( Fig. 2). Male and female HPs that died of starvation were all in a poor or emaciated condition. Both male HP starvation cases (sample mean = 20.7 mg/kg lw; sample median = 12.5 mg/kg lw; n = 26) and female HP starvation cases (sample mean = 27.9 mg/kg lipid; sample median = 9.74 mg/kg lw; n = 16) had higher ∑PCB concentrations than the physical trauma group. For UK-stranded HPs (both sexes), mean ∑PCB mg/kg lipid was 150-328% greater in infectious disease or starvation cases as compared to trauma cases. The increase in ∑PCB on a sample median basis was 130-198% greater for HP disease or starvation cases as compared to HP trauma cases.

Population Dynamics

Such sustained and elevated PCB burdens are likely to have significant effects at the population level in European BNDs, SDs and KWs. Although we cannot directly and causally link high ∑PCB exposures to cetacean population declines, many of these populations are either very small, or show evidence of major and long-term declines or a significant contraction of range. Only very small KW populations are now found in industrialised regions of Europe. There is only one resident population in southern Europe14: a tuna-feeding KW subpopulation in the strait of Gibraltar comprising two pods totalling 36 individuals13. These animals have been monitored every year since 1999. Only six mature females were reported to have calved between 1999 and 2011 and these produced only five calves living more than 1 year. This 6.4% annual female fecundity rate over a 13-year period is one of the lowest recorded reproductive rates for KWs globally and the subpopulation was recommended for listing as "Critically Endangered" due to small population size14. Around NW Scotland and Western Ireland, a small group of only nine KWs is regularly seen15. This group of marine-mammal-eating KWs comprises four adult males, two adult females and three adult female or sub-adult males. Although studied over a 19-year period, no calves have ever been reported within this group15. No other resident or coastal KW groups are found off the Iberian Peninsula or in the Bay of Biscay, although occasional offshore KW sightings occur (e.g.16). Long-term cetacean stranding records in the Netherlands show that KWs were first recorded on the Dutch North Sea coastline in 1783, observed regularly from 1918 to 1963 and completely ceased thereafter until two single KW strandings in 2009 and 201017.

Historic strandings data suggest that multiple BND resident or coastal groups in Europe became depleted or locally extinct in the late-1960s to mid-1970s, including those in the UK (e.g. Morecambe Bay; East coast of England)18 and the North Sea Dutch coast19. The last member of a resident BND population at Arcachon, France, died in 200320 and the small resident BND group (current census n = 27) in the Sado Estuary, Portugal, showed decline due to low calf survival over several decades21. Blubber PCB levels for two females and one male BND (sampled between 1995 and 1997) from this population ranged from 37.1 to 114.0 mg/kg lw. The Mediterranean Sea remains a global PCB hotspot1 where most of the extant marine mammal species, including SDs, BNDs and short-beaked common dolphins (CDs)( Delphinus delphis) have declined over decades14,22. The current IUCN Red List conservation status of BNDs and SDs in the Mediterranean Sea is "Vulnerable". The Mediterranean CD subpopulation is now classified as "Endangered" after experiencing major and generalized decline over the last 40-50 years, particularly in the northern Adriatic Sea and in the eastern Ionian Sea8. Blubber PCB concentrations are significantly higher in CDs and SDs in the Mediterranean Sea compared to the much more abundant NE Atlantic populations23.

For HPs in the NE Atlantic, a single continuous population ranges from France to northern Norway. A separate much smaller Iberian population is estimated to comprise only 4,398 (CV = 0.92) porpoises24. In the Baltic Sea, two separate HP subpopulations exist: one in the western waters (Kattegat, the Belt Seas) estimated at 40,475 individuals (CV = 0.235) and listed as "Vulnerable" (by HELCOM)25 and another in the Baltic Sea proper, which is now very small and has suffered decades of decline and is listed as "Critically Endangered"8. A history of hunting, high chemical pollutant exposure and accidental entrapment in commercial fishing gear (bycatch) are thought to have been responsible for this decline24. A total of 375,358 (95% CI = 256,304-549,713) porpoises was estimated for EU Atlantic continental shelf waters including the Iberian and western Baltic (sub-)populations (July 2005)16. For the NE Atlantic (UK) HP population, a longer calving interval, lower pregnancy rate and later maturation and higher rates of reproductive abnormalities were also recently identified in a necropsy study of 329 female UK-stranded HPs (1990-2012), as compared to HP populations in much less PCB-polluted regions like Iceland and the Gulf of Maine/Bay of Fundy (NW Atlantic)26. Direct observations of reproductive failure (foetal death, abortion, dystocia or stillbirth) were observed in 25/127 (19.7%) of necropsied mature female HPs in the same study26.

Consideration of alternative causes of mortality

Accidental entrapment in commercial fishing gear (bycatch) is often considered to be the greatest conservation threat globally to small cetaceans8,27. In UK strandings and necropsy data (1990-2013 inclusive) bycatch was diagnosed as the cause of death for 296/615 (48.1%) of UK-stranded common dolphins ( Delphinus delphis) (CDs) and 338/1983 (17.0%) of HPs. In contrast, only 9/130 (6.9%) UK-stranded SDs; 4/71 (5.6%) BNDs and 0/7 (0.0%) KWs were diagnosed as bycatch during the same period. Bycatches recorded under the 2013 UK dedicated bycatch sampling programme included 101 days on pelagic trawls and 346 days on static gear vessels and included observed bycatches of 18 HPs, 11 CDs, two SDs and a single BND. Six common dolphins were recorded in pelagic trawls. All other cetacean bycatches were recorded from static net fisheries. Preliminary bycatch estimates for the entire UK fleet during 2013 from systematic observer-based studies provided estimates of HP bycatch of around 1917 animals assuming no pingers (acoustic bycatch deterrent devices) were used (CV = 0.126) and 1652 animals if all UK boats over 12 m used pingers correctly (under EU regulation 812)(CV = 0.147)28. Other estimates for the entire UK fleet in 2013 indicate around 320 CDs and around 470 seals were bycaught28. Bycatch of BNDs, however, was considered too rare to estimate for the entire UK fleet and KW bycatch was not recorded28. A bycatch observer study for the Irish albacore tuna ( Thunnus alalunga) drift-net fishery undertaken in offshore waters of the NE Atlantic in the 1990s found the highest bycatch rates in CDs followed by SDs, with much lower numbers of bycaught BNDs and no KW bycatch29. The highest bycatch rates generally occur in the most numerically abundant species in the NE Atlantic region, HPs and CDs, but with much lower rates of bycatch in BNDs and bycatch unrecorded in KWs in recent years30. Very low numbers of bycatches were found in the long-term observer-based studies of the resident European BND populations:- BNDs in Shannon estuary, Ireland (population size = 107-140; zero bycatch recorded since 1993)[S. Berrow, unpublished data]; Sado estuary, Portugal (population size = 27; zero bycatch recorded from 1983-2013) (Manuel E. Dos Santos, pers. com.) and Slovenia, northern Adriatic Sea (population size estimate = 74; 95% CI = 57-90; 2 cases of bycatch since 2002) [T. Genov, unpublished data, see also31]. Collectively, these necropsy and fisheries bycatch observer studies from approximately 1990 onwards indicate that bycatch is unlikely to be driving population declines of BNDs or KWs around the UK or in the wider NE Atlantic during this period.

Other potential causes of cetacean mortality include ship-strike; acoustic disturbance from high-intensity/anthropogenic acoustic sources; nutritional limitation caused by reduced prey availability (e.g. overfishing or climate change); biotoxins (harmful algal blooms) and infectious disease (e.g. cetacean morbillivirus, CeMV)32. Using CSIP UK strandings and necropsy data (1990-2013 inclusive) ship-strike was a relatively infrequent cause of death accounting for only 17/1983 (0.8%) stranded HPs; 0/130 (0.0%) SDs; 0/71 (0.0%) BNDs and 0/7 (0.0%) KWs necropsied in the UK. All tests for biotoxins in UK-stranded cetacean tissues were negative (or at trace levels) since 199032,33.

The potential impacts of anthropogenic high-intensity acoustic sources on cetaceans include mass stranding events (MSEs) that have occurred on a global basis, predominantly involving beaked whales exposed to mid-frequency active sonars in naval exercises. Non-beaked whale MSEs with a probable acoustic cause have occurred, such as a 2008 CD MSE in the UK, but are very rare in European waters32. No acoustically-induced MSEs were recorded in HPs, SDs, BNDs or KWs in European waters. Acoustically-induced cetacean MSEs have been recorded in the Mediterranean Sea but also primarily involved beaked whales36,37.

In the Mediterranean and Black Seas, all cetacean species where population assessments have occurred are considered to be declining. A range of threats has been proposed including hunting (historically); bycatch; overfishing/prey depletion; habitat degradation; morbillivirus infection and chemical and acoustic pollution. Empirical evidence for the effects of prey depletion (e.g. due to overfishing or climate change effects) leading to nutritional limitation is limited for cetaceans, but diet/nutrition effects have been linked to recruitment in KWs and fin whales ( Balaenoptera physalus) [reviewed in26]. Such processes would potentially increase PCB toxicity in malnourished animals by mobilising lipophilic PCBs from blubber to other body compartments7,23. A recent global review of Cetacean Morbillivirus (CeMV) found that SDs have been negatively impacted by CeMV epizootics in 1990-1992 and 2006-200838 and probably again in 2011 in the western Mediterranean39. The largest CeMV epizootic in 1990-1992 was associated with very high PCB concentrations9. A CeMV epizootic also occurred in long-finned pilot whales ( Globicephala melas) in the Mediterranean Sea in 2006-200740. In contrast, only small periodic CeMV-related mortalities have occurred in individual cetaceans in NE Atlantic region38. No large scale epizootics of CeMV38 or of any other pathogens are known to have occurred in any cetacean species in NE Atlantic region since 1990.

Discussion

The stabilised temporal trends in blubber PCB concentrations in cetaceans in Europe contrast with other industrialised northern hemisphere regions, such as North America, where PCBs have continued to decline in BND41 and KW11,23 populations. PCBs were banned in closed systems in the USA in 19791 and in 1981 in the UK4, but were not phased out until 1987 in European countries that border the Mediterranean Sea1,42. Mean ∑PCB concentrations in adult female KWs in our study substantially exceeded concentrations in both southern resident and transient female KWs off British Columbia11,23. Where females are reproducing normally, blubber PCB concentrations are known to fall significantly with successive pregnancies due to maternal offloading through gestation and lactation. The relatively high ∑PCB concentrations in many adult female KWs in this study is consistent with reproductive failure in at least some individuals, due to either adult females failing to offload PCBs to calves during pregnancy and lactation7 or re-accumulation of dietary PCBs in some post-parturient and reproductively senescent KWs11.

Mean blubber ∑PCB concentrations in male KWs in this study were higher than male "southern resident" KWs off British Columbia (NE Pacific) in the 1990s and were only slightly lower than male "transient" (marine mammal eating) KWs from the same region11,41. Very high mean PCB concentrations have been recorded in blubber biopsies of adult male (n = 4) marine mammal-eating West Coast "transient" KW ecotype off California, USA (mean ∑PCB = 630 mg/kg lipid)43; in individual stranded KWs in Oregon State23; and in a transient adult female that stranded in Washington State in 2002 with ∑PCB = 1,100 mg/kg lipid44. For BNDs, only a population in a highly polluted Superfund site in Brunswick, Georgia (USA) exposed to a major point source of PCB pollution with Aroclor 1268 45 had mean exposures similar to many BND groups in our study. HP blubber ∑PCB concentrations in this study were similar to those associated with immunosuppression and infectious disease mortality in previous multivariate studies of UK-stranded HPs (statistically independent of age, sex, sexual maturity and two quantitative indices of nutritional status)5 and similar to those associated with impaired reproduction in adult female HPs in UK waters26.

A large number of factors can influence PCB exposure and subsequent PCB concentrations in blubber including, but not limited to, blubber sample condition, diet, degree of maternal offloading, year of sampling, season, age, gender, location, species, body condition and whether or not an animal was sampled in good health. Some of these we can control for statistically as potentially confounding factors, others we cannot. Nevertheless, for SDs, BNDs and KWs around the Iberian Peninsula (both NE Atlantic and Mediterranean regions), mean and median ∑PCB concentrations levels were among the highest recorded for cetaceans globally, regardless of sample type/nutritional condition/sex/year of sampling, etc. In terms of blubber sample quality, we are able to obtain 954/1081 ( =  88.3%) fresh or slightly decomposed stranded or biopsied blubber samples, effectively negating the potential bias of sample degradation on PCB determination. Nor did we see any significant overall differences in mean ∑PCB concentrations between stranded and biopsied BNDs, SDs or KWs. This shows that the ∑PCB concentrations in stranded animals were not fundamentally biased in comparison to the free-living biopsied cetaceans.

In relationship to nutritional status, we can show that both male and female HPs that died of trauma had the lowest ∑PCB concentrations and this increased by 190-198% for infectious disease cases and by 130-328% for starvation cases. Although we lack data to test this "nutritional loss" effect rigorously on ∑PCB concentrations in SDs, BNDs and KWs, we can show that the excessively high ∑PCB concentrations occurred in virtually all individual SDs and BNDs around the Iberian Peninsula and for virtually all individual KWs across the NE Atlantic region, irrespective of blubber sample quality, year of sampling, season, gender, nutritive condition and whether or not an animal was sampled in good health.

There are also marked species-specific variations in susceptibility to PCB exposure. The lower PCB toxicity threshold used in this study was predominantly derived from non-cetacean species such as seals, otters and mink7. It is well recognized that mink are one of the most PCB-sensitive species and so it is highly possible that cetaceans are less sensitive to PCB exposure than species like mink. If so, the use of the Kannan threshold7 is likely to overestimate the true PCB risk to cetaceans. However, this lower PCB toxicity threshold is equivalent to only 9 mg/kg lipid (as ∑PCB) - whereas many BND, SD and KW groups in this study had one order of magnitude greater mean or median ∑PCB concentrations (50-350 mg/kg lipid). So, even if the lower PCB toxicity threshold does significantly over-estimate the likely toxicity of PCBs on cetaceans - the very high ∑PCB exposures recorded empirically in European cetaceans in this study (>1,000% greater than 9 mg/kg lipid) still provides compelling evidence for the inherent PCB toxicity risk in this study.

Current threats to cetaceans from persistent organic pollutants (POPs) in Europe appear to be restricted to PCBs. Marked and ongoing declines in tissue concentrations of organochlorine pesticides (e.g. DDTs) have occurred in UK-stranded HPs4 and in western Mediterranean SDs3 and BNDs42. Similar marked declines in tissue contaminants have occurred in UK HPs for butyltins46 and brominated flame retardants following a 2004 EU-ban4. Other newer chemical compounds including the alternative brominated and chlorinated flame retardants such as tetrabromo-p-xylene (TBX), tetrabromo-o-chlorotoluene (TBCT) and 2,3-dibromopropyl-2,4,6-tribromophenyl ether (TBP-DBPE) and Dechlorane Plus (DDC-CO) isomers47, perfluoroalkyl substances (PFASs) such as perfluorooctane sulphonate (PFOS) and perfluorooctanoic acid (PFOA) in48 and emerging organophosphorus flame retardants (PFRs) and plasticisers49 were detected in some UK-stranded HP samples and with the possible exception of PFOS, were at either very low levels or occurred below levels of analytical detectability.

Many European seas (e.g. Baltic, Mediterranean and North Seas) are surrounded by highly industrialised land masses with high human population densities which may at least partly explain the high PCB exposures we have detected1. Mass production of PCBs commenced in 1929, reaching peak levels in the 1960s and 1970s, when many European cetacean populations already appear to be declining. Nearly 97% of the historical use of PCBs occurred in the northern hemisphere50, with the marine environment acting as the ultimate PCB "sink". By the late 1990s, however, only 1% of globally manufactured PCBs were estimated to be in sea water, with 6-7% in marine sediments (reviewed in1), while as of 2005, 1.1 million tons of PCB contaminated material still required disposal by EU Member States, most notably France and Spain51. PCB effects are likely to be particularly pernicious in highly exposed European BND, SD and KW populations by markedly suppressing reproduction and probably subsequent recruitment at individual and population levels. EU regulations to mitigate PCB pollution currently appear insufficient to protect cetacean top predators in the NE Atlantic and Mediterranean Sea. As sentinels of marine ecosystems, monitoring PCB concentrations in marine mammals should be considered for inclusion as an indicator under descriptor 8 within the European Marine Strategy Framework Directive (MSFD) (Directive 2008/56/EC). Our results indicate that European marine mammal habitats are unlikely to meet Good Environmental Status within the MSFD by 2020 and that several dolphin species are unlikely to achieve Favourable Conservation Status now under EC Habitats Directive (Council Directive 92/43/EEC). Further steps to reduce PCB inputs into the European marine environment should include stricter controls on disposal of PCBs, e.g. in buildings with sealants containing PCBs4,52. Also, measures should be taken to limit PCB bioavailability to marine food webs, such as improved management of dredging of harbours/ports, as well as improved containment of PCB environmental contamination from industrial buildings/equipment and domestic waste disposal (e.g. landfill).

In conclusion, this pan-European meta-analysis of stranded or biopsied cetaceans demonstrates that several European cetacean species, specifically BNDs, SDs and KWs, currently have markedly elevated blubber PCB concentrations. Particular "PCB hotspots" included the western (SDs and BNDs) and central (BNDs) Mediterranean Sea and SW Iberia, the Gulf of Cadiz (BNDs) and the Strait of Gibraltar (BNDs and KWs). Despite an EU ban on the use and manufacture of PCBs in the mid-1980s, blubber PCB concentrations are still very high, possibly having reached a "steady state" between environmental input and degradation, meaning that high PCB exposures are set to continue for the long-term in cetacean top predators in Europe. These high and stable PCB exposures are associated with small populations, long-term population declines or contraction of range in several dolphin species in Europe (NE Atlantic and Mediterranean Seas) that were not adequately explained by other factors (e.g. bycatch or other anthropogenic causes of mortality). Bycatch is common in the most abundant cetacean species in Europe, but is comparatively rare in BNDs and virtually unrecorded in recent years for KWs, suggesting that the ongoing population declines in these two species are predominantly driven by other processes, with bioaccumulation of PCBs through marine food chains being the predominant factor. A lack of recruitment in monitored KW and BND populations is also consistent with PCB toxicity as the likeliest cause of their declines. In the Mediterranean Sea, the SD has suffered recurrent CeMV mortalities, which may have been exacerbated by the high and immunotoxic level of PCB exposure. Without significant mitigation, PCBs will continue to drive population declines or suppress population recovery in Europe for many decades to come. Measures to significantly reduce inputs of PCBs into the marine environment from terrestrial and other sources are urgently needed. Further studies are also needed to better assess PCB exposure and quantify toxic effects in marine apex predator populations in Europe. Finally, the potential impact of PCB bioaccumulation in marine ecosystems may extend beyond European waters, particularly in globally distributed marine apex predators such as KWs, false killer whales ( Pseudorca crassidens) and great white sharks ( Carcharodon carcharias).

Methods

Between January 1990 and December 2012, HPs (n = 706), SDs (n = 220); BNDs (n = 131) and KWs (n = 24) in NE Atlantic and Mediterranean Sea regions of Europe were necropsied (HP n = 706; SD n = 119; BND n = 93; KW n = 9) or biopsied (SD n = 101; BND n = 38; KW n = 15) for skin and blubber samples using standardised methodologies5,9.

Necropsies of stranded cetaceans (n = 929)

A total of 929 cetaceans were sampled or necropsied after stranding on a beach or being found dead at sea. These comprised 706 HPs; 121 SDs 93 BNDs and 9 KWs. In the UK, stranded or bycaught HPs (n = 706), BNDs (n = 38) and KWs (n = 7) were necropsied using methods based on a standardised European necropsy protocol53. Necropsies were also conducted on stranded SDs (n = 119) from western Mediterranean, BNDs from Galicia, Spain (n = 11) and Portugal (n = 5) and a single KW stranded in the Strait of Gibraltar, Spain using similar methods. Wherever possible, stranded cetaceans in fresh or slight decomposition were prioritized for necropsy to limit decomposition of tissue samples including blubber. Fresh or slightly decomposed carcasses comprised 802/929 (86.3%) of all necropsies and included 660 HPs; 103 SDs; 33 BNDs and 6 KWs. Moderately decomposed carcasses accounted for 72/929 (7.75%) of necropsied carcasses including 37 HPs; 21 BNDs; 11 SDs; and 3 KWs. Only 18/929 (1.94%) carcasses were in advanced decomposition comprised 8 HPs; 5 BNDs and 5 SDs. State of decomposition could not be determined for 37/929 (3.98%) carcasses comprising 34 BNDs; 2 SDs and 1 HPs. Sex was determined on necropsy. Sex was not recorded or could not be determined (e.g. due to loss of gonads from scavenging animals) for 65 SDs and 6 BNDs. Causes of death in UK-stranded animals were determined by specific and established diagnostic criteria5. Additional chlorobiphenyl congener and other data were included from a single KW that stranded at Roches Point, Cork harbour, Ireland and was sampled on 8 th July 200112.

Blubber samples from biopsied cetaceans (n = 152)

Blubber biopsies of free-living SDs from the western Mediterranean Sea (n = 101), BNDs from the Gulf of Cadiz (n = 11), Strait of Gibraltar (n = 11) and northern Adriatic Sea (n = 6) and skin/blubber biopsies of KWs from Canary Islands (n = 8) and Strait of Gibraltar (n = 7) were obtained using a range of standardised techniques including use of a cross-bow54. For BNDs and SDs in the western Mediterranean Sea, the skin and blubber tissue biopsy was excised from bow-riding dolphins using a sterile biopsy dart of the butterfly valve type, shot with a spear gun or a compressed air pistol from a boat, a non-destructive technique commonly used in cetacean research54. Darts were aimed at the region posterior to the dorsal fin. The precise age or sex of the individual samples was unknown. The samples collected contained about 0.8 g of blubber. Once collected, the skin and blubber biopsy samples were wrapped in aluminum foil and preserved at −20 or −80C until analysis. Sex of biopsied animals was determined using molecular techniques55. Additional chlorobiphenyl congener and other data were included from BNDs (n = 8) biopsied in the Shannon estuary, Ireland in September 200056.

Assessment of sexual maturity in cetaceans

Sexual maturity was determined in necropsied animals by analysis of gonadal material57. For those individuals where sexual maturity status data based on gonadal assessments were unavailable, published data on age and length at both sexual and physical maturity were used as criteria for assessing maturity status - BNDs58, SDs59,60 and KWs61. Animals assessed as sexually immature comprised HPs (n = 413), SDs (n = 62), BNDs (n = 35) and KWs (n = 2). Animals assessed as sexually mature comprised HPs (n = 280), SDs (n = 59), BNDs (n = 41) and KWs (n = 21). Sexual maturity status could not be determined for HPs (n = 13), SDs (n = 99), BNDs (n = 55) and KWs (n = 1).

Determination of ∑PCB in blubber samples (mg/kg lipid)

Two different laboratories conducted chemical determination of PCB concentrations in 1073/1081 (99.3%) blubber samples. The Centre for Environment, Fisheries and Aquaculture Science (Cefas) in Lowestoft, UK conducted analyses of ∑PCB for 706 HPs, 82 BNDs and 24 KWs stranded or biopsied in the NE Atlantic or central Mediterranean Sea between 1990 and 2012. The Department of Animal Biology and Biodiversity Research Institute (IRBio), University of Barcelona, in Barcelona, Spain analysed 220 SDs and 41 BNDs, predominantly from the western Mediterranean Sea. A third laboratory, the Marine Institute, in Galway, Ireland examined blubber biopsies from 8 BNDs from the Shannon estuary, Ireland56.

Cefas data

The blubber samples at the Cefas lab were analysed for 25 individual chlorobiphenyl congeners (IUPAC numbers: 18, 28, 31, 44, 47, 49, 52, 66, 101, 105, 110, 118, 128, 138, 141, 149, 151, 153, 156, 158, 170, 180, 183, 187, 194) (sum25CBs mg/kg lipid) using internationally standardised methodology4,5. After thawing, the homogenised subsamples (~5g) were dried by mixing with anhydrous sodium sulphate and storing in a freezer for a minimum of 12 hours prior to further analysis. The samples were subjected to Soxhlet extraction using of acetone: n-hexane 1:1 ( v:v) for 5.5 hours. The total extractable lipid content was determined gravimetrically after evaporation of the solvent from an aliquot of the uncleaned extract. Depending on the lipid content of the samples, varying volumes of the biota extracts were cleaned to have ~50 mg of lipid in the samples for PCB analysis. For marine mammal blubber samples, this was usually 1mL out of the 100mL extract.

For clean-up and PCB analyses, an aliquot of the blubber extracts were cleaned up and fractionated using alumina (5% deactivated) and silica (3% deactivated) columns, respectively. The final GC-ready fractions were spiked with PCB53 internal standard and made up to a final volume of 1 ml. PCB concentrations were determined with an Agilent 6890 GC with μECD. The separation of analytes was performed on a 50.0 m × 200 μm, 0.33-μm-film-thickness DB-5 capillary column (J&W). The carrier and ECD make-up gas were hydrogen (32.2 psi constant pressure, initial velocity 50 cm/s) and argon/methane (95:5), respectively. The initial oven temperature was 90 °C, held for 2.00min, then increased to 165 °C at 15 °C/min, to 285 °C at 2 °C/min and finally held for 23 min. The injector temperature and detector temperature was 270 °C and 300 °C, respectively. A 1-μl extract was injected in splitless mode with a purge time of 2 min. The PCB standard solutions contained the following 27 compounds in iso-octane: Hexachlorobenzene; p,p'-DDE; CB101; CB105; CB110; CB118; CB128; CB138; CB141; CB149; CB151; CB153; CB156; CB158; CB170; CB18; CB180; CB183; CB187; CB194; CB28; CB31; CB44; CB47; CB49; CB52; CB66; together with the internal standard CB53. Quantitation was performed using internal standards and 7 calibration levels (range 0.5-100ng/ml).

For Quality Assurance and Quality Control the Cefas laboratory participates biannually in proficiency testing scheme Quasimeme (Quality Assurance of Information for Marine Environmental Monitoring in Europe) as external quality assurance. All analyses were carried out under full analytical quality control procedures that included the analysis of certified reference material(s) and a blank sample with every batch of 10 samples analysed so that the day-to-day performance of the methods could be assessed. If levels of target analytes in the samples were outside of the range of the instrument calibration, extracts were diluted to be within range and re-analysed. Reference material used was BCR349 (cod liver oil; European Bureau of Community reference), which has been used in the Cefas lab since 1993. The results obtained for the reference materials were plotted as Shewhart quality control charts for each compound determined. The charts had previously been created by the repeated analysis of the above certified reference materials in the Cefas Lowestoft Laboratory using the North West Analytical Quality Analyst software™ (Northwest Analytical Inc., USA). Warning and control limits had been defined for the charts as 2σ and 3σ - 2x and 3x the standard deviation from the mean for each compound. The results obtained for all samples analysed were accepted as valid as the results for the certified reference materials were within the limits set by the control charts.

IRBio data

Lipid weight concentrations (mg/kg lipid) of 23 individual chlorobiphenyl congeners (IUPAC numbers: 95, 101, 136, 110, 151, 144, 149, 118, 153, 141, 138, 187, 183, 128, 174, 177, 202, 171, 180, 170, 201, 203 and 195) (sum23CBs mg/kg lipid) were generated for 220 SDs and 41 BNDs from western Mediterranean Sea using internationally standardised methodology42,62. Approximately 1 g of tissue was ground with anhydrous sodium sulphate using a mortar. The mixture was extracted with n-hexane for 4 h in a Soxhlet apparatus with a capacity of 125 ml. The solution obtained was concentrated to 40 ml. A portion of this extract (10 ml) was used to gravimetrically determine the quantity of extractable fat per gram of blubber. A further quantity was mixed with sulphuric acid for the purpose of cleaning it and the resulting extract was concentrated to 1 ml and centrifuged for five minutes.

A sample volume of 0.5 μL was injected in splitless mode (1 min splitless vent time) into a Thermo Scientific ITQ 900 GC/MS system equipped with a 30-m DB-5 MS (95% metylsilicone and 5% phenyl) fused-silica capillary column with an ID of 0.25 mm and a film thickness of 0.25 μm, with helium as the carrier gas at constant flow of 1 ml min −1. Injection port and transfer line temperatures were 270 and 300 °C, respectively. The oven temperature program was as follows: 60 °C for 1 min, an increase to 315 °C at 6 °C min −1 and a final hold of 20 min. Source temperature was 200 °C. Mass spectrometry was performed in electron impact mode. Full-scan spectra were acquired over the m:z range 50-550.

The total PCB concentration (tPCB) was calculated as the sum of the 23 congeners known as IUPAC# 95, 101, 136, 110, 151, 144, 149, 118, 153, 141, 138, 187, 183, 128, 174, 177, 202, 171, 180, 170, 201, 203 and 195. Identification and quantification of the individual compounds were performed by comparison with external reference standards calibrated with a 6-point calibration curve encompassing the entire concentration range. The detection limits were between 0.1 to 1 μg kg −1 for the PCBs standards. Analyses were performed in a series of 10 samples and 1 blank. The recoveries of the PCBs were calculated by adding known amounts of a standard to 12 homogenised replicates of the same sample; recovery levels ranged from 82-101%.

Marine Institute (Galway) data

The Marine Institute, in Galway, Ireland examined blubber biopsies collected in 2000 from 8 BNDs from the Shannon estuary, Ireland [ see 56 for Methods].

Data integration

A conversion factor [0.9] was generated to integrate the different sets of PCB data ( Cefas, IRBio and Marine Institute) using a statistical analysis of the 13 CB congeners common to both methods [i.e. sum25CBs = sum18CBs*0.9]. The combined and converged PCB data was referred to as "sum18-25CBs (mg/kg lipid)" or \xAD\xF4PCB ( Supplementary Dataset).

Spatial and temporal trends in ∑PCB exposure

To assess the temporal trends in HP and SD data we fitted a Generalised Additive Model (GAM) to the data using the R package mgcv 63. We smoothed the data using thin plate regression splines and the degree of smoothing was determined by the integrated generalised cross validation function64. The smoothing parameter gamma was set at 1.4 for HPs and set at 2.0 for SDs. To assess the spatial distribution of HP and SD data we used ∑PCB data from 1996-2012 showing spatially smoothed mean values for the ∑PCB data. This period was chosen to maximise data most representing current level of ∑PCB exposure in these species based on their temporal trends. Figures showing the spatial distribution of ∑PCBs (lipid weight) were produced in Esri ArcMap 10.1 ( www.esri.com). Data points are shown along with local averages. These averages were calculated by kernel smoothing using a polynomial order 5 kernel with power = 0, ridge parameter = 50 and bandwidth based on the spatial distribution of the observations for each species: bottlenose dolphin 0.75 degrees; harbour porpoise 0.5 degrees; killer whale 1.2 degrees; striped dolphin 0.5 degrees.

Comparison of ∑PCBs concentrations in relation to marine mammal toxicity thresholds

Two PCB toxicity thresholds were used in this study. A lower PCB toxicity threshold was used for the onset of physiological endpoints in marine mammals of 17 mg/kg lipid weight (lw) (as Aroclor 1254)7, that was calculated to be equivalent to 9.0 mg/kg lw (as ∑PCB) in this study. A higher PCB toxicity threshold, the highest reported in marine mammal toxicology studies, of 77 mg/kg lw (as Clophen 50) for reproductive impairment in Baltic ringed seals ( Pusa hispida)6 was calculated to be equivalent to 41 mg/kg lw (as ∑PCB) in this study. Differences for sex and cause of death in mean ∑PCBs concentrations in HPs only were investigated using univariate analysis of variance (ANOVA). Since only 4 months of ∑PCB data was available for HPs and SDs in 1990, some statistical analyses were conducted from 1991 onwards for these two species. All ∑PCB data were natural logarithm transformed (ln) prior to statistical analysis to satisfy the requirement for the residuals of ∑PCB data to be normally distributed.

We determined long-term spatial and temporal trends in ∑PCB in two cetacean data sets being UK-stranded or bycaught harbour porpoises (HPs) (n = 706) from 1990-2012 and stranded or biopsied striped dolphins (SDs) (n = 220) from 1990-2009 in the western Mediterranean Sea. Secondly, we assessed mean, sample median and population median (predicted by the geometric mean of the sample ∑PCB data) for ∑PCB exposure in male and female HPs of all ages stranded in the UK (n = 706) and stranded/biopsied SDs (n = 220), bottlenose dolphins (BNDs) (n = 131) and killer whales KWs (n = 24) from NE Atlantic and Mediterranean waters. Since our ∑PCB data is lognormal - we can use the geometric mean (GEOMEAN) as the best estimate of the population median together with upper and lower 95% CIs10. ∑PCB concentrations in UK HPs that were "healthy" and died of acute physical trauma (control group) (n = 345) were compared to ∑PCB concentrations in HPs that died of a range of infectious diseases (case group) (n = 252). Sex differences in ∑PCB concentrations of all species were assessed for 3 subsets of animals: sexually mature individuals only; sexually immature animals only; and those individuals where sexual maturity status was undetermined. Finally, we compared mean ∑PCB concentrations between stranded and biopsied BNDs, SDs and KWs (all HPs were stranded).

Population Dynamics

We reviewed the current population dynamics data, mortality data (including bycatch) and reproductive status of the extant KW and other odontocete populations in European waters using strandings and necropsy data from this and other studies and systematic local and international observational surveys and other data on conservation status collected over the past 15-20 years (e.g. IUCN criteria)8.

Permits

The UK Cetacean Stranding Investigation Programme conducts work on UK strandings under contract to the UK Department of Environment, Food and Rural Affairs. It has appropriate licenses from the relevant authorities (Natural England, Scottish Natural Heritage and Natural Resources Wales) to allow it to collect and hold carcasses and samples from European Protected Species, in line with UK legislation enacted under the EU Habitats Directive. The blubber samples analysed at the University of Barcelona were obtained from the biological tissue bank of the University of Barcelona (BMA Tissue Bank) and originated either from biopsies or from naturally stranded individuals. In Spain, no specific permits are required to carry out research on samples stored at public institutions. The Spanish Government translates the competency for wildlife conservation to the regional governments of the various Autonomous Communities involved in this study and the latter provided the permits for attending strandings, obtaining tissue samples and using them for scientific studies. All the technicians that work on marine animal strandings from the Portuguese Wildlife Society are licensed for the capture, handling, tagging and sample collection in mainland Portugal under the Decree-Law n° 140/99 of 24th April, with new redaction given by Decree-Law n° 49/2005 of 24th February and Decree-Law n° 316/89 of 22nd September. The licenses were issued by the Institute of Nature Conservation and Forestries (Government of Portugal) every year and all blubber samples were sent to the UK with the necessary CITES certification. Biopsy sampling in Slovenia was conducted under the permit of the Slovenian Environmental Agency #35601-102/2010-4. The samples in the Shannon estuary were collected under license from the National Parks and Wildlife Service.

Additional Information

How to cite this article: Jepson, P. D. et al. PCB pollution continues to impact populations of orcas and other dolphins in European waters. Sci. Rep. 6, 18573; doi: 10.1038/srep18573 (2016).

Acknowledgements

The research was conducted by the UK Cetacean Strandings Investigation Program (CSIP) and the Centre for Environment, Fisheries and Aquaculture Science (Cefas) under contract to the UK Department for Environment, Food and Rural Affairs (Defra) and the Devolved Governments of Scotland and Wales. Additional funding for toxicological analyses was provided by ASCOBANS under contract "SSFA2010-3" and by the Fundació Barcelona Zoo. The Gulf of Cádiz and Strait of Gibraltar samples collection were sponsored by Loro Parque Foundation, Fundación Biodiversidad, CEPSA and ECOCET Project (CGL2011-25543) and logistical support for sample sexing was provided by Laboratorio de Ecología Molecular, Estación Biológica de Doñana, CSIC (LEM-EBD). Samples from Galicia were obtained with the support of Direccion Xeral de Conservacion da Natureza-Xunta de Galicia, co-financed with European Regional Development Funds (ERDF/FEDER). S. Murphy was supported by a Marie Curie International Outgoing Fellowship within the Seventh European Community Framework Programme. R. de Stephanis and J. Giménez were supported by the Severo Ochoa Programme for Centres of Excellence in R + D + I (SEV-2012-0262). A. D. Foote was supported by a Lawski Foundation stipend.

Author information

Affiliations

1. Institute of Zoology, Zoological Society of London, Regent's Park, NW1 4RY, London, UK

Paul D. Jepson, Rob Deaville, Sinéad Murphy, Andrew A. Cunningham, Matthew W. Perkins & Robin J. Law

2. Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, NR33 0HT, Lowestoft, UK

Jonathan L. Barber, Jon Barry, David L. Maxwell, Alexandra Papachlimitzou & Robin J. Law

3. Department of Animal Biology and Biodiversity Research Institute (IRBio), University of Barcelona, Barcelona, Spain

Àlex Aguilar & Asunción Borrell

4. Scottish Marine Animal Stranding Scheme, SRUC Veterinary Services Drummondhill, Stratherrick Road, Inverness, IV2 4JZ, UK

Andrew Brownlow, Nicholas J. Davison & Mariel ten Doeschate

5. Environment and Sustainability Institute, University of Exeter, Cornwall Campus, TR10 9EZ, Penryn, Cornwall, UK

James Barnett

6. Marine and Freshwater Research Centre, Galway-Mayo Institute of Technology, Galway, Ireland

Simon Berrow

7. CIRCE, Conservation, Information and Research on Cetaceans, Cabeza de Manzaneda 3, 11390, Algeciras, Spain

Ruth Esteban & Philippe Verborgh

8. Marine Animal Tissue Bank - Portugal, Soc. Portuguesa de Vida Selvagem, Dep. Biologia - Univ. do Minho & CESAM, Portugal

Marisa Ferreira

9. Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE‐752 36, Sweden

Andrew D. Foote

10. Morigenos-Slovenian Marine Mammal Society, Kidričevo nabrežje 4, Piran, 6330, Slovenia

Tilen Genov

11. Institute for Biodiversity Studies, Science and Research Centre, University of Primorska, Koper, Slovenia

Tilen Genov

12. Department of Biodiversity, Faculty of Mathematics, Natural Sciences and Information Technologies, University of Primorska, Koper, Slovenia

Tilen Genov

13. Department of Conservation Biology, Estación Biológica de Doñana (EBD-CSIC), Americo Vespuccio S/N, Isla Cartuja, 42092, Sevilla, Spain

Joan Giménez & Renaud de Stephanis

14. Cornwall Wildlife Trust Marine Strandings Network, Five Acres, Allet, TR4 9DJ, Truro, UK

Jan Loveridge & Nick Tregenza

15. Coordinadora para o Estudio dos Mamíferos Mariños (CEMMA), Apartado 15, Gondomar, 36380, Pontevedra, Spain

Ángela Llavona

16. Sociedad de Estudios de Cetáceos en Canarias (SECAC), Lanzarote, Spain

Vidal Martin

17. Marine Environmental Monitoring, Penwalk, Llechryd, Cardigan, SA43 2PS, Ceredigion, UK

Rod Penrose

18. The Natural History Museum, Cromwell Road, SW7 5BD, London, UK

Brian Smith

19. Facultad de Veterinaria, Universidad de Las Palmas de Gran Canaria, Gran Canaria, Spain

Antonio Fernandez

Contributions

P.D.J., R.D., À.A., A.B., S.M., J.B., A.B., J.B., S.B., N.D., R.E., M.F., T.G., J.G., J.L., Á.L., V.M., R.P., M.W.P., B.S., R.de.S., N.T., P.V. and A.F. assisted with collection of blubber samples and associated biological and other data. P.D.J., R.D.,. J.L.B., À.A., A.B., S.M., J.B., A.B., M.tD., D.L.M. and R.L.L. helped to analyse the data. J.L.B., À.A., A.B., D.L.M., A.P. and R.J.L. conducted chemical analysis of blubber samples. P.D.J., R.D., J.L.B., À.A., A.B., S.M., J.B., A.B., J.B., S.B., A.A.C., M.tD., N.D., R.E., M.F., A.D.F., T.G., J.G., J.L., Á.L., V.M., D.L.M., A.P., R.P., M.W.P., B.S., R.deS., N.T., P.V., A.F. and R.J.L. helped with writing and editing final manuscript.

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Jepson, P., Deaville, R., Barber, J. et al. PCB pollution continues to impact populations of orcas and other dolphins in European waters. Sci Rep 6, 18573 (2016). https://doi.org/10.1038/srep18573
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Further reading

Kirsty Blackburn

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PCB pollution continues to impact populations of orcas and other dolphins in European waters - Scientific Reports

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Paper: What caused the UK's largest common dolphin mass stranding event?

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Paper: What caused the UK's largest common dolphin mass stranding event?

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This paper published in the journal PLoS One in 2013, describes the investigation of the UK's largest mass stranding event (MSE) of short-beaked common dolphins, which occurred in Falmouth Bay, Cornwall in June 2008.

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Paul D. Jepson,

Robert Deaville,

Karina Acevedo-Whitehouse,

James Barnett,

Andrew Brownlow,

Robert L. Brownell Jr.,

Frances C. Clare,

Nick Davison,

Robin J. Law,

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Abstract

On 9 June 2008, the UK's largest mass stranding event (MSE) of short-beaked common dolphins ( Delphinus delphis) occurred in Falmouth Bay, Cornwall. At least 26 dolphins died, and a similar number was refloated/herded back to sea. On necropsy, all dolphins were in good nutritive status with empty stomachs and no evidence of known infectious disease or acute physical injury. Auditory tissues were grossly normal (26/26) but had microscopic haemorrhages (5/5) and mild otitis media (1/5) in the freshest cases. Five lactating adult dolphins, one immature male, and one immature female tested were free of harmful algal toxins and had low chemical pollutant levels. Pathological evidence of mud/seawater inhalation (11/26), local tide cycle, and the relative lack of renal myoglobinuria (26/26) suggested MSE onset on a rising tide between 06∶30 and 08∶21 hrs (9 June). Potential causes excluded or considered highly unlikely included infectious disease, gas/fat embolism, boat strike, by-catch, predator attack, foraging unusually close to shore, chemical or algal toxin exposure, abnormal weather/climatic conditions, and high-intensity acoustic inputs from seismic airgun arrays or natural sources (e.g., earthquakes). International naval exercises did occur in close proximity to the MSE with the most intense part of the exercises (including mid-frequency sonars) occurring four days before the MSE and resuming with helicopter exercises on the morning of the MSE. The MSE may therefore have been a "two-stage process" where a group of normally pelagic dolphins entered Falmouth Bay and, after 3-4 days in/around the Bay, a second acoustic/disturbance event occurred causing them to strand en masse. This spatial and temporal association with the MSE, previous associations between naval activities and cetacean MSEs, and an absence of other identifiable factors known to cause cetacean MSEs, indicates naval activity to be the most probable cause of the Falmouth Bay MSE.

Citation: Jepson PD, Deaville R, Acevedo-Whitehouse K, Barnett J, Brownlow A, Brownell Jr. RL, et al. (2013) What Caused the UK's Largest Common Dolphin ( Delphinus delphis) Mass Stranding Event? PLoS ONE 8(4): e60953. https://doi.org/10.1371/journal.pone.0060953

Editor: Andreas Fahlman, Texas A & M University-Corpus Christi, United States of America

Received: January 17, 2013; Accepted: March 4, 2013; Published: April 30, 2013

Copyright: © 2013 Jepson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This investigation was funded by the UK Department for Environment, Food and Rural Affairs under under a variation to contract number CR0364. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: All acoustic and nonacoustic data relating to the naval exercises in this investigation was provided exclusively by the Royal Navy (UK Ministry of Defence). This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

Introduction

Cetacean mass stranding events (MSEs) are commonly described as two or more cetaceans (excluding a cow-calf pair) of the same species coming ashore, usually alive, at the same time and place [1]. Historically, at least 19 cetacean species have been affected, with some species, including false killer whales ( Pseudorca crassidens), long-finned pilot whales ( Globicephala melas), short-finned pilot whales ( Globicephala macrorhynchus), Atlantic white-sided dolphins ( Lagenorhynchus acutus) and white beaked dolphins ( Lagenorhynchus albirostris), stranding more often than others [1], [2]. The cause of these strandings has puzzled humans for centuries and, although numerous theories have been developed, few MSEs have been studied in detail and seldom has a definitive cause been established (reviewed in [1]).

Cetaceans that mass strand are generally pelagic odontocetes with a highly evolved social structure (reviewed in [1]). Proposed causal factors for cetacean MSEs are numerous and include: becoming trapped on a receding tide (e.g. in long meandering channels, broad tidal flats, strong or unusual currents, extreme tidal flow or volume) [3]; navigational errors associated with topographical features forming natural "whale traps" (such as Wellfleet Bay, Cape Cod, Massachusetts, USA; [4]); geomagnetic disturbances and errors in navigation while following geomagnetic contours [5]; disturbance of echolocation by multiple reflections in bays [6]; pelagic cetaceans following prey close inshore [7]; escaping from predators [1]; disease in one or more individuals in a social group leading to some or all of the remainder of the group stranding [8]; algal toxins and unusual environmental conditions such as electrical storms and other meteorological events; earthquakes and high-intensity acoustic fields (reviewed in [1]). Species-specific behaviour in response to "panic" is also suggested as a potential factor explaining why some cetacean species mass strand more frequently than others (reviewed in [1]).

Prior to the common dolphin ( Delphinus delphis) MSE in Falmouth Bay on 9 June 2008, only three common dolphin MSEs, all of undetermined cause, have been recorded in the UK. One of these consisted of five dolphins on 3 November 1915 at Tean, Isles of Scilly; another, of five animals near Invergordon, Ross and Cromarty on 8 February 1937, and a third involved 15 dolphins near Pembroke, Wales on 11 August 1938 (listed by [9]). Between1990-2011, 1,714 common dolphins were reported stranded in the UK and 976 in Cornwall (data: Cornwall Wildlife Trust Marine Strandings Network, UK). In the same period over 500 systematic post-mortem examinations have been conducted on UK-stranded common dolphins (over 300 of them from Cornwall), with the majority (approx. 51%) determined to have died due to incidental capture in commercial fishing gear (by-catch) [10], [11], [12]. By-catch events, in which dead stranded animals are widely distributed, are very different spatially and temporally from the MSE reported in the present study.

The Falmouth Bay Mass Stranding Event (9 June 2008). At 08∶21 hrs (BST) on 9 June 2008, an initial report was received by Falmouth Marine Coastguard Agency of a dead short-beaked common dolphin in Porth Creek, a small tributary of the Percuil River. The Percuil River is a large tributary of the Fal Estuary, a deep tidal ria in Cornwall, in the southwest of the UK that has a tidal range of around 5 m (see Figures 1 and 2). Subsequently, additional live and dead dolphins were found in the area and trained volunteers worked during the day on foot, in boats and swimming, to recover more carcasses and to try to herd larger groups of live common dolphins back out to sea. The earliest eyewitness report of a live dolphin stranding was at Cellar Beach in the village of Place at approximately 08∶15 but the exact timing of this sighting is unconfirmed (Table 1, Figure 1). This live dolphin was refloated twice and later joined a larger free-swimming dolphin group. The total number of dolphins involved in the MSE is not definitely known, but 24 common dolphins were eventually found dead in Porth Creek on 9 June and another two that stranded alive (one in Porth Creek and one near Trelissick approx. 6.5 km NNW of Porth Creek in the River Fal) were euthanized. All of the dead dolphins found in Porth Creek were below the high water level (some closer to mid-water) and all were retrieved for necropsy. A dead dolphin was reported floating at the mouth of the Helford River on 11 June but this was not confirmed and no carcass was obtained for necropsy. The stranding locations on 9 June covered approximately 20 km of the Falmouth Bay and estuary coastline from Gillan Creek to Trelissick (Figures 1 and 2).

Red box show location of 25/26 dead and necropsied animals found in Porth Creek, Cornwall (see Figure 2). Data sourced from Cornwall Wildlife Trust Marine Strandings Network.

Solid line  =  high water mark. Dotted line  =  low water mark. Data sourced from Cornwall Wildlife Trust Marine Strandings Network.

Groups of free-swimming common dolphins were also reported to be seen in Falmouth Harbour at 09∶30 hrs and Gillan Creek at 16∶30 hrs on 9 June and at the mouth of Porth Creek and near Place through the morning of 9 June (see Figures 1 and 2 and Table 1). Eyewitnesses consistently described their behaviour as "swimming continuously in tight circles, being vocal, fluke slapping, leaning sideways, and often with one or more individuals attempting to strand". The dolphins in Falmouth Harbour were also described by eyewitnesses as "appearing to ignore boats and the presence of humans and were resistant to attempts to herd them out of the harbour" [17]. An unknown number of dolphins was successfully guided into the main estuary and swam towards conspecific(s) further out in deeper water. It is not known whether any of the dolphins that were seen free-swimming close to shore on the morning of 9 June at Place and Porth Creek were any of those seen later that morning in Falmouth Harbour and that afternoon at Gillan Creek and Trelissick. High water in Porth Creek on 8 June was at 09∶44 and 21∶52 hrs and on 9 June at 10∶37 hrs (BST). The dolphins furthest into the creek were at a point where the only water present at low tide is a shallow stream. The weather on the day of the MSE and on the days immediately prior to the event was dry and sunny with no unusual storm activity in the region (data: UK Met Office).

The objectives of the present study were to conduct pathological investigations on the individuals that died and to investigate all available evidence to determine possible causes of the MSE.

Materials and Methods

Information on live and dead dolphins involved in the MSE

Information on the times of discovery and locations of live and dead-stranded common dolphins involved in the MSE was gathered from a variety of opportunistic sources. Volunteers from Cornwall Wildlife Trust Marine Strandings Network (CWTMSN) and British Divers Marine Life Rescue (BDMLR) were interviewed after the event and further information was solicited from bystanders and local maritime agencies including the Coastguard, the Falmouth Harbour Authority and the Cornwall Sea Fisheries Committee. Anecdotal reports from eyewitnesses of the events prior to, during and after the MSE were examined to try to clarify the timing and significance of any activities that may have had a bearing on the event. Collated information is shown in Figs. 1 and 2 and Table 1.

Gross necropsies

Carcasses were recovered from the intertidal zone and all 26 dead common dolphins were systematically examined using a standard necropsy protocol [11] beginning within 8 hours of being found dead and terminated 3 days later. Ambient temperature was 14-25°C on the day of stranding. Sexual maturity was determined from gonadal material. Five-ten teeth were obtained from the middle of the left mandible for age estimation and stored frozen (−20°C). Six ears (tympanoperiotic complexes) from the freshest dolphins ( Table 2) were grossly dissected and preserved in 10% formalin for further examination. A standard set of tissue samples was taken, depending on carcass condition, for a range of standard diagnostic tests including microbiology, histopathology, immunohistochemistry (anti-fibrinogen and anti-myoglobin), molecular detection of morbillivirus and other diagnostic studies (see below).

Bacteriology

Tissue samples or swabs of selected tissues, including liver, kidney, lung and brain, were taken aseptically for bacteriological examination using aerobic, anaerobic and capnophilic incubation and standardised methods [11]. Swabs or tissues were inoculated directly onto either Columbia blood agar base (Oxoid, CM331) with 5% horse blood and incubated aerobically, anaerobically or capnophilically at 37°C and observed at 1, 2 and 5 days, or inoculated onto 5% sheep blood agar (Oxoid, CM0271) and MacConkey agar (Oxoid CM0007) and incubated at 37°C in a capnophilic atmosphere and examined daily for 7 days. Lung samples were incubated aerobically on Columbia blood agar base (Oxoid, CM331) with 5% horse blood and on Sabourauds dextrose agar (Oxoid CM41) in an aerobic atmosphere for fungal isolation. Any organisms recovered were identified using conventional methods including growth characteristics, colony morphology, staining properties and biochemical characterisation using the API identification system (bioMérieux, France). Culture methods and identification of Brucella species isolated from tissues utilised standardised methodologies similar to those described by [18] and were confirmed as Brucella ceti using (i) Polymerase Chain Reaction (PCR) amplification of an IS 711 element downstream of the base-pair (bp) 26 gene, (ii) PCR amplification of the outer membrane proteins ( omp) 2 locus and (iii) analysis of the genome using restriction enzymes [19], [20].

Histopathology

A range of tissue samples from all 26 dead dolphins was preserved in neutral buffered 10% formalin, embedded in paraffin, sectioned at 2-6 µm and stained with haematoxylin and eosin (H&E) for histological examination according to standardized protocol [11]. Five tympano-periotic complexes and one cochlea were investigated microscopically at the Institute of Terrestrial and Aquatic Wildlife Research in Büsum, Germany using a similar protocol to that used to examine UK-stranded harbour porpoises [21] and based mainly on methods described by [22]. Briefly, decalcification was conducted by immersion in 0.27 M EDTA (disodium ethylenediaminetetraacetate) at room temperature. The decalcification solution also contained 1% formalin and was changed weekly. Complete decalcification was determined by X-ray radiography. All specimens were then neutralised in tap water for 24 hours, then in distilled water for 4 to 6 hours followed by progressive dehydration with 50%/70%/80%/95%/100% alcohol. The last two solutions (95% and 100%) were changed twice before the samples were finally placed into ether-alcohol (1∶1). The ear tissues were then embedded in 1.5% celloidin for a week, 3% celloidin for another three weeks, then 6% celloidin for three to four weeks and finally into 12% celloidin for another three to four weeks. The formed celloidin-block was hardened with chloroform in a desiccator at 4°C for two weeks, after which the celloidin was hardened with fresh chloroform for another two weeks. At this stage the block was trimmed and set in cedarwood oil for at least one week then set into fresh cedarwood oil for another week. The blocks were then sectioned (20 µm thick sections) using a microtome. Every 10th section was stained with H&E and mounted while the rest of the tissue sections were stored in 80% alcohol. Around one hundred stained slides per ear were examined microscopically.

Immunohistochemistry

Sections of formalin-fixed lung (n = 25) and mesenteric/pulmonary associated lymph node (n = 26) were processed by sectioning and staining with osmium-tetroxide post-fixation followed by embedding in paraffin using standard methods [23] to demonstrate fat emboli. Immunohistochemical techniques to demonstrate fibrinogen in skeletal and cardiac muscle and myoglobin in kidney were identical to those described by [24]. As a positive control for myoglobin in cetacean kidney, renal sections were used from a striped dolphin ( Stenella coeruleoalba) with stranding (capture) myopathy associated with prolonged beach stranding and in which myoglobinuria had been reported by [24].

Assessment of sexual maturity and age

Females were classified as sexually mature if, on gross examination, one or more ovarian corpora scars were present [25]. Samples of testicular tissue were processed for histology and males were classified as mature if testicular tissue showed evidence of spermatogenesis, i.e. spermatozoa were present (after [26]). The straightest and least worn teeth were selected for age estimation using the wax embedding technique outlined in [27]. This consisted of fixing teeth in 10% neutral buffered formalin for at least two weeks, decalcifying using the rapid decalcifier RDO©, and using standard histological processing techniques teeth were dehydrated, embedded in paraffin wax, sectioned at 5 μm using a microtome and stained using 60% Harris's haematoxylin.

Morbillivirus detection

Total RNA was extracted from sections of frozen (−80°C) lung (n = 26) and brain (n = 22) samples and the presence of morbilliviral RNA was tested using reverse transcriptase polymerase chain reaction targeting the conserved N terminus of the morbillivirus N gene [28]. All reactions were conducted in duplicate.

Analysis of chemical toxins

All tissue samples were collected using standard methodology and stored at -20°C prior to preparation and analysis. Frozen liver samples were analysed for a range of trace elements (Cr, Ni, Cu, Zn, As, Se, Mn, Fe, Ag, Cd, Hg and Pb) and butyltins (monobutyltin, dibutyltin and tributyltin) (mg/kg wet weight). The hepatic molar Hg:Se ratio was also calculated [11], [29], [30]. Wet weight concentrations (mg/kg) of 25 individual chlorobiphenyl congeners (IUPAC numbers: 18, 28, 31, 44, 47, 49, 52, 66, 101, 105, 110, 118, 128, 138, 141, 149, 151, 153, 156, 158, 170, 180, 183, 187, 194) and a range of organochlorine pesticides and metabolites were determined in blubber samples according to previously established and validated protocols using internationally standardised methodologies (see [11], [29], [30], [31]). The sum of the concentrations of the 25 CB congeners (Σ25CBs) and organochlorine pesticides tested were determined and were then converted to a lipid basis (mg kg −1 lipid) using the proportion of hexane extractable lipid (%HEL) in individual blubber samples. For all analyses, appropriate quality control materials (certified or laboratory reference materials) were analysed within each sample batch in order that the day-to-day performance of the methods could be monitored.

Analysis of algal toxins

Frozen samples (−80°C) of liver were analysed for the presence of harmful marine algal toxins using high performance liquid chromatographic techniques deployed for the purpose of toxin monitoring in commercial shellfish. For amnesic and paralytic shellfish poisoning toxin groups (ASP and PSP), liquid chromatographic (LC) methods were applied with photodiode array detection [for ASP see [32]] and fluorescence detection [for PSP see [33] and [34]]. Liquid chromatography with tandem mass spectrometric (MS/MS) detection was also used to confirm any analytical observations from these initial analyses (for ASP; [35]). For the diarrhetic shellfish poisoning (DSP) toxins as well as other, co-extracted lipophilic toxins, a LC-MS/MS method was also applied following [36]. The range of ASP toxins included domoic acid (DA) and its associated isomers - epi-DA, isoDA-A, isoDA-D and isoDA-E. The targeted toxins of the DSP were okadaic acid (OA), the dinophysistoxins (DTX1, DTX2 and acyl esters of OA and DTXs), pectenotoxins 1, 2 and 11, azaspiracids (AZA1, AZA2 and AZA3), yessotoxin (YTX) compounds - YTX, homo YTX, 45 OH YTX and 45 OH homo YTX and representatives of the cyclic imine groups - 13-desmethyl spirolide C (SPX1) and gymnodimine. For the PSP group, the following toxins were included - saxitoxin (STX) and its derivatives neosaxitoxin (NEO), gonyautoxins (GTX) 1 to 5, decarbamoyl (dc)STX, dcGTX2,3 and the N-sulphocarbamoyl gonyautoxins -2 and -3 (C1 and C2) toxins.

Results

The locations of the live and dead dolphins reported in this MSE are shown in Figs. 1 and 2. The temporal distribution of all common dolphin strandings recorded in Cornwall throughout 2008 is shown in Fig. 3.

Gross and histopathological examinations

Twenty-six common dolphin carcasses composed of 13 sexually immature males, 8 sexually immature females and 5 sexually mature females) were retrieved from the MSE for necropsy ( Table 2). Individuals ranged in age from 2 to 25 yrs (males 2 to 9 yrs, and females from 3 to 25 yrs). All 26 dead dolphins found on 9 June were in freshly dead condition when initially recovered. The time between death and necropsy varied and not all carcasses could be refrigerated, so the state of decomposition at necropsy was fresh (n = 10); fresh-slight decomposition (n = 2); slight decomposition (n = 8); slight-moderate decomposition (n = 1); moderate decomposition (n = 4) and moderate-advanced decomposition (n = 1). The main gross and microscopic findings were similar in all cases ( Table 2). All dolphins appeared to be in good nutritional condition and showed no significant evidence of acute physical injury or disease. No acute traumatic lesions characteristic of by-catch [12], boat impact [11] or bottlenose dolphin attack [11], [37], [38] were seen in any of the dead (or surviving) common dolphins. One dolphin had a chronic but relatively minor (old) injury associated with granulation tissue and localised fibrosis on the rostral tip of the maxillae (upper beak). Low intensity parasitic infestations were typical, most frequently in the lungs, and these were associated with relatively mild host tissue reactions which are commonly found in stranded common dolphins in UK waters [11], [12]. The stomachs were free of recently-ingested prey in all cases. Gonadal tissue was grossly and histologically normal in all 26 animals. However, three immature females (SW2008/94.3, SW2008/94.4, SW2008/94.15) ranging from 3 to 5 years in age had colostrum-type fluid present in their mammary glands. All five adult females were actively lactating. A small (1-2 cm) raised, pale, plaque-like like lesion was found on the dorsal aspect of the penis in one immature male common dolphin, 1.8 years of age (SW2008/94.18). Histopathology of this lesion confirmed focal thickening of the epithelium (resembling a plaque) without associated inflammatory response or presence of inclusion bodies, consistent with viral (papilloma virus) infection.

Eleven dolphins had moderate to copious quantities of muddy substrate in the trachea and bronchi macroscopically ( Table 2; Fig. 4A). Particulate matter (mud) and plant material could be seen grossly and microscopically in many bronchial, bronchiolar and alveolar spaces ( Fig. 4B). Another three dolphins had traces of particulate matter visible microscopically within bronchiolar and alveolar spaces ( Table 2). Acute vascular congestion of internal organs and acute cortical tubular necrosis (kidney) were common microscopic findings. Widespread acute myocardial degeneration (heart muscle) ( Fig. 4C) and acute rhabdomyolysis (skeletal muscle) ( Fig. 4D) were confirmed by anti-fibrinogen immunohistochemistry. Anti-myoglobin antibodies demonstrated only minimal to absent myoglobin urinary casts in formalin-fixed kidney samples of 26/26 (100%) of dolphins examined ( Fig. 4E) when compared to the positive control material ( Fig. 4F). There was no gross or microscopic evidence of adrenal enlargement in any of the 26 dolphins necropsied. Four out of 26 adrenal glands were grossly normal but too autolysed for histopathology. Of the other 22, 12/22 (54.5%) were markedly congested (mainly within the inner adrenal cortex), 9/22 (40.9%) had acute haemorrhages within the inner adrenal cortex and 2/22 (9.0%) had haemorrhages in the adrenal capsule.

Gross and microscopic examinations of the cerebrum, cerebellum and spinal cord failed to detect inflammatory or infectious processes in any of the animals examined (n = 26) (Table 2). A small number of gas bubbles were seen in the mesenteric veins of some fresh carcasses but other tissues were generally free of macroscopic or microscopic bubbles. Intravascular bubbles were seen in some of the more-decomposed carcasses, but these were considered to be consistent with post-mortem change rather than ante-mortem gas embolism. Lung and lymph node samples were negative for the presence of fat emboli in all 26 dolphins examined.

Gross and histopathological examinations (Tympanoperiotic complexes)

The ears (tympanoperiotic complex) appeared to be grossly normal in all cases, apart from two dolphins that had silt lining the outside of the tympanic bullae. Parasites were not seen macroscopically in all auditory complexes but were seen in 1/4 animals examined microscopically (SW2008/94.2 left). Only six dolphin ear specimens were examined microscopically and these consisted of the complete tympano-periotic complexes of SW2008/94.2 (left ear only); SW2008/94.10 (left and right ear), SW2008/94.14 (left and right ear) and SW2008/94.12 (periotic bone only) ( Table 3) ( Fig. 5A-D). Parts of the auditory tube were visible only in the left ear of SW2008/94.2 and the right ear of SW2008/94.10. The corpus cavernosum tympani was present in all investigated ears, apart from the left ear of SW2008/94.10. The results of the microscopic examinations of the ears in these cases are summarised ( Table 3) and illustrated ( Fig. 5A-D).

Bacteriological findings

Bacteriological findings were unremarkable and, microscopically, there was no evidence of host reaction to bacterial organisms, indicating only probable post-mortem colonisation of tissues. The bacterium Brucella ceti, which has been associated with lesions in cetaceans [18], was isolated from a grossly and microscopically normal sample of testis from a single juvenile dolphin, but this bacterium was not isolated from any other tissues from this animal.

Morbillivirus detection

There was no evidence of morbillivirus RNA in lung (n = 26) or brain (n = 22) samples tested (Table 2).

Chemical and algal toxin detection

Analyses of frozen blubber samples from five sexually mature lactating females, one immature female and one immature male dolphin were determined with mean levels of 10.7 mg/kg lipid weight (range: 2.80-30.6) for the sum of 25 chlorinated biphenyls (Σ25CBs). Mean concentrations (mg/kg lipid weight) were 0.17 (range: 0.05-0.45) for p,p′-DDT; 0.86 (0.13-2.86) for p,p′-DDE; 0.09 (0.02-0.23) for p,p ′-TDE, 0.01 (0.011-0.014) for alpha-hexachlorocyclohexane; <0.01 (all cases were below limit of analytical detection) for gamma-hexachlorocyclohexane; 0.02 (0.01-0.04) for dieldrin and 0.05 (0.01-0.04) for hexachlorobenzene. Analyses of frozen liver tissues from the same adult animals determined mean concentrations (in mg/kg wet weight) of 31.9 (range: 9.20-53.0) for Hg; 0.29 (0.10-0.50) for Cd; 0.03 (0.02-0.03) for Pb; 0.08 mg/kg (0.07-0.11) for Cr; 3.14 (2.70-3.80) for Mn; 168.1 (109.0-221.0) for Fe; 0.07 (0.05-0.08) for Ni; 6.40 (5.40-7.90) for Cu; 49.6 (40.0-90.0) for Zn; 0.36 (0.28-0.43) for As; 13.8 (5.40-22.0) for Se and 1.41 (0.72-2.60) for Ag. The molar Hg:Se ratio was less than 1.0 in all common dolphin livers examined. The mean concentration of the summed hepatic concentrations of monobutyltin, dibutyltin and tributyltin was 0.03 (range: 0.021-0.054) mg/kg wet weight.

Using available toxicological data from adult female UK-stranded common dolphins in 1990-1992 for comparison [12], [29], mean levels of all organochlorine contaminants examined (including Σ25CBs; Figure 6) were lower in the five 2008 MSE adult female common dolphins which were examined for contaminants. The 1990-92 sample was composed of five mature females, including one female with a Σ25CBs of 95.6 mg/kg lipid weight. Mean hepatic concentrations of all butyltins and Cr, Ni, Cu, Zn, As, Se, Ag, Cd, Hg and Pb were again lower in the adult (male and female) dolphins from the MSE in 2008 than in the adults from the 1990-1992 group. No toxicological data were available for comparison from the 1990-1992 period for Mn, Fe or Ag.

Analyses of frozen liver tissues from the immature and mature animals selected for analysis (six female, one male) were negative for the presence of a suite of algal toxins including neurotoxic compounds e.g. domoic acid and saxitoxin, and also for toxins known to cause gastrointestinal disturbance including okadaic acid and dinophysis toxins (Table 2).

Disturbance by marine noise

The hypothesis that marine noise coincident with the strandings could have been responsible for the MSE was considered. No acoustic recordings from the time of the stranding were available to allow assessment of sound levels. Information on high-intensity acoustic activities in the general vicinity of the MSE showed the nearest earthquake to the stranding date and location was recorded off the coast of northern France near Le Havre (approximately 330 km from Cornwall), 2.8 ML magnitude recorded at 10 metres below seabed on 30 May 2008 (source: European-Mediterranean Seismological Centre) (Figure 7). The UK Department of Energy and Climate Change (DECC) confirmed that no geophysical surveys, including surveys involving seismic methodologies, were licensed to take place in the English Channel or Southwest Approaches, or in any adjacent sea area, either immediately prior to or during the stranding event. A research vessel ( RV Celtic Explorer) was conducting a high-resolution 2-D seismic survey in the Celtic Sea south of Cork, Republic of Ireland, using a relatively small sized array of 150 cubic inches (commonly arrays can exceed 3000 cu inches for 2-D surveys), from 31 May until 14 June at a distance from the MSE of over 200 km with intervening land (data: Marine Institute) (Figure 7). Falmouth Harbour Authority records show no unusual commercial shipping movements in or out of the harbour prior to the MSE. No record of movements of small pleasure craft or the use of non-military sonars exists, but no particular large-scale events likely to cause significant noise inputs (e.g. a powerboat race) occurred in the area during the relevant period. A commercial dockyard exists in Falmouth Harbour within the estuary. The dockyard is mainly a dry dock and the management reported no unusual activity around this time.

Naval exercises involving up to 20 Royal Navy surface and submarine vessels and 11 vessels from seven other countries (Holland, France, Germany, USA, Belgium, Chile and Denmark) was conducted in the South Coast Exercise Area off the southern coastline of Cornwall, Devon and Dorset from 1-9 June, with peak activity on 4-5 June (see Figures 7 and 8) (data: UK Ministry of Defence). Routine naval exercises using live munitions, helicopters and fixed-wing aircraft and mid-frequency sonars (deployed from surface ships and helicopters) in the South Coast Exercise Area - which extends from Dorset to Cornwall - occur on about 46 weeks of each year (data: UK MoD). The MoD provided records of activity of all MoD vessels and foreign naval vessels, including associated maritime aircraft, operating under UK Operational Control in the South West and central Channel regions during the 7 days preceding the MSE (1-9 June 2008). The MoD does not hold records of the movement of other vessels in these areas, either UK or foreign registered nor does it have records of the activities of foreign naval vessels not under UK Operational Control. Foreign naval vessels do have the right of innocent passage through UK Territorial Waters but innocent passage would preclude operation of sensors such as active sonar which are not required for navigation (data: UK Ministry of Defence).

During the period from 1 June until the afternoon of 9 June, intermittent and occasional acoustic outputs from naval vessels were reported, including the use of standard echosounders (35/50/200 kHz), other sonars, acoustic modems, autonomous sonobuoys, and the firing of live, inert and blank ammunition, including a single live Seawolf missile (data: UK Ministry of Defence). Antisubmarine warfare (ASW) activities using mid-frequency sonars (2-4 kHz and 5-8 kHz) were conducted in the areas Foxtrot, Golf and Hotel (Admiralty Chart 442, UK Hydrographic Office) at least 45-50 km from the stranding locations in Falmouth Bay (Figure 7) up to 6 June, some 60 hours before the discovery of the stranded dolphins. On 8 June, a high frequency side-scan sonar trial (100 kHz) was conducted in an area approximately 15 and 50 km from the location of the MSE between 10∶00-14∶32 hrs (Figure 7). A submarine was operating in the South Coast Exercise Area, including Falmouth Bay, on the 8 June and from 0900-1500 hrs on 9 June used an echo sounder (at 50 KHz) and passive sonar. A low-power (70 Watts) short range underwater telephone (8 kHz) was scheduled to be used from 08∶00 hrs on the morning of 9 June (Figure 8) but this device was only used in the Plymouth area and after the dolphins had first started to strand at Porth Creek (data: UK Ministry of Defence). The unspecified echo-sounder deployed on 9 June (Figure 8) was a standard navigational system deployed on a troop carrier (RNLN Amphibious Ship Johan De Witte) in the vicinity of Plymouth.

A helicopter flying exercise (not anti-submarine warfare) was scheduled to take place between 08∶00-13∶30 hrs on 9 June in the Falmouth Bay (North), Falmouth Bay and Mounts Bay exercise areas. The entire Falmouth Bay area on 9 June was allocated to RNAS Culdrose Squadrons (Royal Navy helicopter base), but the first flight reported to leave RNAS Culdrose was at 08∶58 hrs on 9 June (37 minutes after the MSE was first reported to Falmouth Coastguard). This helicopter returned to RNAS Culdrose at 09∶31 hrs and the first incoming flight from RAF Netheravon landed at RNAS Culdrose at 09∶17 hrs (around one hour after the discovery of the MSE) and did not use the Falmouth Bay area. There were no helicopter flights from UK or non-UK naval ships reported in the Falmouth Bay region immediately prior to the onset of the MSE and no sonar of any type was deployed by the helicopters (data: UK Ministry of Defence). Naval activities incorporating anti-submarine mid-frequency dipping sonars and firing of inert rounds resumed (after a 3-4 day period of abstinence) on the afternoon of 9 June after the MSE (data: UK Ministry of Defence). The airspace in the vicinity of St Mawes is uncontrolled and available to all civilian aircraft and such aircraft do not need to file a flight plan or utilise an air traffic service (data: Civil Aviation Authority) and so the possibility that civil aircraft were in the vicinity cannot be discounted.

Disturbance by other cetacean species

Marine mammal top predators like bottlenose dolphins ( Tursiops truncatus) and killer whales ( Orcinus orca) are known to attack and kill or, in the case of killer whale, predate on other marine mammal species [37], [38], [39]. The nearest reported sightings of killer whales to the MSE were in waters off west Wales on 1 June 2008 and off northern Scotland on 7-8 June 2008 (data: Janet Baxter pers com.; Seawatch Foundation). An unconfirmed report of two bottlenose dolphins in Falmouth Bay was made by naval Marine Mammal Observers (MMOs) at 17∶22 hrs on 8 June. Other confirmed and unconfirmed reports of bottlenose dolphins were made within 25km of Falmouth Bay both before and after the MSE (Table 1). These sightings are consistent with the presence of a small group of inshore bottlenose dolphins that is known to have been continuously resident around the southwest peninsula, including the Falmouth Bay area, since 1991 (data: Cornwall Wildlife Trust).

Geographical and other factors

The strandings were in shallow, tree-lined, narrow tidal creeks with muddy substrates where common dolphins are not normally seen. Only two records among 7,700 cetacean sightings reports held by the Cornwall Wildlife Trust were from this area and involved two common dolphins that stranded alive at different times in different tributaries of the Fal. Common dolphins are frequently present in the open bay, although usually two kilometres or more from the shore (data: Ray Dennis pers com./Cornwall Wildlife Trust Marine Sightings Database). There were no reports from local fisheries of unusual bait fish distribution or fish die-offs in the Falmouth Bay region (data: Cornwall Sea Fisheries Committee). The tidal cycle for Porth Creek where most of the live and dead dolphins were found is shown in Figure 9. The 8-9 June period was close to the peak of the spring tides (data: UK Met Office).

Discussion

The purpose of this investigation was to conduct rigorous necropsies and additional diagnostic tests on all 26 dead dolphins to try to determine the main cause, or identify a number of likely causal factors, of the Falmouth common dolphin MSE on 9 June 2008. Falmouth Marine Coastguard Agency telephone records show that the first call reporting a common dolphin in Porth Creek was received at 08∶21 hrs. More live and dead dolphins were discovered as the morning progressed but there remained some uncertainty about the actual onset of the MSE in the absence of any earlier eyewitnesses prior to approximately 08∶00-08.15 [17], [40]. It therefore fell under the remit of the investigation not just to identify likely causal factors, but to try to more accurately determine the most likely time of onset of the MSE.

All necropsied common dolphins in this MSE were either sexually immature individuals of both sex or sexually mature lactating females. Sex and/or length data are not available for individuals that were successfully refloated. The calving/mating period for common dolphins in the North-east Atlantic ranges from May to September inclusive [25], and although three of the mature females were heavily lactating, and one showed evidence of recent gravidity, no newborn/neonates were observed in the necropsied sample. All immature necropsied dolphins ranged between 1.9 and 9 years in age. A lactation period of 10.35 months has been proposed for this population [13] and both milk and solid food have been observed in the stomachs of calves between three and six months of age [41]. This all suggests that newborns/neonates would have been part of the original group.

A common dolphin mass mortality event in 1994 in the Black Sea [44] was linked to distemper caused by cetacean morbillivirus infection and occurred over a much longer period of time and a wider geographic area than this event. No lesions consistent with distemper were found in the dolphins in this MSE and all molecular tests for morbillivirus infection were negative. Brucellosis (associated with Brucella ceti) has been associated with high polychlorinated biphenyl (PCB) exposure in bottlenose dolphins that stranded in Cornwall in recent years [45]. Meningoencephalitis associated with Brucella ceti infection was also reported in a striped dolphin that stranded alive in Cornwall [46]. However, the Brucella ceti isolated from the testis of one common dolphin in this MSE was from grossly and microscopically normal tissue and therefore considered incidental to the dolphin's cause of death. A longstanding hypothesis of a "sick lead animal" that may lead other cetaceans in its social group to strand [8] also appears unlikely here both because no such individual was found among the dead (although one unconfirmed dead dolphin was reported but not retrieved). Analytical tests of tissue samples showed that they were free of harmful algal toxins and the levels of organochlorines (polychlorinated biphenyls and pesticides such as DDT), trace metals and butyltins were at relatively low levels and generally lower than levels recorded in by-caught common dolphins stranded in south-west England in 1990-92. However it should be noted that all mature females in the MSE sample were actively lactating and so would be offloading most of their body PCB burden.

Overall, the pathological investigations undertaken indicate that the animals were healthy until they stranded, but provide no further indicators as to the likely cause. The possibility exists that the MSE occurred due to some intrinsic error of "navigation" within a social group of dolphins. The 8-9 June period was close to the peak of the spring tides and this could theoretically have part-influenced the onset of a MSE due to navigational error via the occurrence of rapidly receding tides in the coastal creeks where the MSE occurred. Spring tides are part of normal tidal cycles, however, and there is no established correlation between spring tides and cetacean MSEs. One study has modelled coastlines where low frequency cetacean echolocation signals might be distorted by purely geometric effects [6] but this was considered unlikely to explain this MSE because Falmouth Bay is not a "hotspot" for cetacean MSEs and common dolphin echolocation is at frequencies far higher than those that might fit a model for larger whales using lower frequencies [6].

For centuries humans have made sounds to induce mass strandings of otherwise healthy small cetaceans in "drive fisheries" for food [49]. High intensity acoustic energy sources were therefore considered as a potential trigger of the MSE. The seismic survey by the RV Celtic Explorer appears to have been too distant, and the earthquake occurred too long before the event for these to be likely causes. Nearby dockyard activity was reported to be normal and is therefore unlikely to have attracted animals into its proximity. Military acoustic sources (particularly mid-frequency anti-submarine sonars) have been causally linked with cetacean MSEs predominantly involving beaked whales [23], [50], [51], [52], [53], [54]. It has been proposed that beaked whales (and possibly other cetaceans) may show hazardous behavioural changes in response to some sonar frequencies, potentially leading to nitrogen supersaturation and risk of gas and fat embolism similar to decompression sickness in humans [23], [54], [55], [56]. Cases of acute and chronic gas embolism have occasionally been found in the UK, including in common dolphins stranded in SW England [11], [54], [56] but no evidence of significant gas or fat emboli were found in any of the 26 examined dolphins. Neither were any lesions found consistent with intense sound or shock wave exposure (reviewed in [21], [51], [57], [58], [59], with the possible exception of the haemorrhages in the auditory apparatus (middle and inner ears). Acute physical trauma (such as a boat impact) could have theoretically caused the haemorrhages in the ears but such boat impacts are associated with marked subcutaneous and other non-auditory bruising and/or other injuries [10], [11], but none were found. Apart from some form of acoustic trauma, the haemorrhages in the ears may be caused by hemodynamic changes in vascular perfusion associated with "stranding stress" and hypertension linked to progressive cardiovascular and pulmonary collapse following stranding alive.

It is theoretically possible that a MSE can be acoustically-mediated via the induction of a panic/flight behavioural response(s) that directly leads to stranding and death in the absence of gas or fat embolism or other "acoustically-induced" lesions. Such a mechanism may underpin this common dolphin MSE. However, such a phenomenon is difficult to demonstrate conclusively because there are no analytical or forensic tests available to qualitatively or quantitatively assess the history of acoustic exposure in a dead animal and, by definition, these types of MSEs would not present any lesions (e.g. gas embolism) suggestive of acoustic exposure. One well studied non-beaked whale cetacean MSE causally linked to high-intensity anthropogenic sound was the "near-MSE" of melon-headed whales ( Peponocephala electra) in Hawaii in 2004, that most likely resulted from mid-frequency sonar deployed by the U.S. Navy where a group of normally pelagic animals entered a shallow bay very soon after active mid-frequency sonars were deployed nearby. They exhibited "milling" behaviour and remained (even after the sonar was turned off) until humans finally herded them out of the bay [60]. A behavioural review of this species strengthened the case that mid-frequency sonar played a major role in this near-MSE [61]. Near-shore "milling" behaviours were seen in several small groups of common dolphins around the time that dolphins were first reported at Place and Porth Creek around 08.15 and later in the inner part of Falmouth Harbour and Gillan Creek. This milling behaviour, together with the flight/panic behaviours seen in other individuals is therefore highly consistent with acoustic disturbance [60], [61] although milling behavior is a common finding in other common dolphin mass stranding hotspots such as Cape Cod, USA where no linkage between acoustic stressors and mass stranding events has been shown [1].

Although not always causally linked, a growing number of cetacean MSEs had spatial and temporal associations with naval activities in recent years. These include 145 long-finned pilot whales ( Globicephala melas) which stranded and died in a series of three events in the Marion Bay region in the Southeast of Tasmania, 25-27 October 2005 [62]; a mixed-species mass stranding event involving 33 short-finned pilot whales ( Globicephala macrorhynchus), two dwarf sperm whales ( Kogia sima) and one minke whale ( Balaenoptera acutorostrata) in North Carolina on 15-16 January 2005 [63] and a mass stranding of four Cuvier's beaked whales in Almeria, Spain in January 2006 [64]. A mixed-species unusual mortality event involving multiple deep-diving cetacean species and 23 individual cetaceans in Taiwan between 19 July and 13 August 2005 was spatially and temporally linked to naval exercises [65] and another nine beaked whale MSEs were causally associated with naval sonars or other high-intensity acoustic naval activities in the Mediterranean and Caribbean Seas [52]. Increased numbers of stranded harbour porpoises were associated with naval sonars in Washington State, USA from 02 May to 02 June 2003 [66] and increased numbers of stranded harbour porpoises (n = 85) (mainly bycatches) were spatio-temporally associated with an international naval exercise in Danish waters from 7-15 April in 2005 [67]. A mass mortality of four long-beaked common dolphins ( Delphinus capensis) was caused by primary blast injuries (barotrauma) induced by naval activities (munitions disposal) in Southern California on 4 March 2011 [57]. Most recently, a mass stranding of long-finned pilot whales in Scotland in July 2011 was correlated in space and time with munitions disposal in northern Scotland, although the investigation into the cause of this MSE is currently ongoing (unpublished data).

Naval exercises or some other unknown anthropogenic or natural event could have caused the large group of 50-60 common dolphins to come unusually close to shore in the days leading up to this MSE. This large dolphin group was seen unusually close to shore in the 3-4 days leading up to the MSE and may have been the same group that subsequently mass stranded on 9 June. This MSE may even have been a "two-stage process" where a group of normally pelagic common dolphins entered a coastal bay (possibly to avoid a perceived acoustic threat) and then, after 3-4 days in/around Falmouth Bay, a second acoustic or other disturbance event occurred nearer to shore that caused them to subsequently strand en masse. Several consecutive days of anti-submarine warfare training using mid-frequency active sonars ended on 6 June, some 60 hours before the MSE was first reported. Based on available evidence, active mid-frequency sonars may have been a factor causing a large dolphin group to move closer to shore several days prior to the MSE, but were considered unlikely to have directly triggered the event on 9 June.

Similarly, high-frequency sonars were considered unlikely to have triggered the MSE. The towed high-frequency (100 kHz) side-scan sonar trial conducted at 10∶00-14∶32 hrs on 8 June was between 15-50 km from the location of the MSE and terminated approximately 18 hours before the MSE was discovered. Such high-frequency acoustic devices are commonplace in the marine environment, occurring on almost all vessels and, although high-frequency systems are often components of both naval exercises and seismic surveys, they have not been directly implicated (causally) in any cetacean MSE.

Establishing the precise timing of the onset of an MSE is critical to determining a potential cause, particularly if an immediate behavioural response to an anthropogenic acoustic event is suspected. Although in the 2008 MSE the first dolphin was reported at 08∶21 hrs on 9 June, the tide cycle would have allowed dolphins to enter Porth Creek much earlier (see Figure 9a). A period around the ebb tide was also considered inaccessible for stranding due to the lack of water. There are therefore two scenarios for when the dolphins could have entered Porth Creek, one on an ebbing tide and one on a rising tide (Figure 9a-c). Based on the times of high and low water, it is possible that the common dolphins entered the inner parts of Porth Creek (where they were first discovered) either after 06∶30-07∶00 hrs on 9 June or sometime shortly after 00∶00 hrs on 9 June. The period around two hours either side of low tide at 04∶32 would have prevented dolphins entering the creek between 02∶30-06∶30 (approx.) because the water level would be too shallow (see Figure 9a-c). All of the dead dolphins retrieved for necropsy in Porth Creek were found below high water level as the incoming tide was still rising. This is consistent with the dolphins not ever being in the creek at high tide (dead or alive) otherwise at least some might have been found stranded high on the bank at the high water level. It is therefore most likely that they entered sometime after 06∶30 hrs on 9 June on the incoming tide once the water was sufficiently deep for them to swim into the creek.

The MSE was close to spring tide but there is no scientific evidence linking cetacean MSEs with spring tides as causal factors. Furthermore, the dolphins in this MSE stranded on the incoming tide mid-way in the tide cycle - so there was neither a particularly high or low tide at the beginning of the MSE onset. The presence of water and mud in the lungs in 11 of the dead dolphins was consistent with them stranding alive on a rising (incoming) tide and drowning. Based on 20 years of necropsies of UK-stranded and by-caught cetaceans, water is rarely, if ever, found in the lungs of cetaceans that have died of asphyxiation in fishing nets (by-catch), demonstrating that water is not aspirated during the by-catch process and that the blowhole probably remains closed after death preventing entry of seawater [11], [12]. In our experience, water in the lungs is usually only found in necropsies of cetaceans that have stranded alive and where an incoming tide eventually submerges the blowhole resulting in drowning by terminal aspiration of seawater (P.D. Jepson, pers obs). A similar mechanism is likely to explain the agonal inspiration of water and mud found in the lungs of 11/26 dead dolphins in this MSE.

Diseases such as stress cardiomyopathy [68], capture myopathy [69] and contraction band necrosis [70] may cause rapid death in mammals exposed to extremely stressful events (such as capture, violent assault or beach stranding), with cetaceans appearing particularly vulnerable to these conditions [71]. Evidence of rapid death was universally present in this MSE, including acute congestion of internal organs, widespread acute myocardial degeneration (heart muscle), acute rhabdomyolysis (skeletal muscle) and acute cortical tubular necrosis (kidney). These lesions are consistent with acute cardiovascular failure and stranding (capture) myopathy associated with beach stranding [24]. However, these acute changes occurred largely in the absence of renal myoglobin urinary casts (myoglobinuric nephrosis). Extensive myoglobinuric nephrosis has been described in a striped dolphin ( Stenella coeruleoalba) that was beached alive for a prolonged period before death [24]. A study of humans with severe muscle injury also showed development of myoglobin urinary casts in kidneys within 1 hour between muscle injury and death [72]. The absence of myoglobin urinary casts is, therefore, suggestive of fairly rapid death without a prolonged period of being beached alive. If dolphins had entered the Creek on the ebb tide then stranded overnight and drowned on the incoming tide we would have expected to see myoglobin urinary casts in the kidneys. Conversely, if dolphins had entered the creek on the ebb tide and quickly died, there would not have been enough seawater in the Creek to have drowned 12 of them. Collectively, these findings are most consistent with a relatively rapid death after entering Porth Creek on the incoming tide on the morning of 9 June. This technique of combining inhalation of seawater (drowning), the tidal cycle and the presence or absence of myoglobin urinary casts in kidneys could be used to help determine the precise timing of stranding in future MSE events.

The number of detailed studies on the impacts of aircraft on cetaceans is relatively small and forms part of a wider overall knowledge gap on how cetaceans respond to anthropogenic noise [77]. Detailed long-term flight records (other than take-off and landing times) are not routinely kept for small civilian aircraft and so it is not possible to identify or exclude any low-flying civilian fixed-wing/rotary flights in the Falmouth Bay region on 8-9 June 2008 (data: Civil Aviation Authority). The low power/short range underwater telephone (8 kHz) system scheduled to operate on the morning of 9 June was not used until after the dolphins had started to strand, and was only used in the Plymouth area, so can largely be discounted as a potential cause. The "unspecified" echo sounder deployed on the morning of 9 June during and after the MSE onset was a standard navigational echo-sounder also deployed in the vicinity of Plymouth and is therefore highly unlikely to have triggered the MSE.

In relation to the military activity in the vicinity of the Falmouth MSE, the times and positions of all military sonar transmissions and aircraft flights were provided by the UK MoD under the Freedom of Information Act and no unclassified records of sonar transmission or naval flights were withheld. This provides some degree of confidence that all appropriate active sonar and other naval data has been obtained for the Falmouth MSE. The RN/MoD also provided a very large quantity of information in relation to the naval exercise - probably far more data than has been released in any previous cetacean MSE. Concerns have been raised within the scientific community about the veracity of acoustic information provided by military sources in relation to other cetacean MSEs, given that the military is usually the only source of data on naval acoustic activities in most of these events (e.g. [53], [58]). For example, the occurrence of an entire North Atlantic Treaty Organisation (NATO) naval exercise that was coincident with a Cuvier's beaked whale MSE on the west coast of Greece in October 1997 was withheld by NATO for over a decade and only relatively recently reported [52].

The Royal Navy uses a range of measures to mitigate potential impacts on cetaceans and other marine mammals including "soft starts" (the gradually progressive ramping up of active sonar source levels to allow cetaceans to move away from the vessel conducting the exercise), use of trained marine mammal observers and reduction of sonar source levels (i.e. power) when cetaceans are sighted close to a vessel operating sonar transmissions (data: UK MoD). The naval mitigation of potential impacts of helicopters and fixed-wing aircraft includes maintaining a 500 m minimum flight altitude, wherever practicable, if any cetaceans are seen on the surface (data: UK Ministry of Defence). The rarity of cetacean MSEs in this area indicates that either normal methods of mitigation of naval impacts on cetaceans in the naval exercise area are generally effective, or the conditions necessary for an acoustically-driven MSE have not previously occurred.

Following this MSE and recommendations from the organisations involved in the rescue of dolphins in the MSE, the UK Ministry of Defence initiated the Marine Underwater Sound Stakeholders Forum in the UK to regularly meet with all interested stakeholders (scientists, other Government Departments like Defra and a range of non-Governmental organisations) to discuss these issues in some detail. A direct line of communication was also established after the Falmouth MSE to facilitate rapid exchange of information between cetacean strandings/sightings organisations and Royal Navy Naval Command Headquarters to report groups of pelagic cetaceans seen unusually close to shore and potentially at increased risk of stranding. This was used to report a near-MSE of over 20 common dolphins in the Fal estuary in April 2009 that was seen 15 minutes after RN sonar trials were initiated in the region. The RN immediately modified the naval exercise (including use of active sonars) until the group of dolphins had returned to open sea several hours later. The need to alter training excercises due to the presence of dolphins has not subsequently occurred in this region. Such continual improvement of mitigation strategies by the military themselves is probably the best way to limit future environmental impacts of naval activities, including cetacean MSEs.

Conclusions

In conclusion, a number of potential causes of the 2008 common dolphin MSE can be either excluded or considered highly unlikely. These include infectious diseases, gas embolism, fat embolism, boat strike, by-catch, attack from killer whales or bottlenose dolphins, feeding unusually close to shore immediately prior to stranding, ingestion of harmful algal toxins, abnormal weather/climatic/tidal conditions and high-intensity acoustic inputs from seismic airgun arrays, recreational craft and geological events. A group of 50-60 common dolphins was seen unusually close to shore in the days leading up to the MSE and may have been the group that subsequently mass stranded. It is therefore possible that the MSE was a "two-stage process" where a large group of normally pelagic dolphins initially entered a coastal bay (possibly to avoid a perceived acoustic threat) and then, after 3-4 days in/around Falmouth Bay, a second acoustic or other type of disturbance event occurred, causing them to strand en masse. The international naval activities that took place in very close temporal and spatial proximity to this MSE are the only established cause of cetacean MSEs which cannot be eliminated and is ultimately considered the most probable (but not definitive) cause.

Acknowledgments

We would like to thank the members of the CSIP Steering Group, particularly the contract managers and policy officials within Defra (Jo Myers, Carole Kelly, Jim Gray and Trevor Perfect), for their comments and advice during the period of this report. The UK Ministry of Defence and Cdr. Andy Robinson and Lt Cdr Lez Hardy (Royal Navy) provided detailed information and comments on acoustic inputs into the marine environment from Royal Navy vessels/activities involved in the South Coast Exercise Area during the period 1-9 June 2008 along with comments on the structure and content of the investigation.

Individuals and organisations that assisted with necropsies and provided information or comments to support this investigation include Adrian Colloff (Animal Health and Veterinary Laboratories Agency, Truro), Jeff Loveridge (Cornwall Wildlife Trust Marine Strandings Network) and Dave Jarvis (British Divers Marine Life Rescue), Lesley Jarvis (BDMLR) and Debs Wallis (CWT MSN). Characterisation of the Brucella ceti isolate was conducted by Lorraine Perrett and Claire Dawson, Brucella Department, Animal Health and Veterinary Laboratories Agency, Weybridge UK. Data on the event were also provided by British Divers Marine Life Rescue, Cornwall Wildlife Trust Marine Strandings Network, RNLI Falmouth, Marine Coastguard Agency (Falmouth) the residents of Gillan Creek and a range of other agencies and members of the public. Sightings data were provided by Ray Dennis (Cornwall Wildlife Trust Sightings Database Coordinator). Useful scientific information was also provided by Sarah Dolman (Whale and Dolphin Conservation Society). Constructive comments on earlier drafts of this report were kindly provided by Professor Peter Tyack (Sea Mammal Research Unit, University of St Andrews) and several anonymous scientific reviewers.

Finally, we would like to pay special tribute to those who worked tirelessly during the numerous rescues throughout the day that led to a large number of dolphins being successfully returned to open water, in particular members of British Divers Marine Life Rescue, Cornwall Wildlife Trust Marine Strandings Network and the RNLI and Conrad Birnie, Seth Neil and local members of public in and around Froe for their unstinting efforts throughout 9 th-10 th June.

Author Contributions

Conceived and designed the experiments: PDJ RD KAW ND RJL S. Morris S. Murphy HS US ES AAC AF. Performed the experiments: PDJ RD KAW JB FCC ND RJL SKM S. Morris S. Murphy RP MWP HS US ES VS AF. Analyzed the data: PDJ RD KAW JB AB RLB FCC ND RJL JL SKM S. Morris S. Murphy RP MWP EP HS US ES VS MLT NT AAC AF. Contributed reagents/materials/analysis tools: PDJ RD KAW JB ND RJL SKM S. Morris S. Murphy RP MWP HS US ES VS AF. Wrote the paper: PDJ RD KAW JB AB RLB FCC ND RJL JL S. Morris S. Murphy RP MWP EP HS US ES VS MLT NT AAC AF.

References

1. 13. Murphy S (2004) The biology and ecology of the common dolphin Delphinus delphis in the North-east Atlantic [P.hD.]: University College Cork.

2. 27. Murphy S, Perrott M, McVee J, Read FL, Stockin KA (2012) Deposition of growth layer groups in dentine tissue of captive common dolphins Delphinus delphis. NAMMCO Scientific Publications Series. In review.

3. 41. Brophy J, Murphy S, Rogan E (2009) The diet and feeding ecology of the common dolphin (Delphinus delphis) in the northeast Atlantic. Report to the International Whaling Commission, SC/61/SM14.

4. 63. Hohn AA, Rotstein DS, Harms CA, Southall BL (2006) Report on marine mammal unusual mortality event UMESE0501Sp: Multispecies mass stranding of pilot whales (Globicephala macrorhynchus), minke whale (Balaenoptera acutorostrata), and dwarf sperm whales (Kogia sima) in North Carolina on 15-16 January 2005. NOAA Technical Memorandum NMFS-SEFSC-537, 222 p. NOAA website. Available: http://www.nmfs.noaa.gov/pr/pdfs/health/umese0501sp.pdf Accessed 2013 Mar 11.

5. 66. Norman SA, Raverty S, McLellan B, Pabst A, Ketten D, et al.. (2004) Multidisciplinary investigation of stranded harbor porpoises (Phocoena phocoena) in Washington State with an assessment of acoustic trauma as a contributory factor (2 May - 2 June 2003). U.S. Dep. Commerce, NOAA Tech. Memo. NMFS-NWR-34, 120 p.

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What Caused the UK's Largest Common Dolphin (Delphinus delphis) Mass Stranding Event?

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On 9 June 2008, the UK's largest mass stranding event (MSE) of short-beaked common dolphins (Delphinus delphis) occurred in Falmouth Bay, Cornwall. At least 26 dolphins died, and a similar number was refloated/herded back to sea. On necropsy, all dolphins were in good nutritive status with empty stomachs and no evidence of known infectious disease or acute physical injury.

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Website: Cornwall Wildlife Trust Marine Strandings Network

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Website: Cornwall Wildlife Trust Marine Strandings Network

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Website of the Cornwall Wildlife Trust Marine Strandings Network (Cornwall strandings), part of the CSIP consortium

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The Marine Strandings Network is a flagship project of Cornwall Wildlife Trust, and has been recording and monitoring dead marine wildlife strandings in Cornwall and the Isles of Scilly for over 25 years. We work in partnership with the Cetacean Strandings Investigation Progamme (DEFRA Funded) and University of Exeter to lead on marine strandings work for Cornwall.

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Website: Marine Environmental Monitoring (Welsh strandings)

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Website: Marine Environmental Monitoring (Welsh strandings)

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Website for Marine Environmental Monitoring (Welsh strandings), part of the CSIP consortium

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RSPCA: 0300 1234 999

or Welsh Marine Life Rescue (WMLR) on: 01646 692943

or British Divers Marine Life Rescue (BDMLR) on: 01825 765546.

If the animal is dead then contact The Cetacean Strandings Investigation Programme (CSIP) on:
The National Strandings Line: 0800 6520333.

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Strandings Wales - Report Cetacean Strandings in Wales

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RSPCA: 0300 1234 999 or Welsh Marine Life Rescue (WMLR) on: 01646 692943 or British Divers Marine Life Rescue (BDMLR) on: 01825 765546. If the animal is dead then contact The Cetacean Strandings Investigation Programme (CSIP) on: The National Strandings Line: 0800 6520333.

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Film: Field examination of 'Lulu' the killer whale in January 2016

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Film: Field examination of 'Lulu' the killer whale in January 2016

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Short film produced by the Scottish Marine Animal Strandings Scheme covering the initial examination of 'Lulu' the killer whale in January 2016. Please note, there are some issues with sound quality in this film.

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BBC Radio 4 programme broadcast in 2015 and presented by the author Philip Hoare, describing the work of the UK stranding programmes.

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Philip Hoare attends the necropsy of a porpoise and thinks about going inside a whale.

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Social media: Strandings programme social media channels

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Social media: Strandings programme social media channels

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Links to social media channels for UK strandings programme partners.

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Strandings programme social media channels

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Paper: Gas bubble lesions in stranded cetaceans

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Paper: Gas bubble lesions in stranded cetaceans

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Paper published in 2003 in the journal Nature, describing pathology in stranded cetaceans with acute and chronic gas embolism (collaboration between the CSIP and colleagues from the Universidad de Las Palmas de Gran Canaria).

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UK strandings programme timeline (1990-2021)

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UK strandings programme timeline (1990-2021)

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Paper: Cetaceans as sentinels for informing climate change policy in UK waters

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Paper: Cetaceans as sentinels for informing climate change policy in UK waters

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Paper published in 2021 in the journal Marine Policy, describing the use of strandings data to help inform climate change policy in UK waters.

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Paper: Persistent pollutants, persistent threats

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Paper: Persistent pollutants, persistent threats

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Paper published in 2016 in the journal Science, describing the persistent and ongoing major threat that polychlorinated biphenyls pose to marine apex predators.

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Paper: Exposure to a more neurotoxic mixture of polychlorinated biphenyls in UK stranded harbour porpoises

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Paper: Exposure to a more neurotoxic mixture of polychlorinated biphenyls in UK stranded harbour porpoises

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Paper published in the journal Science of the Total Environment in 2019, describing research demonstrating that juvenile harbour porpoises in the UK are exposed to a more neurotoxic mixture of polychlorinated biphenyls than adults.

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Juvenile harbor porpoises in the UK are exposed to a more neurotoxic mixture of polychlorinated biphenyls than adults

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Polychlorinated biphenyls (PCBs) are a group of 209 persistent and bio-accumulative toxic pollutants present as complex mixtures in human and animal t…