Northeast Atlantic bluefin tuna by Lars Korslund

2024-04-29 11:26:28 UTC

The bluefin tuna inhabit the pelagic ecosystem of the entire North Atlantic and its adjacent seas, primarily the Mediterranean Sea, North of Norway and from the Black Sea to the Gulf of Mexico.

The Atlantic Bluefin Tuna (ABFT) occupies the surface and subsurface waters of the coastal and open-sea areas. However, both jucenile and adult ABFT frequently dive to depth of 500-1000m.

Age of maturity and spawning

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In Mediterranean Sea, ABFT reproduce for the first time between ages of 3–5 years.

Oviparous batch spawners when sea surface reach 22.5-25 degrees (Mediterranean) between june and August.

13-15 million eggs.

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The Atlantic Bluefin Tuna can been >3 m in length and weigh up to 900 kg.

The Atlantic bluefin tuna can sustain temperature down to 3 °C as well as up to 30 °C, while maintaining a stable internal body temperature.

The International Commission for the Conservation of Atlantic Tunas; ICCAT

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Management of Northeast Atlantic bluefin tuna have been done as a collaboration, because the species are highly mobile.

Source: Adrian W. Leach, Polina Levontin, Johnson Holt, Laurence T. Kell, John D. Mumford,

Identification and prioritization of uncertainties for management of Eastern Atlantic bluefin tuna (Thunnus thynnus),

https://doi.org/10.1016/j.marpol.2014.03.010.

(https://www.sciencedirect.com/science/article/pii/S0308597X14000827)

Migration patterns during breeding for Thermoregulation in the gulf of mexico

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The Bluefin tuna perform vertical migrations during the breeding phase in the Gulf of mexico.

The Bluefin Tuna dives to daily depths of 500+/- 150m and exhibits directed movement paths to and from localized breeding areas. This depth is shallower during the night when Bluefin tuna perform oscillatory dives with 200 +/- 76m depth.

These dives are suggests behaviorally and physiological thermoregulation in the gulf of mexico due to the Bluefin tuna being Ectothermic.

Article Link:

https://rdcu.be/dF71F

Migration follow prey migration patterns (herring, macrell, and squid)

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Development

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Eggs hatch after 2 days.

At 24°C, embryo development lasts about 32 hours and larval stages about 30 days.

Ram ventilators

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Ram ventilators, means they don’t have the ability to pump water through their gills, and have to keep swimming to get sufficient amount of oxygen.

Atlantic bluefin tuna is belived to have two main spawning grounds in the gulf of Mexico and in the Mediterranean sea.

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Slope sea is also a spawning ground for Atlantic tuna

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The adult atlantic Bluefin tuna populations spawn in three major proven locations which includes the Mediterranean, Gulf of Mexico and Slope Sea outside the United states.

Their migration makes research difficult which is important to manage their populations.

Younger western bluefin tuna spawn in the slope sea until they reach a certain size where longer migrations to the GOM is possible (gulf of mexico). The spawning tuna in the Slope sea is estimated to have higher biomass compared to older tuna in the GOM, further strengthened by larval abundances in the slope sea, suggesting the region as a third major spawning ground.

Article link:

https://cdnsciencepub.com/doi/full/10.1139/cjfas-2020-0444

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Experiments in captivity suggest that spawning by an individual might occur only once every 2 or 3 years

Comeback of Atlantic Bluefin Tuna in Norwegian waters

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Bluefin tuna has reestablished in Norway, echibiting its historical migration pattern in Norwegian waters in recet years after being absent for decades
Bluefin tuna har been feeding along the Norwegian coastline and in offshore waters for thousands of years due to high abundance of nutrient-rich schooling prey such as
- Mackerel
- Herring
- Blue whiting
- Lesser sandeel
Historically BFT visited Norwegian waters from early July until late October to feed.
During feeding season majority of individuals were adults (50-520kg)
There was a drastic reduction in the distribiution and extent of BFT migration patterns in Norwegian waters from ca 1965 onwards --> led to gradual decline of the Norwegian BFT fishery.
Reasons for reduction:
- Uncertain however..
- major international overfishing (juvenile and adult fish) ocurring for decades --> possible potential reason for historical decline on BFT in Norwegian waters
- Overfishing of juvenile in spawning areas in the Mediterranean Sea, Bay of Biscay and off the coast of western Africa
  === Possible effect on the migration pattern by interactions between environmental, trophic and fishing processes
Discussion part of the article
- A significant increase in the number of BFT observations in Norwegian waters from 2012 to 2018 + the latitudinal distribution of observations has widened each year between 2016 and 2018
- The comeback coincides with the overall annual increase in BFT spawning stock biomass (SSB) observed during feeding and long-distance migration events.
- Large BFT found in Norwegian waters may be due to different factors:
  - Sea surface temperature (SST) tolerance (small juvenile BFT may not tolerate low SST)
  - Long distance migration of food (herring, mackerel and capelin) has a function of fish length --> increase in body size will determine and increase the extent of migration = food availability is important for the maintenance of growth and body temperature
- BFT seemed to mainly prey on mackerel. Mackerel have expanded their northern distribution in the in the recent years Northeast Atlantic --> important food source for BFT = tuna follow mackerels migration patterns and therefore expand their own northern distribution.

Link: https://www.sciencedirect.com/science/article/pii/S016578362030206X

Built for speed

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Bluefin tuna is one of the fastest fish in the pelagic ocean (75-100km/h). Torpedo schapped body reaching well over 3m long

endothermic

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Figure 1:

Spatial distribution of Atlantic bluefin tuna (gray shading) and main migration routes (black arrows) are deduced from current and historical fisheries data as well as traditional and electronic tagging information (see text for references). The vertical dashed dotted line depicts the stock delimitation between the two current ICCAT management units. The main spawning grounds (dark dotted gray areas) being currently assumed in the literature are located in the Mediterranean Sea and the Gulf of Mexico.

Source: https://doi.org/10.1111/j.1467-2979.2005.00197.x