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Fish counters are automatic devices that count the number of fish that travel through the counter on a given river during a given period of time. Typically, a specific species of fish is intended to be counted and is determined by the counter. There are three principal types of fish counters: resistivity, optical, and hydroacoustic (AZoSensors, W. B., 2013).

Resistivity counters require structures that are placed within a river, often weirs or fish ladders, to embed electrodes which measure conductivity. Because fish have a lower resistivity than water, when a fish passes through the electrodes they are detected and counted (AZoSensors, W. B., 2013).

Optical counters also require in-river structures and traditionally use light beams that, when interrupted, create a pattern or outline of a fish allowing fish to be counted and their size, profile and swimming direction to be determined (AZoSensors, W. B., 2013). High resolution underwater cameras with infrared vision  and LED lights are also used to capture the images of fish traveling through a funneled weir (FISHBIO, n.d.).

Hydroacoustic counters use sonar to determine the presence of a fish traveling past an underwater microphone. The microphone will emit pulses and "listen" for return echos. When the sound waves are interrupted by the passing of a fish, the counter will convert the echos into an image  (Deveron, 2017).

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Yellow Breeches Angler Club (YBAC) stocks trout from their fishery three times a year in the Yellow Breeches in York County, Pennsylvania (McCleary, K., Personal Communication, 2017). I am not yet a member of the YBAC, but I plan to join upon my return to PA this summer. Being able to determine the number of fish presently navigating the Yellow Breeches before each stocking would be helpful in order to know how successfully the aquacultured trout are surviving in the wild, as well as, perhaps an idea of how many are culled  by fisherman during a given season.

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Fish counters have been used in conservation efforts to determine  the population size of the endangered Atlantic Salmon for more than 50 years (Eatherley et al., 2005). Specifically, the VAKI Riverwatch fish counter has been aiding in the conservation of Atlantic salmon since the late 1990s by enabling the compilation of population data automatically and electronically (FISHBIO, n.d.). Riverwatches have been installed in more than 300 rivers in 40 countries worldwide (ATOM01.IS, D. C., n.d.).

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As land mammals, it is very difficult for humans to comfortably navigate in water and to see through the surface, especially in murky waters, to count quickly moving fish. One of the greatest strengths of fish counters is that they are a non-invasive means, that does not stress fish or physically harm them (Costa et al., 2006). Also, fish counter technology is constantly improving to yield more accurate results. Once resistivity electrodes or optical light beams/cameras are installed, they can be left in place in order to conduct long-term data collections from remote locations, recording the exact date and time a fish passed through the counter (ATOM01.IS, D. C., n.d.).

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Large rivers may require several set ups across the river which can become expensive. With some fish counters in large rivers, the instruments must be calibrated every day because conditions are more variable than in a small river (Casselman et al., 1990). Some fish counter technologies are only accurate if the fish are  moving unidirectionally, such as in a migration, or else there will be over estimates of the population (Casselman et al., 1990). Debris can also trigger some fish counters causing false counts (Beaumont et al., 1986). In order for optical fish counters utilizing underwater cameras to be accurate, the water must be clear and with enough light (FISHBIO, n.d.).

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VAKI  is an Icelandic company that produces Riverwatch fish counters which are installed on weirs (ATOM01.IS, D. C., n.d.).

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ATOM01.IS, D. C. (n.d.). The riverwatcher fish counter. Retrieved March 01, 2018, from http://www.riverwatcher.is/

AZoSensors, W. B. (2013). Environmental Sensors: Fish Counters. Retrieved February 22, 2018, from https://www.azosensors.com/article.aspx?ArticleID=224

Beaumont, W. R., Mills, C. A., & Williams, G. I. (1986). Use of a microcomputer as an aid to identifying objects passing through a resistivity fish counter. Aquaculture Research, 17(3), 213-226. doi:10.1111/j.1365-2109.1986.tb00105.x

Casselman, J. M., Penczak, T., Carl, L., & Mann, R. H. (1990). An evaluation of fish sampling methodologies for large river systems.

Costa, C., Loy, A., Cataudella, S., Davis, D., & Scardi, M. (2006). Extracting fish size using dual underwater cameras. Aquacultural Engineering, 35(3), 218-227. doi:10.1016/j.aquaeng.2006.02.003

Deveron, R. (2017). Counting on the Future - Deveron Hydroacoustic Fish Counter. Retrieved March 03, 2018, from http://deveron.org/counting-future-deveron-hydroacoustic-fish-counter/

Eatherley, D.M.R., Thorley, J.L., Stephen, A.B., Simpson, I., MacLean, J.C. & Youngson, A.F. (2005). Trends
in Atlantic salmon: the role of automatic fish counter data in their recording. Scottish Natural Heritage
Commissioned Report No. 100 (ROAME No. F01NB02).

FISHBIO Fisheries Research, Monitoring, and Conservation. (n.d.). Retrieved February 27, 2018, from http://fishbio.com/automated_monitoring

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When a fish swims through an optical fish counter, interrupting the light beams, a profile of the fish is created. In Riverwatch fish counters, photographs are taken  and scanned (FISHBIO, n.d.)