Field Tech: Resistive Fish Counters

Counting fish is a very difficult thing to do and humans, as a rule of thumb, are not particularly good at it. This is of course, because fish are much more at home in the water than humans are. So how do humans, especially scientists who need accurate data, count fish?

It depends on a few things: habitat, species being observed, and funding, to name a few. We are going to focus on automatic fish counters designed to specifically target fish traveling up a river or stream, as opposed to fish in oceans or lakes. The three types of automatic fish counters are resistive counters, optical counters, and hydroacoustic counters. They each utilize a different type of technology to do the same basic thing: count fish passing through a limited space. This particular post is going in depth into the science behind resistive fish counters.

Resistive counters rely on disturbances in the Force which, in this case, is not midi-chlorians but closer to Obi Wan's definition "an energy field created by all living things." That force is a field of low electrical impulses created using electrodes in the water. It is important to note that these electrical impulses are barely. if at all noticeable to the fish. Due to the difference in resistance between fishes and water, the electrodes can pick up on interruptions in the normal current. Resistance is a measure of how difficult it is for electrical current to flow through something, As fish pass over the electrodes, the order and time in which the electrodes experience resistance can provide data on both direction of travel and an approximate size of the fish (Eatherley et al. 2005). In order for a resistive counter to count all of the traveling fish, they must be built into the stream or river, as with a weir, pictured below.

Hydro-Electric Mk Xb resistivity counter spanning the River Frome in Scotland (Freshwater Biological Association, 2015)

As seen in the picture above, the water travels through a tube/tunnel or an open array to control water flow and allow for stronger accuracy while counting fishes. The combination of the layout of the electrodes and the shape of the weirs means that resistive counters are about 97% accurate (Freshwater Biological Association, 2015; Loughs Agency, 2016). The diagram below shows each part of the counter for a better view of the parts that actually make resistive counters work so well.

Resistive Counter Diagram, from the Loughs Agency (2016)

Resistive counters are most commonly used in salmonid counts. Salmonids include more than just salmon and are actually broken down into three subgroups: whitefish, graylings, and Salmoninae, which includes salmon, trout, and char (McPhail & Strouder, 1997). Most salmonids are anadromous, meaning that they spawn in freshwater but mature in the ocean. This means that every year, salmonids make an incredible migration upstream back to where they hatched in order to mate and lay their own eggs. After mating, adult salmonids die. Once hatched, the young salmonids then make their way back down river into the ocean to grow and mature until they are ready to breed.

As mentioned before, resistive counters can measure both the direction and size of fish passing through them, which gives researchers good counts of fish traveling upstream to breed (mature adults) and fish traveling downstream towards the ocean (young). These numbers are extremely important to conservation efforts, especially combined with information on river health and infrastructure (Freshwater Biological Association, 2015; Loughs Agency, 2016). With the accuracy of resistive counters and the reliable and consistent data they produce after being built, research done using these counters has been used to change building policies, dam implementations, and laws surrounding waste dumping, all of which can have massive impacts on breeding salmonids.

There are two major issues with resistive counters. The first is that they are extremely expensive. Because of the amount of work that goes into designing, building, and maintaining resistive counters, they have a large initial cost and can have high upkeep costs (Eatherley et al. 2005; Freshwater Biological Association, 2015; Loughs Agency, 2016). These costs make them impractical for small scale studies or smaller rivers and streams, which leads to the second issue with resistive counters. They are often custom designed and installed along main branches of rivers for the sake of efficiency, but fishes, especially salmonids, travel to very specific tributaries to spawn. While the overall population of salmonids might have healthy numbers within the main stretch of river where the fishes are passing the resistive conductor, the stationary conductor cannot give any data on specific tributaries (Eatherley et al. 2005). In many cases, this lack of specific data means that certain spawning regions can be on the verge of collapse before scientists are aware of the problem.

Despite these issues, resistive counters are a major technological tool used for serious and long term scientific research. Data gathered through the use of resistive counters is often cited as a major factor in saving both Atlantic and Pacific salmon populations from disaster (Eatherley et al. 2005; McPhail & Strouder, 1997).

If you have more questions about resistive counters please send them to us! We would love to discuss them more! Keep an eye out on our Field Tech posts for discussions on optical counters and hydroacoustic counters, both of which are wicked cool in their own rights. We've been through the power of counting fish with electricity, but next we get to discuss using lasers and sounds.


The WTH Wildlife Team


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).

Freshwater Biological Association (2015). The Fish Counter. Retrieved from

Loughs Agency (2016). Fish Counters. Retrieved from

McPhail, J.D.; Strouder, D.J. (1997). Pacific Salmon and Their Ecosystems: Status and Future Options. The Origin and Speciation of Oncorhynchus. New York, New York: Chapman & Hall.

#technology #conservation #wildlife #fish #fishes #salmon #fieldworld #research #science #nature

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