Jul 289 min read

Electrofishing: Understanding one of the most commonly used sampling methods in fisheries science

Updated: Aug 27

Electrofishing is probably the most common method of sampling freshwater fishes. As the name implies, it involves electricity and water, strange bedfellows, to be sure. To quote a landowner in West Virginia when we asked to electrofish his property, "You're putting electricity in water, I've gotta see that!". It is a lot safer than it sounds.

La Crosse fisheries crew — Tommy, Ryan, and Kirk - the La Crosse inland fisheries crew - about to weight and measure fishes after barge electrofishing a Kickapoo River stream reach.

Electrofishing is pretty simple. Most of the time, it is putting direct current (DC) current into water and very temporarily stunning fishes. DC current is typically pulsed - that is, it alternates between on and off cycles. Those fishes are then netted and put in a livewell to recover. And they typically recover pretty quickly - it is much less invasive than you might imagine. There are a few applications of elecrofishing where alternating current (AC) is used but it causes more injuries and is less often used. DC current attracts fishes whereas AC current repels fishes. People doing the electrofishing are safe in waders (preferably non-breathable ones) or in the boat.

Electrical conductivity (specific conductance) of the water is the factor that has the largest effect on electrofishing settings and how well electrofishing works. Conductivity is a measure of the ionic content of water and determines how well electricity is conducted by water. Low conductivity, indicative of soft water, makes electrofishing less efficient because there are too few ions to conduct electricity. Soft water requires a greater voltage to generate the same amperage (electrical current). Typically, the goal is to generate some safe number amps which is done by adjusting the voltage. Similarly, water that is too hard is difficult to sample and may require specialized equipment.

Barge electrofisher — From last summer - a WDNR stream barge electrofsiher. Anodes are carried by three people to cover the stream width.

As with any sampling technique, standardization is important to allow meaningful comparisons among samples over time. And electrical field is created between the anode (positively charged) and the cathode of the electrofisher. Typically the cathode is a probe (backpack and stream barge electrofishing) or a dropper ring (boat electrofisher) and the barge or boat has a region that serves as the cathode. A "rat tail" cathode drags behind a backpack electrofisher. Because conductivity differs among water bodies, biologists alter the voltage and the duty cycle to reach a desired amperage. And then professional judgement and experience are used to be sure that the current is not too low - fishes will escape capture - or too great - physical damage to fishes may occur. This is usually quite evident by how fishes are reacting to the electrofisher.

Types of Electrofishing

There are a number of different ways that fisheries biologists use electrofishing which generally depends upon the environment being sampled. And, at times, it depends upon the species being sampled. For streams, backpack electrofishers are used for the smallest of sites, a tow barge electrofisher is used for larger wadeable streams, and boat or boom electrofishers are used for non-wadeable streams. Pre-positioned electrofishing may be used in cases where specific habitats are being sampled. And sometimes a team of backpack electrofishers are used for larger, wadeable streams.

On lakes, electrofishing is typically employed on near-shore habitats and its effectiveness is limited by water depth. This means that boat electrofishing on lakes is often done at specific times of the year and often at night to most effectively sample the species of interest. For example, the Wisconsin Department of Natural Resources will target Walleye (Sander vitreum) early in the spring and bass and panfish a little later in the year. AC electrofishing with "chase boats" (boats behind the electrofishing boat) in deep water habitats is often used to sample catfishes (Porreca et al. 2013).

Sampling the Milwaukee River with multiple backpack electrofishers.

On wadeable streams, selection of electrofishing gear is usually determined by stream size. Backpack electrofishers are used on small streams. They tend to be less efficient (Bergman et al. 2011) than tow barges but are much lighter and simpler to use. Multiple electrofishers may be used on larger streams to increase efficiency. Likewise, block nets are effective in increasing capture probabilities (Peterson et al. 2005, Price and Peterson 2010, Hense et al. 2010). Tow barges can be limited by stream width or if the stream is too shallow which would make the barge difficult to drag through the stream (Bergman et al. 2011). On larger and deeper streams, a boat may be used. Typically a jon boat is used but sometimes rafts or driftboats are modified for use in rivers.

State and federal agencies tend to have their own electrofishing protocols and use those to determine which electrofishing gear to use and how much effort to expend.

Why Electrofishing is Used

While electrofishing is mostly used as a sampling technique, it may also be used to collect fishes for reintroduction or translocation, to remove fishes without having the effects of rotenone (Olson et al. 2024), or other times when the capture of fishes is necessary.

Sculpin reintroduction — Sculpin being reintroduced into a Crawford County stream (Kirk Olson photo).

Electrofishing is imperfect and has its limitations but being as commonly used as it is, electrofishing the standard method to sample many fishes and habitats.

Sampling Effectiveness and Potential Issues

As with any gear, electrofishing is does not capture fishes with a 100% success rate. However, to be an effective sampling technique, it is more important that any method used samples fishes in proportion to their population and that the method is repeatable across a range of conditions.

Checking Brook Trout for gill lice — Checking a Brook Trout for gill lice during a 2024 electrofishing survey.

In stream sampling, typically a length of stream is sampled. Again, agencies have different protocols they follow to determine the distance and effort. Typically, the length of stream is determined by the average stream width - often 30 to 35 times the mean stream width. And sometimes, a minimum length (100 or 150 meters) and a maximum length (300 to 1000 meters) of stream is determined. This allow for a number of pools, riffles, and runs to be sampled.

This length of stream may be covered in a single pass or in multiple passes (Peterson et al. 2004, Hense et al. 2010, Bergman et al. 2011). Generally, that depends upon the goals of the survey which method is used. Single pass is less effective but it requires much less effort and so long as fishes are sampled in proportion to their density, a single pass is an effective method. Multiple pass - often three passes - or capture-mark-recapture are often used when a more accurate population estimate is needed, for fish community studies (when we want to sample all of the fishes in a waterbody), or to test the relationship between single and multiple pass electrofishing.

Stream Conditions

As mentioned previously, water conductivity plays a large role in electrofishing capture success. Likewise, water clarity, stream width and depth, habitat complexity and gradient, the amount of aquatic vegetation, fish species, and other factors affect capture probabilities (Snyder 2003, Dauwalter and Fisher 2007, Hense et al. 2010, Price and Peterson 2010, and others).

A turbid stream — A rather extreme example - we would never electrofish a stream this turbid - but it shows how water clarity matters.

Often capture probabilities are calculated and models may be developed to determine what factors are associated with those capture probabilities. An example of that is from a study I was part of in West Virginia where we tested single pass and three pass electrofishing. Hense et al. (2010) found that for a number of commonly captured species; conductivity, age (a measure of fish size - more below), and stream width were the factors that were included most often in the models. Capture efficiency went down in wider streams and went up as conductivity increased. It should be noted that conductivities in this study were quite low compared (range of about 20 to 130 microsiemens /cm^3) to those typical of many Wisconsin streams. Other factors that are often an issue are water clarity and depth - capture efficiencies increase in clear, shallow waters.

Fish Size

As mentioned a couple of times now, size matters in electrofishing capture efficiency and larger fishes are more susceptible to electrical current. There is a catch however - larger, older fishes of some species are better able to sense and avoid the electrical field. I remember a musky we encountered several times on the Monogahela River in Morgantown, WV and were never able to capture. As so as we got close, it would jump out of the electrical field and escape. But typically, larger fishes are easier to capture as they have a greater electrical potential. One of the issues with many of the fisheries surveys is that young-of-the-year (YOY) fishes are almost certainly underrepresented. If capturing smaller fishes is a goal of the study, researchers typically alter their methods - like using block nets or altering the electrofisher's duty cycle and power - to improve capture success.

Safety

I know what you are thinking - how can electrofishing be safe? First, it is done by trained professionals that have undergone animal care and use and electrofishing safety training (repeatedly). And we wear non-breathable waders to protect us from the electrical field. Given normal safety procedures, it is a very safe activity. Anyone that has electrofished for any amount of time has stories but all of the stories I know are more humorous then they are scary or physically damaging.

Is it safe for the fishes? The answer to that question, I would say, is yes, but with caveats. Fishes can be injured by electrofishing, mostly due to convulsive muscle spasms which can cause spinal injuries (Snyder 2003, Pottier et al. 2020) that can occur due to the shock. This can be reduced by observing how fishes are affected by the electrical field. Most crews have certain numbers - usually the number of amps - that they are shooting for based on their experiences in those waters. They will adjust settings to better capture fishes is they are not being shocked well enough (low capture probabilities) or if they are being shocked too hard, which is what generally causes physical harm to fishes. Fish injuries are much more common in larger fishes and in very high or very low water conductivity.

More often than fish injuries, I have experienced issues with water temperatures and dissolved oxygen. Captured fishes are typically put in a livewell and biologists do what they can to keep that water well oxygenated. In boats, an air stone is often used to replenish oxygen and in steams, replacing water in the tub has the same effect. They also do things like sample only part of the reach before "working up" (weighing, measuring, identifying and enumerating) the fishes. The effects can also be minimized by not sampling in streams that are too warm or by sampling early in the morning before streams warm later in the day. Again, professional judgement should minimize negative effects.

The Wrap-Up

That, in a nutshell, is electrofishing. It is the most commonly used sampling techniques in freshwater lakes and streams. Like any sampling technique, it is not perfect and has its been long used and has become more standardized than other sampling techniques. This standardization allows fisheries biologists to make meaningful comparisons among samples between streams and over time. This is the "holy grail" of fisheries science because it is difficult to make those meaningful comparisons. With a few caveats, we can do that with electrofishing data.

Brown Trout being measured — A chucky 15 inch Brown Trout - captured by electrofishing - on the measuring board.

I could have written a lot more about electrofishing and its strengths and weaknesses and how we use the data and make comparisons among samples and species. But as a blog that aims to educate the non-biologist, the above is hopefully enough and you can hopefully say you know more about electrofishing than you did before reading this. That is always my goal.