A recently published study that evaluated underwater video cameras for assessing brook trout in Appalachian streams piqued my interest. Brook trout do not swim in the warm lakes or streams in the Deep South, and few streams in Mississippi resemble the rock and bedrock, clear-water streams of the Virginia’s Shenandoah National Park, where the study was conducted.
But the study provided a head-to-head comparison of estimates of fish populations obtained via direct observation by divers and electrofishing, thus allowing insight into a question I have wondered about for years: the efficiency of electrofishing and how well samples represent what is actually in the water.
Electrofishing, usually using backpack electrofishers, and direct observations by snorkel divers clad in wet suits or dry suits, are the go-to methods for assessing fish populations in shallow, wadable streams. The term “electrofisher” refers to the power source, a control box that manipulates the electric current, and the electrodes that transfer the current to the water. The U.S. Geological Survey and U.S. Forest Service fisheries researchers compared brook trout counts obtained by diver counts, electrofishing captures and fixed-location video cameras.
The video-camera methods, when averaged over many sampling stations with a wide range of environmental conditions, provided estimates of brook trout abundance similar to both diver counts and backpack electrofishing. Although the researchers’ focus was the effectiveness of video cameras, the study answered my question; the number of fish captured by electrofishing was, on average, about the same as divers’ counts.
Well, sort of. The stream pools where fish counts were compared were sampled by single-pass electrofishing — once through the pool — and triple-pass electrofishing — three successive passes through each pool with captured fish not released until the last pass is completed. The average counts of larger adult brook trout were the same for single-pass electrofishing, triple-pass electrofishing, and divers. For smaller, young-of-the-year trout, only the triple-pass electrofishing counts were similar to diver counts, but the single-pass counts were much lower.
Not a big surprise. Small fish are less affected by electric current. A threshold amount of electric current is required to affect a fish, and the amount of current that flows through the fish to elicit a response increases as the fish grows.
The counts achieved by electrofishing and those from divers were not in perfect accord for each individual sample. Fish can hide from divers, and not all fish in the field of an electrofisher are stunned. But the brook trout study reinforces the notion that electrofishing provides as good an estimate as can be achieved by direct observation.
All charged up
Electrofishing has been the standard method for sampling black bass and sunfish in the Southeast for at least 50 years. Advances in the equipment have improved both operator safety and sampling efficiency.
Many early electrofishers relied on alternating current (AC) output from generators. They worked for some fish, like black bass, sunfish, crappie and white bass. Most biologists today use pulsed direct current (DC) outputs that combine the fish-attracting effect of DC — fish in a DC field swim to the positive electrode — with the fish-stunning effect of AC.
Advanced electrofishing control boxes allow operators to adjust power output, pulse form and pulse frequency. Adjusting power output allows standardization of power in the water and facilitates comparison of catches across environmental conditions that vary with a body of water over time or among different water bodies.
The ability to adjust pulse frequency has made electrofishing an effective tool for sampling catfishes. AC or 60-pulses-per-second DC outputs that are typically used to sample many scaled fish do not work for sampling catfish, but low frequency (15 pulses per second) DC is very effective and has become the standard method for assessing blue and flathead catfish populations in many states. The response to low-frequency current is why old hand-crank telephone generators effectively captured catfish.
Electrofishing has limitations. It does not work in water with very low conductivity (few dissolved salts) or very high conductivity water (brackish water). Fortunately, most waters throughout the Southeast have mid-range conductivities conducive to effective sampling.
Depth is also a limitation. Most of the electric field in the water flows between the electrodes. If you want to create an electric field on the bottom in 25 feet of water, you need 25-foot long electrodes.
If you shock a fish on the bottom, it has to swim or float to the surface to be detected. Despite all that has been learned about both electric current in water and the physiology of fish, exactly how fish are attracted to or stunned by electric current remains a mystery.
You can learn a lot when shocking relatively clear water: not all fish are affected, and some of the stunned fish remain on the bottom. Electrofishing does not collect all fish.
Electrofishing has exposed another mystery, a largemouth bass mystery. Electrofishing bass quickly reveals areas and habitats where bass congregate. Yes, I’m guilty of trying some of these areas on my next fishing trip. Wouldn’t you?
But here’s the deal: I can always collect bass with an electrofisher in an area where I catch them, but in only a very few instances have I found new places to fish based on electrofishing catch. In other words, just because fish are in an area doesn’t mean you can catch them there.
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