Just as anglers can fill their tackle boxes with everything from jigs and crankbaits to sinkers and bobbers, biologists also depend on different “gear types” to sample Oklahoma’s fish communities. Electrofishing – the practice of sampling live fish by temporarily stunning them with an electrical current – has long been the recommended gear for studying the state’s blue catfish. But biologists have had doubts about its ability to bring up big fish.  

“There’s an abundance of 30-inch-plus fish getting checked in at lake tournaments,” said Austin Griffin, a fish biologist with the Oklahoma Department of Wildlife Conservation. “But when we sample the same lakes with our shock boats, we’re just not seeing those fish.”  

To find out if additional gear should be used to monitor the state’s blue catfish, Griffin teamed up with other Wildlife Department biologists to intensively sample three lakes using three common catfish gear types. They found electrofishing may catch the largest numbers of blue catfish but misses a “shocking” number of big fish.  

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Provided by Austin Griffin

Blue catfish are considered “preferred-sized” when they reach at least 30 inches in length. These catfish grow slowly relative to many other sport fishes and may take 12-14 years to reach this size class.

⚡The Nuts and Bolts of Electrofishing ⚡ 

The idea of using electricity to capture fish has been around since at least 1863, when a patent for an apparatus using a battery and electrified hooks was unsuccessfully filed in England. But the practice didn’t take off until the 1940s when technology adequately caught up to the concept. Now, fisheries biologists can use a direct current at either a high frequency – 60-120 pulses per second – to sample bass, sunfish, and other centrarchids, or at a low frequency – about 15 pulses per second – to sample catfish. In general, electrofishing is used when water temperatures are at least 60 degrees Fahrenheit. High-frequency electrofishing is typically conducted in April and May while low-frequency is used throughout the summer months.  

“Electrofishing is the best way to momentarily stun and capture fish; collect length and weight data; and return the fish to the water unharmed,” Griffin said.  

For the blue catfish study, Griffin’s team used three aluminum-hulled boats: one “shock boat” and two “chase boats.” The shock boat was equipped with a generator and two booms with arrays of stainless-steel electrodes.  

“The generator creates a field of electricity that arcs from the two booms back to the boat, with the water conducting electricity.”  

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When the shock boat is active, it moves at a slow, constant pace while the catch boats are constantly moving to net the affected fish. Depending on the temperature and amount of suspended particles in the water, catfish may come to the surface anywhere within 100 yards of the shock boat. Some fish may lay stunned on the top of the water long enough to be collected while others may make a quick roll on the surface before swimming away.  

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Electrofishing may be an effective means of sampling fish for biological purposes, but it is not a legal means of take in Oklahoma. According to state statute, no person may use any electrical device for shocking purpose for taking, catching, capturing, or killing any game or nongame fish. 

Three Gears, Three Lakes, Three Years 

To test for bias among blue catfish gear types, three Oklahoma lakes were chosen that varied in size, had large blue catfish populations, and relatively good conductivity for electrofishing. Beginning in 2021, Lake Ellsworth, Arcadia Lake, and Wiley Post Lake were visited on an annual schedule, with weekly visits made to an individual lake in summer months to evaluate electrofishing, and multiple visits made each week in winter months to weigh the value of gill nets and jug lines in monitoring catfish populations.  

Gill nets are large, mesh nets that are suspended and left overnight. Nets can vary in size and mesh dimensions, but for this study were about 133 yards long and 8 yards deep with 3- to 6-inch mesh that targeted larger catfish. Jug lines are baited hooks attached to a buoy, often an empty plastic jug. Jug lines are a legal means of take in Oklahoma and must be attended at least once every 24 hours. 

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Over the three-year study, Griffin’s team sampled just over 15,000 blue catfish, including nearly 600 catfish more than 30 inches in length. Regardless of how it was caught, total length and weight was recorded for each fish, and a series of identification tags was added between two bones that cover the gills on any fish measuring more than nine inches in length. 

Low-frequency electrofishing brought up about 90% of the study’s blue catfish but less than 25% of the 30-inch-plus fish documented with all gear types. Gill netting, an admittedly harsher and less productive sampling gear, captured only 6-7% of the total fish, but accounted for about 65% of the study’s 30-inch-plus fish. Jug lining also captured big fish, but significantly fewer individuals than either electrofishing or gill netting.  

Though Griffin’s team is still running the numbers, they’ve been able to estimate the catfish populations at Lake Ellsworth and Wiley Post Lake. 

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Provided by Austin Griffin

More than 15,000 blue catfished were sampled over a three-year period to test the effectiveness of three different gear types. Biologists tagged each fish greater than nine inches in length to identify any recaptured fish. (Provided by Austin Griffin.) 

Considering all gear types, biologists estimate 120,000 blue catfish can be found in Lake Ellsworth. If they looked only at the electrofishing results, they would estimate 1,400 of those catfish would be preferred-sized. But the gill netting results show there are closer to 5,500 preferred-sized catfish in the lake, which equates to about one 30-inch-plus fish per surface acre. Similar densities of 30-inch-plus fish are thought to also occur in Wiley Post Lake.

A companion study in 2022, also conducted by Wildlife Department biologists, looked at the surfacing response of catfish exposed to low-frequency electrofishing in a controlled setting. Only half of the fish surfaced in one-third of the trials, and half of the fish were stunned but remained below the surface in another one-third of the trials.  

“You can sample more fish with electrofishing than any other way,” Griffin said. “But it is biased to smaller individuals, and you may not get the same response in all individuals. To effectively sample blue catfish, a multiple-gear approach is needed instead of just electrofishing.” 

Evaluating the effectiveness of three gear types in monitoring blue catfish was funded by the Wildlife Department and U.S. Fish and Wildlife Service through Sport Fish Restoration Grant F22AF00257 with matching funds provided by Oklahoma’s hunting and fishing license holders.