Electrofishing surveys provide biologists with data for assessing, managing Texas bass fisheries
KURTH LAKE -- It was shortly after sunset and the surface of Kurth Lake had slicked off smooth as a sheet of glass. Todd Driscoll and Dan Ash got their game faces on as they evaluated their gear and prepared for a late-night mission with high hopes of counting coop on as many largemouth bass as possible.
Interestingly, there was not a single rod, reel or bass bait in the boat.
Driscoll are inland fisheries biologists with the Texas Parks and Wildlife Department's District 3D office in Jasper. Their crew rides shotgun over nearly a dozen public reservoirs spanning nearly 317,000 surface acres across southeast Texas.
Kurth is the third smallest of the bunch. But don't be fooled by its meager 726-acre size. With its gin-clear water and generous abundance of high quality fish habitat such as hydrilla, cattails and lily pads, this is one East Texas gem whose name locals had rather not see in print.
Driscoll, Ash and the rest of the 3D inland fisheries staff paid visits to Kurth and nearby Lake Nacogdoches a couple of weeks ago to conduct their springtime "electrofishing" surveys. They invited me along for a crash course in how modern-day fisheries managers go about keeping tabs on bass populations in lakes across Texas and beyond.
"In terms of the field work we do, our springtime electrofishing surveys are by far the most enjoyable," said Driscoll, the District 3D supervisor. "We don't gather forage species like sunfish and shad, or weigh fish like we do in our fall surveys. During spring, we target only bass. We measure them, count them and put them back in the lake."
Always performed after dark, when lakes are less crowded, electrofishing surveys are one of several reliable tools fisheries scientists utilize for assessing public bass fisheries. Coupled with angler creel surveys, electrofishing helps provide managers with an accurate clue about the overall health of a specific fishery and how well the public is satisfied with it.
Biologists use electrofishing surveys to gather critical data related to numbers and size structure of largemouth bass and various forage fish populations from one lake to the next. The data helps them establish a "relative abundance estimate."
Electrofishing is a much more fish-friendly process than antiquated fisheries management practices such as rotenone (killing fish by poisoning), mainly because it does not kill the fish. Instead, it temporarily stuns fish so they can be captured in hand nets for counting and measuring.
"Years ago, state agencies used rotenone to kill everything in a specific area," Driscoll explained. "This gave them an 'absolute abundance' per acre. While very effective, this method came with a tremendous negative to override the good. Obviously, it meant killing a bunch of fish."
TPWD's electrofishing boats are sturdy, aluminum rigs. Boats are equipped with a large, elevated front deck, water level spotlights off the bow and a spacious holding tank that is oxygenated to keep fish lively. GPS/depthfinders help scientists feel their way around in the dark.
A floor mounted gasoline powered generator is used to send electrical current through a pair of booms mounted to both sides of the bow. Once it passes through the booms, the current is displaced into the water through a series of steel cables called anodes.
The set-up closely resembles a daddy longlegs spider, only this one packs a whole lot more punch. Driscoll likes to limit the electrical current to about 3-4 amps. He calls it "responsible shocking." Anything stronger could harm the fish.
Once the current reaches the water, it causes fish finning around within a 6-8 foot radius of the anodes to experience electrotaxis, otherwise known as "forced swimming" toward the anode. Think of it like steel to a magnet.
As the fish swims closer, the electrical field intensifies. Driscoll says this causes muscles to go limp and a loss of equilibrium in any aquatic life caught within the barrier. This is called narcosis. Bass generally float to surface in a belly-up posture with their gills flared.
The driver uses the power of the outboard engine to push the boat along at idle speed, usually following the contour of the shoreline at water depths no deeper than four feet. Meanwhile, two net men are posted on the front deck. It is their job to gather stunned bass and place them in the holding tank.
Lakes are divided into random quadrants that vary in number according to the size of the reservoir. Two shocking boats sampled six sites apiece at Nacogdoches and Kurth lakes.
"It's no problem to survey either of those lakes in a few hours," Driscoll said. "But lakes like Toledo Bend and Sam Rayburn take much longer, mainly because they are so large. The low water conditions made things more difficult this year because it is harder to get around the shallow water."
Each shocking sequence is carried out for five minutes. Once the time is up, the generator shuts down and the interior lights come on so biologists can log pertinent data before moving to the next station.
Most of the fish are released. However, Driscoll says a few fish from lake samplings must be sacrificed so biologists can determine their exact age. This is done by examining the odilith, or inner ear bone.
As a rule, Ash says bass fully recover from hypnotic effects of electrofishing fairly quick. "They are usually back to normal within a couple of minutes," Ash said. "It doesn't hurt them at all."
In two nights of shocking I saw common carp, needlenose gar, channel catfish, gizzard shad, suckers, sunfish and dozens of largemouth bass drift to the surface after getting hit by the current. The biggest fish by far was a 25 1/2-inch Nacogdoches bass. Ash estimated the fish to weigh about nine pounds.
Driscoll says trophy class fish are more the exception than the rule when it comes to electrofishing surveys.
"We know going in that electrofishing is not going to collect fish upwards of four pounds in proportion to their actual abundance," Driscoll said. "We do collect a few 9-10 pounders. But it doesn't happen very often, mainly because those bigger fish are more prone to occupy deeper water. You hear the rumors all the time about us shocking up 15-17 pounders, but it just doesn't happen."
The biologist says the electrofishing technique is most effective for collecting fish four pounds and under, mainly because of the water depth factor.
"Over time it gives us a pretty accurate snap shot of a fish population four pounds and under," Driscoll said. "If we do happen to shock a big one, we never use that data to assess the big fish populations. That's why we do angler creel surveys (questioning anglers one-on-one) and trophy bass surveys like those that are in place at lakes like Nacogdoches and Fork. Those are tools we use to get information on populations of larger fish -- the stuff that electrofishing does not provide."
Matt Williams' email address is mattwilliams@netdot.com.
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