May 4, 2016

Game and Fish tagging and studying unwanted local walleye

Only a light breeze ruffles the surface of the water, but falling snow accentuates a soggy night for the two boat crews doing very wet work.

The Wyoming Game and Fish Department employees are catching, tagging and returning hundreds of walleye in Buffalo Bill Reservoir to help determine if suppressing the unwanted fish is feasible.

On a recent Wednesday night, the employees were working west of Buffalo Bill Dam.

Jacob Scoville (at left) and Riley Gallagher, Wyoming Game and Fish Department fisheries technicians, net some squirming walleye. Cody News Co. photos by Gib Mathers

A portable generator provides electricity for the electrical arrays on booms they lower into the shallow water where the walleye spawn. The arrays send a jolt of electricity to stun the fish, the men net the dazed walleye near the surface and deposit them in a live well.

If they haven’t been tagged before, each fish receives two tags that resemble short, wire antennas jutting from their sides. Five hundred of the tags will offer either a $10 or $100 reward if they're brought back to Game and Fish.

“Here’s one of the bigger females,” said Game and Fish Department fisheries biologist Jason Burckhardt, showing off a walleye that would tickle any angler. It weighed 2.58 pounds.

Next, Burckhardt notes a tagged male of smaller proportions, but worth its weight in gold with a tag that's worth $100.

Sean Cooley, fisheries technician for the department, determines each walleye’s sex and size and then inserts the tags before returning them to the lake.

“Looks like seven recaps,” Cooley said.

“Recaps,” or recaptures, are walleye previously caught and tagged.

The higher the recapture rate, the less variance in the abundance estimate, said Daniel Kaus, a Montana State University-Bozeman graduate student. Kaus is working on the project so he can build a walleye population model.

The guys on the boat wear protective gear to avoid being shocked. They tag at night because the walleye are easier to catch where they are spawning, Burckhardt said. The crew launches at dark, sometimes remaining on the lake until 3 a.m.

Jason Burckhardt, Game and Fish Department fisheries biologist, examines a previously captured and tagged walleye female while Riley Gallagher (at left) mans the net.

Game and Fish workers have caught females drained of eggs, so they know the fish are spawning.

As of April 26, the department had tagged about 350 walleye. The plan is to keep going until 700 of the fish are tagged or the spawn or the spawn ends.

The potential $10 or $100 walleye bounty on the tags is meant to encourage fishermen to report the tagged walleye they catch. The more anglers reporting their marked walleye, the more accurate the department's mortality estimate will be, Burckhardt said. The study is aimed at measuring both angler-caused mortality and Game and Fish-caused mortality.

The estimating technique works like this: a portion of the population is captured, marked and released. Later, crews go back out, capture more fish and count the number of tagged walleye.

The number of marked walleye within that later sample should be proportional to the number of marked walleye in the whole population, so you can come up with an estimate of the total population by dividing the number of marked walleye by the proportion of marked walleye in the second sample.

For example, if Game and Fish tag and release 100 walleye and 50 tagged fish are caught by anglers, they can assume that is half the population and further deduct that the population totals 200 walleye, Burckhardt said.

Sean Cooley, Game and Fish fisheries technician, measures a captive walleye.

Walleye were illegally introduced into the reservoir, likely occurred from 2002 through 2004.

They prey on the trout that inhabit the wild, unstocked fishery in Buffalo Bill. Previous studies have shown walleye eat a lot of juvenile trout, Burckhardt said. Game and Fish wants to maintain the wild fishery in Buffalo Bill to in turn support trout on the North Fork of the Shoshone River.

A walleye female bears 20,000 to 200,000 eggs, Kaus said. Although the walleye hatchling survival rate is relatively low, the sheer number of offspring one female can produce makes a big difference.

If Kraus can obtain the vital rates (survival and death), then he can run a simulation model to determine population growth each year. From there, he can calculate the rate of mortality needed to suppress the walleye population, he said.

If enough effort is exerted, walleye can be suppressed in Buffalo Bill, but the question remains: How much would that effort cost? Game and Fish wants to know the degree of suppression needed and the price tag, Kaus said.

If tagged walleyes are caught, anglers are asked to call the Cody Game and Fish office at 307-527-7125 or the phone number provided on the tag.

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