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Colorado Parks and Wildlife researching walleye at southwest Colorado reservoir
CPW News Release
DURANGO, Colo. – In the ongoing effort to provide more sport-fishing opportunities for anglers, Colorado Parks and Wildlife is conducting a research project on walleye that have been stocked into Narraguinnep Reservoir in southwest Colorado.

Because walleye are a predator fish that could harm endangered native fish species in the San Juan River Basin, CPW stocking work is restricted. But aquatic biologists have developed a technique which renders walleye sterile and therefore unable to reproduce in the wild. The research study is aimed at confirming the effectiveness of the sterilization technique and how well the walleye survive in the wild. If the sterilization is shown to be successful, sterile walleye would lessen the threat to native species.

“Our concern, which we share with the U.S. Fish and Wildlife Service, is that if walleye escape into the San Juan River they would pose a major threat to the endangered razorback sucker and pike minnow,” said Jim White, aquatic biologist in Durango. “If we can show that these fish are sterile and can’t reproduce that would greatly reduce the risk of escaped walleye starting a population in the San Juan or Colorado rivers.”

The walleye experiment actually started in 2008 when CPW began stocking walleye fry and fingerlings in Narraguinnep. Since then the agency has stocked 1.5 million walleye in the reservoir. To make fish sterile, fertilized eggs ‒ harvested from walleye at Pueblo Reservoir ‒ are placed under high pressure in a special container which alters the chromosome structure. The technique has been shown to be 95 percent effective.

In work that began March 18 at Narraguinnep Reservoir, nets will be set every night for two weeks, walleye will be removed and a blood sample will be taken from each fish. A blood test, to be done by a lab in California – will determine if the fish was sterile. About 400 fish will be removed during the operation.

Researchers will also be measuring and aging the fish, and examining their stomach contents to determine what they are eating. Some fish were captured last year and they were found to range in size from 14 inches to about 24 inches. Walleye can easily live for 10 or more years.

White said Narraguinnep hasn’t been heavily fished because motorized craft are not allowed on the reservoir and most anglers troll for walleye.

If tests show that 95 percent of the fish are sterile, White said that CPW will be able to make a good case to the U.S. Fish and Wildlife Service for stocking walleye here and at other Western Slope reservoirs where similar stocking restrictions exist. The U.S. Fish and Wildlife Service, CPW and other agencies work together on the endangered fish recovery program in the intermountain west.

White hopes a full report on the testing will be completed late this year.