In the ongoing effort to provide more sport-fishing opportunities, Colorado Parks and Wildlife is researching walleye that have been stocked in Narraguinnep Reservoir in Southwest Colorado.
Walleye are predators that could harm endangered native fish species in the San Juan River Basin; therefore, CPW stocking work is restricted.
But aquatic biologists have developed a technique that renders walleye sterile and unable to reproduce in the wild, said Joe Lewandowski, CPW public information officer.
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 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, CPW 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 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 that alters the chromosome structure. The technique has been shown to be 95 percent effective.
In work that began March 18 at Narraguinnep Reservoir, nets were set every night for two weeks, to remove walleye and take blood samples. A blood test, to be done by a lab in California, will determine whether the fish was sterile. About 400 fish were removed during the operation.
Researchers also will measure the fish and examine their stomachs to determine what they are eating. Some fish captured last year were found to range from 14 inches to about 24 inches long. Walleye can live more than 10 years.
White said Narraguinnep hasn’t been heavily fished because most anglers troll for walleye, and motorized craft are not allowed on the reservoir.
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.