Sterile Walleye Stocked in Southwest Colorado Reservoirs

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As an experimental project aimed at protecting native fish and expanding recreational fishing opportunities in western Colorado, the Colorado Division of Wildlife has stocked sterile walleye in Puett and Narraguinnep reservoirs near Cortez. Stocking of the sterile fish in these southwest Colorado reservoirs will continue annually for five years.
Several lake management plans in northwest Colorado are being rewritten and the DOW hopes to obtain permission to stock triploid walleye in those lakes in 2009.
Walleye are a popular sport fish that are stocked by the DOW in plains and foothill reservoirs on the eastern slope of Colorado. In western Colorado, however, stocking of warm-water predator fishes is restricted by agreement between the members of the Upper Colorado River Endangered Fish Recovery Program to protect endangered fishes in the upper Colorado River and San Juan River drainages. Walleye are predators and can take a toll on native fish, specifically the endangered Colorado pikeminnow, the razorback sucker and the humpback chub.
The fish recovery program is a partnership between the states of Colorado, Utah and Wyoming, U.S. Fish and Wildlife Service, irrigators and other water users, National Park Service, Bureau of Reclamation, and environmental groups. Through the program, endangered fish are recovered in a way that allows water development to continue in the Upper Colorado and San Juan river basins.
“The Division of Wildlife stocked fertile walleye for many years at Puett and Narraguinnep reservoirs,” explained Mike Japhet, senior aquatic biologist for the DOW’s southwest region. “The switch to stocking sterile walleye is something new. It will be a couple of years before we know how the experiment to produce and stock sterile fish will work. The payoff for anglers is that sterile fish grow faster and reach larger size than their fertile counterparts.” 
The endangered fish recovery program may also benefit from this stocking trial. In mid-March, the DOW was given permission by the U.S. Fish and Wildlife Service to stock sterile walleye in Narraguinnep and Puett reservoirs. These are irrigation lakes far from the San Juan River, so it’s very unlikely that the fish could make it to the river if they escaped from the reservoirs. But if they did, the fish would eventually die without reproducing and would present less risk to endangered native species.
The walleye were made sterile through a process that exposes fertilized eggs to extreme pressure – 9,500 pounds per square inch. The eggs were from spawn taken at Carter Lake, located near Loveland, in mid-April. Within three minutes after being fertilized eggs were placed in a reinforced steel cylinder filled with water. A hydraulic jack was used to raise the pressure. The eggs were held for about 10 minutes. The pressure alters early cell division by producing an extra female chromosome. The fish, then, have three chromosomes and are known as triploid. The third chromosome renders them sterile.
The technique has been used for years on a variety of other fish in other states and in the private aquaculture industry. Triploid trout and catfish are fairly common; but triploid walleye are more challenging to produce. 
About 450,000 walleye fry were stocked at the two reservoirs on April 23. DOW biologists expect that about 10 percent of those fish will survive and grow to catchable size in several years time.
“When people start catching them, they won’t notice any difference in appearance of the triploids—they look the same as regular diploid walleye. The differences can only be determined at the genetic level. In time, however, we hope there will be more lunker walleye at these reservoirs,” Japhet said.
In upcoming years DOW biologists will conduct netting surveys to determine the growth rate of the fish. Also, some of the pressure-treated walleye are being raised in hatchery ponds to determine what percentage actually develops into triploid fish.  

 – Colorado Division of Wildlife –

The Colorado Division of Wildlife is the state agency responsible for managing wildlife and its habitat, as well as providing wildlife related recreation. The Division is funded through hunting and fishing license fees, federal grants and Colorado Lottery proceeds through Great Outdoors Colorado.