Sterile Walleye Stocked in Southwest Colorado Reservoirs
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.
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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.
