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Stocking Gets Fishy

Illegal Introduction of Fish Worries Biologists
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Author's note:
This article was published a couple of years ago, so some things have changed.

The program to make sterile walleye was stopped temporarily in 2009 while the DOW looks at alternatives for developing triploid fish. The DOW hopes to start producing sterile walleye again in the near future.

At Blue Mesa Reservoir, over-predation of kokanee by lake trout has occurred. Consequently, the DOW is working to remove some 17-30 inch lake trout in order to reduce the predation. Kokanee salmon numbers have decreased significantly in the past five years. Blue Mesa, however, will continue to produce trophy-sized lake trout.

To learn more about the situation at Blue Mesa Reservoir, go to:

Stocking fish in Colorado's lakes and reservoirs can be a tricky business. Put the right fish in the right water and the results can be amazing. But if just one small piece of the aquatic puzzle is wrong, a fishery can crash.

Through years of education and on-the-job experience, Colorado Division of Wildlife aquatic biologists and researchers have learned that there is a fine balance between predator and prey in the aquatic world. Deciding what fish are best suited for each body of water involves a fundamental understanding of ecology and primary productivity. In lakes and reservoirs, biologists attempt to match the available food supply by stocking the proper fish species - or combinations of species - in the correct numbers to produce fish for anglers to catch.

DOW's aquatic biologists however, are facing a major problem that threatens the state's lake, pond and reservoir fisheries: The work of "Bucket Biologists": people who illegally dump fish into waters hoping to start a population of their favorite game fish.

In recent years, DOW biologists have learned that illegally stocked warm- and cool-water fish are showing up in some of Colorado's finest trout and salmon fisheries. The species most commonly used for illegal stocking are northern pike, smallmouth bass, yellow perch and walleye. Unauthorized stocking of these predator species has the potential to cause irreversible damage to some of the most popular trout and salmon fishing waters the state has to offer. That costs Colorado's anglers millions of dollars and lost fishing opportunity, explains Mike Japhet, senior aquatic biologist for the DOW in the southwest region.

"The DOW strives to provide a variety of fishing opportunities to match the diverse interests of Colorado anglers," Japhet says. "Some lakes and reservoirs are better suited for trout and salmon. While other waters provide conditions where warm-water and cool-water species like pike, walleye, bass, and crappie can thrive. We can't put just any fish in any water."

The reservoirs in Colorado, especially on the Western Slope, aren't like the waters in the Mississippi River basin, the Great Lakes or even the Eastern Slope. While warm- and cool-water fish species may be able to survive and reproduce in these colder waters, they grow very slowly and are typically prone to stunting. High-altitude reservoirs, however, provide optimal conditions for cold-water species like trout and kokanee salmon.

"When an angler takes matters into his own hands by illegally stocking his favorite fish, it is like playing Russian roulette with the food chain. There are unintended consequences and bad things can happen," Japhet says.

Blue Mesa Reservoir provides a prime example of the problems caused by illegal fish stocking.

Blue Mesa is one of Colorado's most productive reservoirs. Its waters are rich with plankton – the food on which kokanee salmon and rainbow and brown trout thrive. In turn, lake trout - a predator – feed on those fish. Consequently, fishing for all the species is outstanding; and surveys have shown consistently that most anglers prefer to fish for those species at Blue Mesa.

But several years ago, someone illegally stocked yellow perch in the big reservoir west of Gunnison. While the perch don't grow very large they are prolific spawners. Perch eat plankton and small kokanee salmon, and their population is growing rapidly. The consequences? Perch have forced significant extra expense on the DOW's established stocking program. The DOW is spending $125,000 to raise more kokanee at the Roaring Judy Hatchery to compensate for predation losses to perch.

"Anglers who like to fish for lake trout should be concerned. If the kokanee population declines, lake trout will be right behind," Japhet said.

But the effect doesn't end there. Blue Mesa is Colorado's top kokanee producer with 6 million eggs collected during the spawning run each year. Many of those eggs are then shipped to hatcheries throughout the state and used to replenish a dozen other reservoirs.

A proliferation of perch at Blue Mesa could have a devastating effect on the kokanee population. A collapse of the kokanee population would also have a huge negative impact on the Gunnison economy which derives an estimated $15 million per year from anglers.

And that's just one example.

Other predator fish – walleye, smallmouth bass and northern pike – have been illegally stocked at 18 other Colorado reservoirs, most located on the Western Slope.

A look at some basic facts from the world of fish biology provides further evidence of potential negative effects. A single 40-inch northern pike will consume 14 pounds of trout and salmon per year. So the cost for the DOW - and indirectly to anglers - to grow a pike to that size is more than $400 if the primary forage in the lake is trout or salmon.

Predator fish also present human health concerns in some reservoirs where mercury is present. Predators "bioaccumulate" mercury when they feed on smaller fish in reservoirs where mercury is present. Consequently, the Colorado Department of Health and Environment has issued a warning that consumption of northern pike and walleye taken from certain waters be limited or eliminated from the diet.

Another big issue involving illegal stocking comes in the form of the Upper Colorado River Endangered Fish Recovery Program that is administered by the U.S. Fish and Wildlife Service, a federal agency. In three western Colorado rivers – the Colorado, the San Juan and the Yampa - three fish are listed as endangered species: the pikeminnow, the razorback sucker, and the humpback chub. To make sure these native fish survive, the non-native predator fish must be kept out of rivers.

Unfortunately, smallmouth bass and northern pike have moved into the Colorado and Yampa rivers and are preying upon native fish, including the endangered species. The DOW is working cooperatively with the fish and wildlife service to remove those fish.

"We're asking fishermen to work cooperatively with us on the endangered fish program," Japhet says. "We can help to save endangered fish and continue an outstanding recreational fishing program in Colorado."

Japhet, a 30-year employee, also admits that the DOW has caused some of the problems with predator fish.

"We've made our share of mistakes; we aren't without blame," Japhet says.

A major example: The DOW probably erred in introducing northern pike to Colorado waters back in the 1960s. Back then, biologists saw that northern pike, a cool-water species, could help to consume nuisance fish in reservoirs, and provide a great recreational opportunity. But they underestimated pikes' ability to reproduce in Colorado's high-country reservoirs.

"At this point, the important thing is to learn from the past and take precautions with stocking predator fishes." Japhet said.

Now northern pike are thriving in many reservoirs and have escaped into the species-sensitive waters of western Colorado. And, ironically, northern pike is now the species that is used the most for illegal stocking. Northern pike were recently found in McPhee Reservoir in southwestern Colorado where they were placed illegally. Biologists believe the fish were probably brought to the lake by anglers from nearby Narraguinnep Reservoir where they were also stocked by the DOW. McPhee contains kokanee, smallmouth bass and trout – all forage for the voracious northern pike.

Statewide, there is no bag limit for northern pike.

So, with illegal stocking documented in more than 40 reservoirs in Colorado, the DOW Aquatic Section is working to discourage the practice among anglers. One idea is to impose serious financial penalties on bucket biologists.

"We need to make the fine commensurate with the damage that an illegal stocking can cost; and it can potentially cost the DOW hundreds of thousands of dollars," Japhet says.

He cited a recent case in Wyoming in which a Colorado man was fined $100,000 for illegally transporting and stocking rusty crayfish.

"That's the kind of fine that truly shows the seriousness of the crime," Japhet said.

The DOW knows, however, that it must do more than just use a bigger hammer.

"We also need to expand fishing opportunities for various species," Japhet said.

One species that biologists are concerned about is walleye. The fish, a top predator, is a very popular sport fish and has been stocked by the DOW in numerous low-elevation reservoirs along the Front Range and on the eastern plains. They‘ve also been stocked in four waters in western Colorado: Puett and Narraguinnep reservoirs near Cortez, Sanchez Reservoir in the San Luis Valley, and at Rifle Gap in Garfield County.

But unfortunately, some anglers have illegally placed walleye in other western Colorado reservoirs where they pose a threat to the native threatened and endangered species.

So, in order to protect native fish and to enhance recreational fishing opportunities, the DOW is now engaged in a long-term experiment: The agency is working to produce sterile walleye.

In April, for the first time, fish culturists from the Pueblo hatchery used a special technique to produce what is known as triploid walleye. Immediately after walleye eggs were fertilized they were placed in a cylindrical pressure chamber filled with water and subjected to high pressure - 9,500 pounds per square inch. Hydraulic pressure on the eggs causes a third set of chromosomes to develop in the walleye embryos and the resulting fish are rendered sterile. The technique has been used on a variety of fish species throughout the country for years.

There's a bonus to the procedure for walleye anglers: Because the fish don't need to put energy into sexual development, they grow bigger and faster.

"For biologists the use of sterile predator fish offers a two-fold advantage: fish numbers are controlled by stocking alone, greatly reducing the risk that a sport fishery will collapse from predation. And if they escape into endangered fish habitat, their negative impact is diminished because they are incapable of natural reproduction" Japhet explains.

In late April about 400,000 sterile walleye fry were stocked for the first time in Colorado at Narraguinnep and Puett reservoirs near Cortez. In two to three years the fish should reach catchable size. Aquatic biologists and fish culturists will evaluate their progress to determine if they should be stocked in other waters.

The DOW has been working with the U.S. Fish & Wildlife Service for more than a year to develop lake management plans that will allow sterile fish to be stocked in other appropriate waters in western Colorado. If the plans are approved, more sterile walleye could be stocked in reservoirs in the northwest area of the state in 2009.

In the meantime, the DOW is urging cooperation from people throughout the state. Anglers are asked to spread the word to others about the dangers of illegal stocking, and to report any suspicious activity around water to Operation Game Thief at 1-877-265-6648.

"This is a complicated issue and there are no perfect solutions," Japhet says. "I believe we can have an endangered fish recovery program and still provide plenty of sport-fishing opportunities. The use of triploid or sterile hybrids is one example where a win-win solution may be possible. But we need cooperation from anglers."

And for those who long to fish for a particular species found elsewhere, Japhet suggests that they take a close look at everything Colorado has to offer.

"Blue Mesa Reservoir offers world-class fishing for kokanee, trout and lake trout. Why not give it a try?"
© 2024 Joe Lewandowski (CDOW)
Reprinted from Colorado Outdoors 2007, The publication is the official magazine of the Colorado Division of Wildlife. Written by Joe Lewandowski for the Colorado Division of Wildlife. Joe is the public information specialist for the DOW's southwest region. He's based in Durango.
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