Lake trout causes decline in salmon population | PostIndependent.com

Lake trout causes decline in salmon population

GUNNISON — Colorado Parks and Wildlife biologists are keeping a close eye on the fishery at Blue Mesa Reservoir and continue work to rebuild the kokanee salmon population.

On April 2, CPW made the annual release of about 3 million fingerling kokanee from the Roaring Judy Hatchery. Those fish that reach maturity will return to spawn in two to five years. Kokanee at Blue Mesa Reservoir are critically important for CPW’s fishery program statewide. Of any reservoir in Colorado, Blue Mesa produces the largest number of kokanee eggs annually. These eggs are essential for stocking kokanee in 26 waters throughout the state.

The kokanee population has been on a steady decline for the last decade. In the early 2000s, the population was estimated at more than 1 million fish. In 2013, the last time biologists conducted a sonar survey, they found fewer than 200,000 kokanee in the reservoir. Predation by lake trout on kokanee is the major factor in the population decline. Kokanee are the primary prey for lake trout, providing the food source that allows the lake trout to grow to trophy size. And while the kokanee population has declined, the lake trout population has grown significantly.

“Managing the fishery in Blue Mesa is a challenging task because of a variety of factors,” said Dan Brauch, aquatic biologist in Gunnison. “CPW is working to restore a balance in the fishery.”

The challenge was compounded by low-water conditions during the summer of 2013. The low water affected water quality in one area of the reservoir. An algae bloom depleted oxygen and caused a die off of some fish. More significantly, the low water caused kokanee and lake trout to be crowded close together, which likely led to greater predation by the lake trout.

Because of those conditions, anglers had little success catching kokanee this past summer.

Fewer fish headed back upstream during the annual spawning run to the Roaring Judy Hatchery. CPW harvested only about 3 million eggs last October from the Blue Mesa kokanee, a decline of about 5 million from the previous year.

“The water level improved somewhat last year, but the runoff will be down again this spring,” Brauch said. “We can’t say for sure what’s going to happen this year or how kokanee will be affected.”

Predation of kokanee by lake trout continues to be a major concern for CPW. To encourage harvest of lake trout, CPW regulations allow anglers to keep all of the fish caught that are under 38 inches. Anglers can keep just one fish over 38 inches.

“The smaller and more numerous lake trout appear to be having a much greater impact on kokanee than the larger trophy-sized lake trout,” Brauch said. “So CPW encourages anglers to harvest and keep lake trout that are under 30 inches.”


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