Predator-prey balance vital to egg supplyBy Karl Licis, Special to The Denver Post
01/27/2010Fishing for kokanee salmon in Colorado is only as good as the supply of eggs available to the Colorado Division of Wildlife, and for many years, Blue Mesa Reservoir has been a major source of salmon for the state. In recent times that egg basket has become less reliable, and the DOW has had to look to other waters to help meet the demand for kokanee.
An annual spawning run of sal-mon from Blue Mesa up the Gunnison and East rivers to the Roaring Judy Hatchery above Almont, more than 20 miles away, traditionally has produced 5 to 8 million eggs toward a statewide goal of 11 to 12 million. The Blue Mesa salmon run has been declining because of a growing population of kokanee-eating mackinaw in the reservoir, according to DOW fisheries biologists. With fewer salmon making the run, the supply of eggs at the hatchery has dropped significantly.
In 2008, the Roaring Judy facility collected a little under 5.8 million eggs, less than a bumper crop but well above the 2.1 million gathered last fall. To make up the shortfall, the DOW expanded its egg-taking efforts to Cheesman, Shadow Mountain and Elevenmile reservoirs, in addition to established brood waters including Granby, Williams Fork, Vallecito and McPhee reservoirs.
The effort yielded a little more than 10.1 million eggs last fall, compared with just under 11.6 million in 2008, down roughly 14 percent. However, with an excellent hatching rate, the actual difference in the supply of young fish is about 10 percent, according to Matt Nicholl, the DOW's chief of hatchery production.
With fewer young salmon available, stocking brood waters will be a priority, Nicholl said. Lakes with incidental populations of kokanee are likely to receive fewer salmon this year.
In the longer term, the DOW last fall began removing mackinaw from Blue Mesa by netting. Mackinaw were introduced to the reservoir in 1968, with subsequent stocking in the 1970s, 1980s and 1990s. The lake trout have been naturally reproducing, and salmon numbers have been going down. DOW population studies indicate the number of kokanee in the reservoir has gone from 1 million in 1994 to a current 200,000, despite increased stocking in years when supplies permitted.
While catch rates for salmon have been going down, the catch rate for mackinaw has increased. Mackinaw also are becoming more slender, another indication that their food supply is declining.
The DOW hopes to restore a predator-prey balance between the mackinaw and salmon, and will continue the fall netting until biologists determine the salmon population no longer is threatened.
The salmon situation
Salmon egg counts for Roaring Judy Hatchery
2008
5,778,039
2009
2,102,004
Difference
3,676,035
Statewide salmon egg take
2008
11,558,569
2009
10,139,875
Difference
1,418,694
Approximate egg yields for 2009
Roaring Judy/Blue Mesa
2.4 million
Granby/Shadow Mountain reservoirs
2.3 million
Vallecito Reservoir
2.1 million
Williams Fork Reservoir
2.0 million
McPhee Reservoir
700,000
Cheesman Reservoir
500,000
Elevenmile Reservoir
121,000
Source: Colorado Division of Wildlife
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