Lake Granby kokanee are dwindling
December 20, 2013
Kokanee salmon populations in Lake Granby have dropped to an alarming low, and Colorado Parks and Wildlife officials are exploring solutions.
This year's fish count found only 357,000 kokanee eggs, down from 4 million in 2006 and from a population peak of 16 million eggs in 1982. Many of Colorado's kokanee stock come from eggs collected in Lake Granby, and state biologists said finding management objectives to bring back their numbers is a top priority.
"Lake Granby kokanee need to produce 1.2 million eggs just to sustain their population there," said Colorado Parks and Wildlife biologist Jon Ewert in a statement. "We are well below that number, so we won't be stocking other waters with eggs from here until we can get this situation turned around."
According to Ewert, kokanee numbers have dropped because of an increase in populations of the fish's natural predator, the lake trout, and a decline in its primary food source, zooplankton.
The number of lake trout has increased because of favorable spawning conditions after last spring's heavy runoff. The trout spawn around October in relatively shallow water with a rocky bottom. During drier periods, water levels drop and dry the eggs or cause them to freeze over the winter. But this last season was wet, so a large number of lake trout successfully hatched.
The kokanee's food source has run out because of growing competition from mysis shrimp, a species introduced in the 1960s to help predatory fish thrive in the lake.
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Kokanee salmon are native to the Pacific Northwest and were introduced to Colorado in 1951. Lake Granby was the first place the fish were stocked, Ewert said.
Fish out of water
Grand County's man-made reservoirs have long presented unique challenges to fish and state biologists trying to maintain their populations. Fish aren't adapted to a situation like Lake Granby, where levels drop as much as 50 feet in a year.
"No natural lake does that," Ewert said. "So it's figuring out how we manage a fishery in this highly unnatural situation. That's why all the different non-native species have been brought in."
Ewert said Lake Granby will probably never see the 16 million-egg spawning season of 1981 again. Statewide, the kokanee demand is for about 11 million eggs. Kokanee in Blue Mesa Reservoir near Gunnison and Vallecito Reservoir near Durango are faring better, producing about 7.5 million and 1.5 million eggs, respectively.
"They are eggs that will go to help out our lakes up here, which is rare," Ewert said. "Usually we're providing eggs for that part of the state instead of the other way around."
Ewert expects to hold public meetings on the issue in March after he completes more analysis over the winter. Possible solutions Colorado Parks and Wildlife might explore in Lake Granby include increasing the numbers of lake trout fishers can take. There's not much biologists can do about the mysis shrimp density, however. To keep Lake Granby as a popular and sustainable fishery, biologists will need to find solutions on both ends of the kokanee problem.
"The issue is a combination, a double-whammy, of competition and predation," Ewert said. "In order to have a trophy lake trout fishery, we need a healthy kokanee population."
Leia Larsen can be reached at 970-887-3334 ext. 19603.