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Blue Mesa Reservoir, located west of Gunnison, is a very productive fishery and the primary water for kokanee salmon production in the state. Lake trout, which are very efficient predators, are consuming kokanee and causing a significant drop in the population of the fresh water salmon. Rainbow trout survival is also declining due to lake trout predation. At the same time, the size of lake trout is declining because competition for food among the fish is increasing.
"This is a predator-prey relationship issue," said Dan Brauch, aquatic biologist for the DOW in Gunnison. "At Blue Mesa Reservoir the predators are overwhelming the prey; the relationship is out of balance."
John Alves, senior aquatic biologist for the DOW's southwest region, explained that this is a serious issue for Colorado's fisheries.
"We're losing kokanee and we're losing the potential for long-term production of trophy-sized lake trout. Without a strong population of kokanee salmon there is no possibility of growing lake trout to trophy size," he said. "The DOW's strategy will be aimed at maintaining a highly productive kokanee fishery, a good rainbow trout fishery and a viable trophy lake trout fishery. There is room for all the species in the lake, but careful management is necessary."
The situation is also causing concerns for 26 other lakes and reservoirs in Colorado where the DOW stocks kokanee salmon. Blue Mesa Reservoir provides more than 60 percent of the eggs needed to stock 10 million kokanee fingerlings in the reservoirs each year. Stocking will be reduced if the annual spawn take at Blue Mesa continues to decline.
Lake trout that are average size, 17-30 inches in length, will eat 13 pounds or more of kokanee each year. A 40-pound trophy-sized fish can consume 50 pounds of kokanee. In 1994, the DOW estimated that the kokanee population was nearly 1 million at Blue Mesa; it is now estimated to be only about 200,000. Each year, the DOW releases 2-3 million fingerling salmon into the reservoir.
Kokanee fishing is very popular at Blue Mesa. According to multiple-year surveys, 40 percent of anglers there fish specifically for salmon, 15 percent fish specifically for rainbow trout and 8 percent fish specifically for lake trout. The catch of kokanee, according to creel surveys, has declined significantly: from 175,000 in 1993 to just 30,000 in 2008. The catch of rainbow trout has dropped from about 74,000 in 1998 to only about 10,000 in 2008.
Catch of lake trout, however, has increased substantially: from about 1,700 per year during the late 1990s to about 8,600 in 2008. The large increase in the catch rate indicates significant growth in the natural reproduction of lake trout.
Lake trout spawn in shallow water during October and November, and they feed close to shore during the spring. Removal will take place at those times of year. This will be a multi-year project. DOW biologists will use nets to target small- to medium-sized lake trout for removal and every effort will be made to release trophy fish unharmed. The number of fish to be removed each year will be determined based on creel surveys and the results of spring population monitoring.
Anglers can also help to reduce the lake trout population by keeping the fish they catch. A creel survey in 2008 showed that anglers released half of the approximately 8,600 lake trout they caught.
"In an environment like Blue Mesa Reservoir where lake trout reproduction is significant, angler harvest of lake trout will help the DOW maintain the balance between lake trout and kokanee populations," Alves said. "If anglers keep the fish they catch the DOW probably wouldn't need to remove fish. Plus, they'll get some great meals."
Detailed plans for how the removal project will be conducted are being developed. Public meetings to explain the project are planned for later this summer. "We've never removed lake trout from the reservoir, so we will plan the operation carefully and work diligently to develop the best techniques," Alves said.
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