Kokanee salmon of concern at Granby
It’s become a familiar refrain for Colorado Parks and Wildlife — a topline predator fish eats itself out of its prey, forcing fisheries managers to find ways to control the predator.
It happened at Blue Mesa, where lake trout plundered the kokanee salmon and eventually became over-populated, leading to some large-scale management and regulation changes at the southwest Colorado lake.
Now, a similar situation has arisen at Lake Granby, once home to some of the largest lake trout swimming in Colorado.
But it’s not just the lake trout that are of concern. It’s also the kokanee salmon, the lake trout’s preferred diet and a favorite target of anglers.
According to Parks and Wildlife, nearly 4 million kokanee salmon eggs were harvested from Lake Granby salmon in 2006. This past fall, Parks and Wildlife managed to collect a little more than 350,000 eggs, a decrease that has fishery officials wondering how to reverse that trend.
The egg harvest at Lake Granby at times was the agency’s second-largest such harvest behind Blue Mesa Reservoir, where in 2012, a record 13.1 million eggs were taken.
The kokanee eggs are used to stock Lake Granby and other waters around the state and Lake Granby’s salmon are key to having enough eggs to meet stocking demands.
It’s thought the lake trout ate the salmon to the point the salmon and their egg production are in trouble.
The question is how to increase the number of salmon and in turn increase egg production.
“Lake Granby kokanee need to produce 1.2 million eggs just to sustain their population there,” said Parks and Wildlife fisheries biologist Jon Ewert. “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.”
Kokanee are land-locked Pacific sockeye salmon that feed primarily on zooplankton. Biologists say the kokanee population has declined because of competition from mysis shrimp that also feed on zooplankton and predation by a “significant” lake trout population.
Ewert says mysis populations rise during high-water years and fall during periods of low-water levels. It’s thought the abundant snowfall this winter might lead to more mysis, and more competition for zooplankton, in the future.
Although Parks and Wildlife can’t affect mysis densities, Ewert said a viable solution may be found by working on the predation side of the equation.
“Based on the data and information gathered, we believe that the lake trout can definitely sustain a higher level of harvest in Lake Granby,” said Ewert. “We increased the lake trout take limit in 2006 but have continued to see their numbers increase while kokanee numbers decrease, so the goal is to manage more effectively.”
Ewert adds that fewer kokanee has led to poor body condition in large lake trout.
“Many of the lake trout I have seen are very skinny, essentially starving because their primary food source has become scarce,” he said. “This is a clear sign that we need to do more to address the current predator and prey imbalance in the reservoir.”
At Blue Mesa, managers are pursuing a combination of more liberal bag limits and the mechanical removal of lake trout to reduce the lake trout population. During the past four years, biologists have removed almost 5,000 lake trout from the reservoir.
Parks and Wildlife officials said they will host a series of public meetings in March of 2014 to discuss the direction of fishery management in Lake Granby.