CPW seeks solutions for decline of Lake Granby kokanee
Kokanee are land-locked Pacific sockeye salmon found in several high-elevation reservoirs in Colorado. The fish feed primarily on zooplankton however, biologists say competition and pressure from both mysis shrimp that also feed on zooplankton and a significant lake trout population that prey on kokanee are the primary reasons for the drop in their numbers in Lake Granby.
Because many of the state's kokanee come from eggs collected in Lake Granby, increasing their numbers to meet management objectives is a priority for Colorado Parks and Wildlife.
"Lake Granby kokanee need to produce 1.2 million eggs just to sustain their population there," said 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."
Ewert says mysis populations rise during high-water years and fall during periods of low-water levels. According to this scenario, the substantial rainfall and snowpack this year may contribute to higher numbers of mysis in the next several years, a situation that will weigh on future management decisions regarding how the issue is eventually addressed.
While Ewert acknowledges that CPW cannot influence mysis densities in the reservoir, he says there is a viable solution for 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."
In March of 2014, Colorado Parks and Wildlife officials plan to invite the public to several meetings where they can review biological data and discuss the direction of fishery management in Lake Granby with state wildlife officials.
For more information, visit www.wildlife.state.co.us/Research/Aquatic/ColdwaterResEcology/Pages/ColdwaterReservoirEcology.aspx