Why did the kokanee crash at Granby?
A “perfect storm” of factors, but primarily lake trout predation. Many years of research show that kokanee at Granby are strongly influenced by a productivity cycle driven by climate and dam operations. In wet years, when the reservoir is full in summer, it tends to be cooler, allowing the cold-adapted mysis shrimp to reach the surface and decimate the zooplankton that kokanee rely on for food. In dry years the reservoir is not so full and it becomes too warm on the surface for the mysis shrimp, and the kokanee have all the zooplankton they need. Several high water years at Granby created very poor food conditions for kokanee. This created slower growing kokanee which were in turn more susceptible to lake trout predation. So, the combination of high water at Granby, competition for food with mysis shrimp, and predation from lake trout appear to be the causes for the kokanee’s near demise at Granby.
