Predicting the effects of environmental change on the behavior, growth, and predation risk of juvenile kokanee salmon

Abstract

Kokanee (Oncorhynchus nerka) have shown great variation in diel vertical migration (DVM) patterns throughout their geographic range. I utilized a bioenergetics-based foraging model to characterize DVM patterns exhibited by juvenile kokanee in Blue Mesa Reservoir (BMR), Colorado, and to address how environmental change (e.g., eutrophication, climate warming, drought, and dam operations) could affect kokanee growth and predation risk. I enhanced an existing bioenergetics-based foraging model by incorporating a light-dependent functional response that utilizes light availability and prey density to better predict feeding rates. By executing a series of simulations that considered either bioenergetic efficiency or predator avoidance, I determined that kokanee DVMs in BMR are controlled more by predator avoidance in early summer. Diel vertical migrations become more bioenergetically efficient in August, when warmer temperatures and thermal stratification provide some protection from cold-water predators such as lake trout (Salvelinus namaycush). I also investigated the effects of drought and dam operations (specifically, the installation of a temperature control device (TCD)) on kokanee growth and predation risk from lake trout in BMR. Predation risk was characterized by lake trout reaction distance (a function of turbidity and light level) and probability of occurrence (a function of temperature). I found that predation risk was lowest under full reservoir conditions and highest in drought conditions. Because drought is characterized by high water clarity, and both drought and TCD conditions had cooler metalimnetic temperatures, the volume of foraging habitat available to lake trout was increased. Additionally, environmental changes like climate warming and eutrophication have the potential to alter the degree and timing of DVMs. Increased eutrophication lowered water clarity and decreased visibility at greater depths, so kokanee needed to inhabit shallower water to see zooplankton prey. Consequently, spending more time in warmer surface waters increased bioenergetic expenses which were further amplified by climate warming. In general, I found that neither eutrophication nor climate warming alone affected kokanee growth, but their interactive effects reduced kokanee growth to a great degree. My results suggest that kokanee behavior must be plastic in response to resource availability and predation risk, which will vary with environmental conditions.