Linking the individual-level foraging interactions of piscivores to food-web dynamics in pelagic systems

Abstract

Aquatic ecosystems are structured by environmental gradients, including temperature, depth, and water clarity. These gradients can mediate the strength of trophic interactions by influencing the distribution, physiology, and behavior of predators and prey. My dissertation addressed the following questions: 1) What factors determine the foraging success of pelagic piscivores during encounters with prey? and 2) How do temperature and depth mediate direct and indirect trophic interactions in lakes? First, I conducted feeding experiments to quantify the effects of turbidity and prey shoaling behavior on the foraging efficiency of Chinook salmon during encounters with Pacific herring. Chinook salmon successfully consumed herring during only 1-4% of encounters, limited by the rates of attacks per encounter and capture success (prey consumed per attack). Capture success declined with increasing turbidity and prey shoaling. These results indicate post-encounter processes can strongly limit feeding rates of pelagic piscivores and they provide the necessary parameters to incorporate these processes into foraging models. Second, I conducted field sampling in Lake Chelan, Washington to quantify how the strengths of trophic interactions between zooplankton, mysid shrimp, kokanee, and lake trout changed along environmental gradients. Bioenergetics and population models revealed strong predation impacts of lake trout on kokanee, which were initially masked from detection by ecological time lags. Mysids influenced kokanee through two negative indirect interactions, which differed in strength between contrasting lake basins. Mysids competed for zooplankton prey more strongly in a deeper, cooler basin due to their low thermal optimum. However, mysids provided greater energetic support to lake trout diet in a shallower basin, where lake trout were greater in density and inflicted greater predation risk on kokanee. A diel vertical migration model predicted mysids were more vulnerable to lake trout predation at shallower sites within the lake, and this prediction was supported by stable isotope analysis of lake trout diets. These findings revealed a mechanism by which mysids could cause greater food-web impacts in shallower systems where they are more vulnerable to predation. In conclusion, my results show strong associations between the physical environment, the behaviors of individual predators and prey, and the dynamics of populations and food webs.