Cyclical prey abundance drives interannual variation in predator diel vertical movement

Abstract

Diel vertical movement (DVM) is a widespread behavior in aquatic ecosystems, occurring across a variety of taxa and water bodies. The factors hypothesized to drive DVM can vary tremendously through time, yet little is known about how DVM changes at interannual timescales. Here we explore how cyclical prey abundance affects predator DVM. Higher consumption levels increase the optimal temperatures for growth in fishes. Thus, annual variation in prey abundance should generate corresponding variation in the depths and temperatures selected during predator DVM. In Crater Lake, one of the deepest and most oligotrophic lakes in the world, Daphnia zooplankton exhibit cyclical patterns of abundance. We compiled data spanning four distinct pulses of Daphnia and analyzed the response of their predator, kokanee salmon (Oncorhynchus nerka). Our data spanned 36 yr for Daphnia abundance and kokanee body condition, and 24 yr for kokanee DVM (measured by hydroacoustic surveys). Kokanee exhibited four pulses in body weight and condition that corresponded to the four Daphnia pulses, suggesting a strong bottom‐up response. Kokanee altered their DVM in years with Daphnia by occurring deeper during the day, where Daphnia were concentrated, and shallower at night, where temperatures were > 5°C warmer. By selecting warmer habitat in years with Daphnia, kokanee increased their estimated overnight digestion by ~ 25%. Understanding how predators alter DVM and other patterns of cyclical habitat use in response to variation in prey abundance has important implications for understanding predator–prey dynamics, which are highly sensitive to prey encounter rates and maximum consumption rates.