Environmental Constraints on Piscivory: Insights from Linking Ultrasonic Telemetry to a Visual Foraging Model for Cutthroat Trout

Management of pelagic food webs under a shifting climate requires an understanding of how behavior, physiological tolerance, and the environment interact to mediate the foraging rates of consumers. However, analyses that treat each of these factors explicitly in a framework that captures interdependencies are lacking. We linked a visual foraging model and bioenergetics simulations with ultrasonic telemetry to mechanistically evaluate how stressful abiotic conditions (high temperature and low dissolved oxygen) influenced the foraging success of piscivorous Cutthroat Trout Oncorhynchus clarkii feeding on juvenile salmonids in Strawberry Reservoir, Utah. Our primary objectives were to (1) determine whether the foraging success of apex predators changes during periods of environmental stress and (2) identify the behavioral mechanisms that either lead to or buffer against shifts in foraging success. During early and mid-August, high epilimnetic temperatures and low hypolimnetic oxygen levels generated divergent diel vertical distributions between predators and prey. Consequently, encounters with prey were restricted to crepuscular or early morning periods, and the potential foraging success of the piscivores was reduced by 53-98%. Conversely, predator-prey overlap increased considerably when the reservoir was destratified during October, allowing the piscivores to achieve up to 98% of their maximum predation rate. Comparison of encounter and predation rate estimates from the visual foraging model with estimates of fish consumption from a bioenergetics model indicated that prey capture success was higher for piscivores during low-light periods. Therefore, as periods of stress impose constraints on the distribution of pelagic fishes, the magnitude of piscivory will depend on the resulting temporal-spatial overlap of predators and prey in complex ways. This study demonstrates how dynamic environmental conditions can mediate foraging success of piscivores and predation risk for prey. Received January 22, 2012; accepted September 17, 2012