Foraging of Cutthroat Trout Tracked with Ultrasonic Telemetry in Relation to Shifts in Reservoir Stratification

In pelagic environments, the potential foraging success of visually feeding piscivores is determined by the intersection of overlapping diel-vertical distributions of predators and prey, and the effects of ambient visual conditions on the predation sequence.  Movements of piscivores can be influenced by temporally dynamic factors that affect both visibility (i.e., light and turbidity) and other physiological processes (e.g., temperature and oxygen).  We hypothesized that spatial-temporal overlap with prey and foraging success would change for piscivores among periods of differing environmental stress.  We addressed this question by combining ultrasonic tracking data on the diel-vertical movement of piscivorous cutthroat trout Oncorhynchus clarki and available information on prey distribution with a visual foraging model to estimate prey encounter rates and predation rates by applying light-dependent capture probabilities in relation to three distinct limnological periods observed during summer and autumn 1997 in Strawberry Reservoir, Utah.  While vertical movements of cutthroat trout were mediated by intense thermal stratification and a hypoxic hypolimnion in early and mid-August, prey fish were only available during short crepuscular or early morning periods.  The visual foraging model indicated that cutthroat trout only achieved 2-6% of their maximum light-adjusted predation rate when confined within the metalimnion during early August and 35-47% during mid-August when the metalimnion became hypoxic and piscivores moved into the epilimnion.  During destratification in October, temperature and oxygen exerted little influence on the vertical movement of tracked cutthroat trout.  During this period, cutthroat trout more consistently maintained overlap with prey (achieving 41-97% of their maximum light-adjusted predation rate), increasing opportunities for piscivory.  Comparison to independently derived fish consumption rates suggested that the timing of and thus the ambient visual conditions during prey encounters affects the capture efficiency of visually feeding piscivores, and could be related to shifts in fish behavior as optical conditions change along a diel sequence.