Adult Bull Trout Exhibit Binge-Feeding Across Temperatures during an Annual Prey Pulse of Juvenile Sockeye Salmon

Understanding the limits of consumption by predators is important for understanding adaptations to variable prey availability and the influences of predation on prey. The annual outmigration of sockeye salmon Oncorhynchus nerka smolts from lakes represents an episodic prey pulse, and predation-induced mortality by bull trout has been hypothesized as an explanation for high rates of loss of smolts from Chilko Lake, British Columbia. Consumption by adult bull trout Salvelinus confluentus was assessed during smolt outmigrations in 2013 and 2014.  Virtually all consumed items were sockeye salmon, and consumption rates generally exceeded daily sustainable maximums (Cmax) calculated with bioenergetics models. In fact, consumption was generally 10-20 times higher than Cmax at low temperatures (3.5° C) and 3-5 times higher at ~8° C at a location where prey availability was extremely high (a counting fence where smolts are funnelled into high densities). The degree of this binge-feeding, or hyperphagia, was still evident upstream of the fence where prey catchability is assumed to be decreased, but stomach fullness was more variable. Our results indicate the ability for predators to maximize consumption during periods of prey availability and that short-term consumption may be less limited by temperature than initially thought.