Bull trout ( Salvelinus confluentus ) exhibit binge‐feeding and digestive flexibility during seasonal resource pulses associated with salmon migrations

Abstract

Resource pulses are infrequent, ephemeral events of resource hyperabundance that can represent important feeding opportunities for consumers. To capitalize on pulsed resources, consumers can exhibit behavioural and physiological traits including binge‐feeding and phenotypic plasticity of digestive physiology, although expression of these traits has not been observed simultaneously. Further, past studies of binge‐feeding have largely focused on times and locations where resources were highly concentrated, ignoring potential temporal and spatial variation in consumer responses. We investigated these traits in bull trout Salvelinus confluentus that experience seasonal resource pulses associated with the spring migration of sockeye salmon Oncorhynchus nerka smolts and their fall spawning migrations in a large lake–river system. We also examined spatial variation in S. confluentus diet and feeding behaviour within seasons to explore associations with proximity to aggregated salmon. To do this, we collected S. confluentus stomach contents and analysed consumption rates across seasons and capture locations. We also investigated if the size of digestive organs changed with season. In the spring and fall, S. confluentus consumed O. nerka smolts and eggs, respectively, at high rates, with consumption often exceeding theoretical daily maximums by up to 21.50‐fold in the spring and 7.69‐fold in the fall. The degree of binge‐feeding was correlated with proximity to the lake outlet where smolts and spawning salmon congregate, increasing by 1.21‐fold in the spring and 2.78‐fold in the late fall for each ~13‐km shift in capture location towards the outlet. Salvelinus confluentus also exhibited larger digestive organs during the spring and fall, while the same organs were atrophied during the summer. Our results indicate that a single consumer population can exhibit both behavioural and physiological responses to resource pulses, and that exploitation can vary along a spatial gradient of presumed resource availability. These responses emphasize the importance of resource pulses to consumers and the potential for intra‐population differences in consumer responses to transient feeding opportunities.

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