Behavioural and physiological responses by bull trout to resource pulses associated with sockeye salmon migrations

Animal migrations transport energy and nutrients to recipient communities and can generate infrequent pulses in resource abundance (i.e., resource pulses), which may represent important feeding opportunities for consumers. Thus, consumers may adopt strategies to maximize exploitation of such pulses. I investigated behavioural and physiological responses by bull trout (Salvelinus confluentus) in relation to the migration timing and spatial distribution of sockeye salmon (Oncorhynchus nerka) in the Chilko Lake system, British Columbia. Sampling of bull trout stomach contents demonstrated that short-term consumption of sockeye salmon smolts in the spring and eggs in the fall represented binge-feeding, particularly for bull trout caught near the lake outlet where salmon availability is highest. Further, bull trout exhibited larger digestive organs in the spring and fall compared to the summer, presumably in response to seasonal variation in prey availability associated with salmon migrations. Using an acoustic telemetry positioning system, I tracked bull trout movements during the early migration of salmon smolts and found that variation in bull trout movements was strongly associated with the diel cycle, but not with smolt density. Bull trout were most active at night and occupied areas closer to a fence that funnels smolts into high densities. Thus, bull trout behaviour may correspond to increased effort to locate and consume smolts, which primarily migrate at night. Finally, acoustic telemetry was used to relate bull trout movements throughout Chilko Lake and the Chilko and Chilcotin rivers to the timing and location of sockeye salmon smolt and spawning migrations over four years. Bull trout generally occupied areas closest to the lake outlet during periods of salmon smolt migration and spawning, with some individuals moving >100 km during the summer interpulse period. Furthermore, bull trout that were present in the upper Chilko River during salmon spawning or smolt migration were larger for their age than bull trout that did not move to the lake outlet. Individually, chapters advanced the fields of movement ecology and feeding ecology, while collectively, this thesis improves our understanding of the mechanisms that allow consumers to exploit animal subsidies and the factors that mediate those responses.