Effect of water temperature, timing, physiological condition and lake thermal refugia on success of migrating adult Sockeye salmon

I related survival of adult Weaver Creek sockeye salmon (Oncorhynchus nerka) to water temperature, migration timing, physiological condition, and lake residence in the lower Fraser River and Harrison System, British Columbia. Late-run sockeye were intercepted in the Harrison River after completing the Fraser River component of their spawning migration. Individual sockeye were tagged with radio or acoustic transmitters (equipped with depth sensor), biopsied for physiological assessment and released. Additional fish were sacrificed to increase sample size and to help evaluate baseline physiology. Fish were grouped by river entry timing (early or normal), migration residency (river or lake), and survival to spawning grounds. Of the early-timed fish, the majority (64%) resided in Harrison Lake and only those that resided in Harrison Lake survived to reach spawning grounds, ie none that resided in the Harrison River survived. In contrast, a majority of normal-timed fish (63%) resided in Harrison River and 72% of fish that survived to reach spawning grounds were river residents. Temperatures encountered by migrants during their Fraser River migration were strongly correlated with their subsequent fate to reach spawning grounds (r = -0.9186, P = 0.0275). Lake residents used the entire water column, but stayed in the cold, deep regions> 80% of the time. Estimates of ‘hypothetical’ degree-day (DD) accumulation revealed that early-river fish could have greatly surpassed (~800 °C DD) a critical disease threshold value of ~450 °C DD. There was no difference in hypothetical DD accumulation between normal-timed river fish, the most successful behaviour, and early-timed lake fish. According to calculations of ‘actual’ DD accumulation, based on knowing the precise fate and temperature exposure of a sub-sample (n = 17), all fish that survived accumulated ≤450 °C DD. Early-timed Weaver Creek sockeye had elevated levels of physiological stress (e.g. plasma lactate, glucose and hematocrit), which was likely related to higher encountered temperatures and may have contributed to high levels of mortality in these fish. This thesis illustrates the influence of temperature on migratory success and the importance of thermal refugia for early migrants and provides insight into future challenges for a vulnerable species.Forestry, Faculty ofGraduat