Heat transfer, thermal preference, and behavioural thermoregulation of adult female Babine Lake Sockeye Salmon

During their upriver migration to spawning ground, many adult Sockeye Salmon (Oncorhynchus nerka) pass through lakes that provide wide ranges of water temperatures and opportunities to behaviourally thermoregulate. There is still uncertainty regarding if and how effectively Sockeye Salmon behaviourally thermoregulate in lakes, what factors might drive the thermoregulation, and how thermoregulatory behaviour impacts fitness. The purpose of this thesis was to study the thermal ecology of adult Sockeye Salmon by estimating their thermal preference and how effectively they behaviourally thermoregulated while migrating through Babine Lake where up to approximately 90% of Sockeye Salmon returning to the Skeena River spawn. Additionally, I investigated what parameters impact the thermal preferences and thermoregulatory behaviour of the salmon, and if more effective behavioural thermoregulators had greater spawning success. External attached temperature logger tags monitored the thermal experience of Sockeye Salmon in Babine Lake, and shuttle box tests were conducted to estimate the thermal preference ranges of the salmon as well as to estimate the coefficient of heat exchange (k) from their body surface to body core. The k coefficient was used in conjunction with the external temperature logger data to estimate body temperatures of the salmon in Babine Lake, and the body temperatures, thermal preference estimates, and lake water temperature data were incorporated to estimate how effectively the tagged Sockeye Salmon behaviourally thermoregulated while passing through Babine Lake. Chapter One of this thesis focuses on estimating k for adult Sockeye Salmon and applying these estimates to predict the body temperatures of the externally tagged Sockeye Salmon in Babine Lake. Chapter Two focuses more so on behaviour, including estimates of the thermal preference and effectiveness of behavioural thermoregulation of Babine Lake Sockeye Salmon. The median estimated k of the salmon was 0.08 min-1 and was slightly lower for fish with higher condition factor (Fulton’s), when the body was cooling, and when water temperatures were closer to the body temperature. Metabolic heat production (Tm) appeared to have a small but notable impact on body temperature. The median model-averaged fitted thermal preference range (interquartile) of the shuttle box fish was 11.84°C to 12.32°C, and the median predicted thermal preference range of the radio tagged fish was 13.39°C to 13.89°C. The salmon appeared to behaviourally thermoregulate quite effectively in Babine Lake, although there was a lot of individual variation. There was high uncertainty for relationships between the thermal preference of the salmon or the effectiveness of their thermoregulatory behaviour in Babine Lake and their relative infectious burden, mass, and gross somatic energy density, or the time of day (day/night). Salmon with higher heat stress scores tended to prefer warmer temperatures, presumably due to an association between acclimation temperatures and temperature selection. Most of the salmon had high spawning success, with more effective thermoregulators tending to have had greater spawning success. Babine Lake provides important thermal habitat to adult Sockeye Salmon, and this habitat and effective behavioural thermoregulation could become imperative as North American rivers and lakes continue to warm with climate change.