Effect of water temperature, timing, physiological condition, and lake thermal refugia on migrating adult Weaver Creek sockeye salmon (Oncorhynchus nerka)

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Abstract
We coupled physiological biopsy and positional telemetry to examine survival to reach spawning grounds in relation to water temperature, timing, physiological condition, and holding location (river or lake) in adult migrating sockeye salmon (Oncorhynchus nerka). We tracked 83 fish across a large temperature range (13.5–21.5 °C), which included record highs. Only early-timed migrants that held in Harrison Lake survived to reach spawning grounds (16%, or n = 4). Normal-timed fish, those that migrated at historically observed times, survived at higher levels if they held in Harrison River (72%, or n = 18). Mortalities were identified on the bottoms of both the lake and river. Hypothetical degree-day (DD) accumulation revealed that early-timed river fish would have greatly surpassed (~800 °C DD) a critical disease threshold value (~500 °C DD). There was no difference in hypothetical DD accumulation between normal-timed river fish and early-timed lake fish. Early-timed sockeye had elevated physiological stress (e.g., plasma lactate, glucose, and hematocrit), which may have contributed to high levels of mortality. By using lakes as thermal refugia, early-timed fish likely reduce rates of disease development and may better recover from physiological stress associated with high encountered temperatures.

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