Linking High and Variable Ocean Temperature Exposure to Coastal Migration Behaviour and Bioenergetics During Homeward Migration of Adult Sockeye Salmon

For the first time we present marine temperature data recovered from approximately 80 individual sockeye salmon that were tagged with thermal data loggers in the Johnstone Strait, British Columbia in 2006 and 2010.  In both years, thermal data loggers were attached to radio tags that registered on receivers as fish migrated into the Fraser River and towards natal spawning grounds.  Individual fish thermal experience was paired with fish movement and location using fixed telemetry receivers. Our data suggests that there is a large amount of individual variation in thermal behaviour of fish migrating through the Strait of Georgia, and that sockeye salmon experience variable ocean temperatures (10-18˚C).  However, salmon exhibit a preference for a narrow range of temperature that corresponds to temperature optimum for maintaining aerobic function. Additionally, salmon are exposed to temperatures above thermal optimum during coastal migration (≈18˚C) that are as high as what they are exposed to during in-river migration.  In order to examine the costs associated with the different observed behaviours, we used bioenergetic models to compare three groups of fish: 1) those that experienced variable ocean temperatures including extreme highs and low values; 2) fish that experience constant temperatures around predicted optimums; and 3) fish that experience variable temperatures but not extreme values. Our results indicate that there is a modest increase in energetic costs associated with fish behaviour that exposes them to variable temperatures in marine environments.  Although our results demonstrate that variability in thermal behaviour between individuals exists, we still do not understand the mechanisms behind different thermal behaviours.  Individual physiology such as stress levels, osmotic state and reproductive hormone concentrations could potentially contribute to observed behaviours.  Also salmon might move to different thermal habitats to aid in navigation/orientation or to avoid predation. Finally, these results are important for understanding how future increases in temperature in the Strait of Georgia could impact reproductive fitness of migrating sockeye salmon.