Behavioral thermoregulation by adult Chinook salmon (Oncorhynchus tshawytscha) in estuary and freshwater habitats prior to spawning

The movements of maturing Pacific salmon (Oncorhynchus spp.) from marine waters into rivers pose challenges for their physiology, especially in basins affected by human structures and increasing temperatures. This study determined the thermal regimes experienced by maturing Chinook salmon (O. tshawytscha) entering the Lake Washington basin in western Washington through navigational locks and a ship canal and migrating through the lake to spawning grounds. Chinook salmon entered in mid-summer, when temperatures were warmest, but often moved back into the cool, marine waters of Puget Sound before travelling quickly through the ship canal and into Lake Washington. In Lake Washington, they swam above and below the thermocline and made little use of the available thermal refuge in deep water before ascending rivers to spawn. The migration timing and tactics of Chinook salmon indicate a greater tolerance for warm water than that of sympatric sockeye salmon (O. nerka, documented in an independent study), but in each species the ability to exploit thermal refuges may be essential for their survival in the face of the increasing temperatures seen in this basin in the past decades and those that are likely to continue rising in the future.