Behavioral thermoregulation by maturing adult sockeye salmon (Oncorhynchus nerka) in a stratified lake prior to spawning

Adult sockeye salmon, Oncorhynchus nerka (Walbaum in Artedi, 1792), return to Lake Washington several months prior to spawning, spending the warmest months of the year in the lake. We proposed that the fish selected a temperature range ideal for final sexual maturation and energy conservation prior to swimming upstream to spawn. The temperature preferences of the adult sockeye salmon in Lake Washington are attributable to physiological factors, as they are not avoiding predators or seeking prey and are not limited by dissolved oxygen. At the Hiram M. Chittenden Locks, 257 sockeye salmon were tagged with temperature loggers in the summer of 2003, and 38 taus with readable data were recovered. The fish spent an average of 6 days swimming through the ship canal's warm water (ca. 18 degrees C) and then experienced a drop to temperatures of 13 degrees C or lower when they entered the lake and descended below the thermocline. Fish remained in the lake for an average of 83 days before migrating upstream to spawn, as indicated by a sudden increase in recorded temperature. Approximately 92% of temperature records in the lake were 9-11 degrees C, corresponding to depths of 18-30 m. The salmon rarely occupied the cooler and warmer waters available to them. Finally, the apparent thermal preference decreased over the summer, perhaps as a function of sexual maturation.