The Influence of Thermal Stratification on the Vertical Movements of Kokanee and Their Predators in Lake Sammamish, Washington

Movement and distribution of fishes are strongly influenced by environmental conditions, ecological constraints, and physical habitat characteristics. We examined the seasonal and diel distribution of kokanee salmon and their potential predators in response to thermal stratification by combining concurrent limnological measurements with acoustic telemetry. Rapid warming of the surface waters caused the lake to stratify in July. Stratification intensified through summer and fall with increasing temperatures in a deepening epilimnion. As the summer progressed, low hypolimnetic dissolved oxygen encroached into the metalimnion, creating a temperature-dissolved oxygen squeeze that reduced the amount of favorable habitat available for salmonids (<17 °C and > 4 mg/L) by as much as 90%. Kokanee responded to changing limnological conditions by moving deeper (16-18 m) within the metalimnion, segregating them from high epilimnetic densities of Daphnia during peak stratification in August-September. Cutthroat trout were also located within the metalimnion during stratification, but were concentrated in more shallow, warmer water (12 m).  Northern pikeminnow moved more freely during thermal stratification (0-18 m) but were rarely found at the same depth as kokanee. As thermal stratification relaxed in November, kokanee and cutthroat trout redistributed throughout the water column.