Diel and Seasonal Patterns of Horizontal and Vertical Movements of Telemetered Cutthroat Trout in Lake Washington, Washington

Abstract

Salmonid fishes are often the top predators in cool lakes, and their movements may reflect the distribution of available prey as well as physical factors such as light and temperature. We used ultrasonic telemetry to examine the vertical and horizontal movement patterns of cutthroat trout Oncorhynchus clarki in Lake Washington, a monomictic lake in Washington State. We hypothesized that the trout would show seasonal shifts in vertical distribution, balancing avoidance of warm surface waters with the need to forage on vertically migrating limnetic planktivores. During the lake's annual stratification period, the trout avoided the near-surface waters (mean fish depth = 15.7 m); however, the trout were nearer to the surface when the lake was mixed (mean fish depth = 7.5 m), consistent with a thermal constraint. The trout did not show notable diel changes in behavior, either in depth or swimming speed. Rather, they swam about 0.5 body lengths per second in open water during the day and were only somewhat slower at night, with fewer vertical excursions. No home ranges were evident, as trout moved throughout the lake. Foraging in littoral areas occurred primarily in spring and exclusively during daylight hours. The results are consistent with foraging by large cutthroat trout in the limnetic zone on the lake's dominant planktivores, juvenile sockeye salmon O. nerka and longfin smelt Spirinchus thaleichthys. Cutthroat trout behavior contrasted sharply with results for sympatric rainbow trout O. mykiss, which occupied the warmer surface waters and the littoral zone and which showed a more pronounced diurnal activity pattern.