Salmonids often display a series of ontogenetic shifts in habitat, and these may also be associated with changes in diet. For example, adfluvial populations rear in streams for several years and then migrate to lakes. The patterns of habitat use, trophic ecology, and movements of such populations are commonly studied during the riverine stages. The lacustrine period is typically less well known, but salmonids may play an important ecological role as lake piscivores. In Lake Washington, Seattle, Washington, cutthroat trout Oncorhynchus clarki are a top native piscivore and may affect the dynamics of the fish and zooplankton upon which they prey. Our objective was to study the growth, diet, and size distribution of cutthroat trout in littoral and limnetic habitats of Lake Washington, with emphasis on consumption of two of the lake's dominant pelagic planktivores: juvenile sockeye salmon O. nerka and longfin smelt Spirinchus thaleichthys. Cutthroat trout entered the lake at approximately age 2. As they grew larger, the cutthroat trout became increasingly piscivorous and tended to occupy the limnetic zone after they reached about 250 mm fork length (FL). Specifically, percentages (by wet weight) of fish in the diet of cutthroat trout increased from 22.5% for cutthroat trout smaller than 200 mm FL to over 95% for cutthroat trout larger than 400 mm FL. Fish made up a higher percentage of cutthroat trout diets in fall and winter in both the limnetic and littoral zones, and a greater proportion of fish was consumed in the limnetic zone than in the littoral zone. Variation in diet was observed among years (1995–2000), apparently reflecting the relative abundance of longfin smelt. The role of cutthroat trout both as the object of recreational fisheries and as a predator on sockeye salmon (valued in commercial and recreational fisheries) complicates management of this large, urban lake.
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.
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