ECOLOGICAL RELATIONSHIPS OF HATCHERY RAINBOW TROUT IN LAKE WASHINGTON (FEEDING, GROWTH, MORTALITY, PREDATOR-PREY)

Rainbow trout stocked as fingerlings occupied a relatively broad niche in Lake Washington, overlapping the feeding guild of facultative planktivores and piscivores. Trout released in late May or June were planktivorous upon entering the lake, feeding heavily on Daphnia, but became mostly piscivorous by mid- to late fall at sizes (GREATERTHEQ)250 mm. Prickly sculpin entered the diet briefly in the fall and represented only a minor portion of the diet while longfin smelt constituted the largest fraction of the fish species eaten. Nearshore, yearling and adult longfin smelt comprised the major fraction of the diet of larger ((GREATERTHEQ)250 mm) rainbow trout during the winter and offshore during the spring, summer and fall. Nearshore, prickly sculpin, yellow perch, and cyprinids comprised the majority of trout diet during the spring. Rainbow trout occupied the nearshore zone from December through late April or early May. Proportionally more of the larger than the smaller trout moved offshore into the epipelagic zone (0-20 m) from late spring through the fall while the majority of the smaller trout remained in the nearshore zone year-round. The inshore-offshore movements and the longitudinal distribution of the rainbow trout were associated with the seasonal and annual abundance and distribution of longfin smelt. The trout selected longfin smelt as prey over juvenile sockeye salmon thus, the smelt act as a buffer to pelagic predation on the sockeye salmon by the rainbow trout . The heaviest predation by the hatchery origin rainbow trout on the sockeye salmon occurred off the mouth of the Cedar River as the salmon fry entered the lake. Rainbow trout predation accounted for an estimated loss of 81,000 sockeye smolt equivalents in 1984 (1983 brood year) and 184,000 smolt equivalents in 1985 (1984 brood year). This corresponded to a 7% and a 2% loss in sockeye smolt production in 1984 and 1985, respectively.