Diet and Bioenergetics of Lake-Rearing Juvenile Chinook Salmon in Lake Washington

Abstract

Abstract.—Use of lake habitats by ocean-type Chinook salmon Oncorhynchus tshawytscha is rare under natural conditions. We studied aspects of the trophic ecology of naturally and hatchery-produced juvenile Chinook salmon rearing in the littoral zone of highly urbanized Lake Washington in Washington State. During February through May, naturally produced juvenile Chinook salmon occupied littoral habitats and consumed mostly epibenthic prey, primarily chironomid pupae (Diptera). In June, they switched to a diet dominated by plankton, specifically Daphnia spp. This diet shift from littoral prey to limnetic prey coincided with increasing temperature, a shift by the fish from littoral to limnetic habitats, the spring bloom of Daphnia, and increasing fish size. Bioenergetics modeling for these populations estimated that naturally produced juvenile Chinook salmon had high consumption rates and were generally feeding close to their maximum ration, even after large numbers of hatchery-produced Chinook salmon entered the lake. The feeding rates, growth rates, and proportions of maximum daily ration from the modeling suggested that under current conditions, both naturally produced and hatchery-produced juvenile Chinook salmon were finding ample food in littoral habitats of Lake Washington. These results further reveal that hatchery-produced Chinook salmon did not compete with naturally produced fish and that this was probably a result of hatchery juveniles enterin