Juvenile Chinook Salmon (Oncorhynchus tshawytscha) Prey and Prey Resources in Lake Washington: the Effects of Shoreline Modification

Unlike most systems in which juvenile chinook rear in rivers and estuaries, juvenile chinook in Lake Washington rear in littoral areas of the lake from January to early June. These shallow littoral areas are highly urbanized, consisting of docks and bulkheads and accompanied by loss of riparian area. The decline of Puget Sound chinook salmon, and their listing as endangered under the Endangered Species Act, suggests that one potential “limiting factor” on juvenile chinook is the effect of such intensive urbanization on juveniles in the lake. To investigate this, we examined stomach contents of juvenile chinook caught in the littoral zone to determine prey composition, and conducted epibenthic and neustonic sampling in the lake to document prey resources available to juvenile chinook.

Preliminary results indicated that juvenile chinook rearing in the littoral zone of Lake Washington primarily consumed dipterans in the family chironomidae, but that they switched to feeding on the planktonic crustaceans Daphnia spp. in late spring. Other crustaceans and terrestrial insects formed a relatively small proportion of the prey.

From April through June, we conducted pilot prey resource sampling over fine and coarse substrates at three sites corresponding to three levels of human impact in the lake: natural, natural/altered, and residential shorelines. Epibenthic samples indicated large variances in prey taxa densities. Taxa richness (mean number of taxa per sample) was highest at the natural/altered site throughout the sampling. This difference was more pronounced in fine sediments than in coarse sediments.

Neuston (organisms associated with the water’s surface) was also sampled at the three study sites. Densities of neuston prey resources were highly variable among the sites. In general, taxa richness was greater at the natural and natural/altered sites than at the residential site, but the magnitude of this difference varied throughout sampling.

We used the results from this pilot study to re-design our sampling in 2000. In addition to continued sampling of juvenile chinook diet composition, we investigated landscape-scale differences between paired residential and forested riparian sites and commercial and marsh sites at the north end of the lake. Our goal is to use the resulting data to evaluate the effects of major shoreline development patterns on prey assemblages of juvenile chinook salmon in Lake Washington that might contribute to population declines.