Zooplankton Community Changes at Pend Oreille Lake, Idaho: Testing Implications for Age-0 Kokanee Prey Selection, Digestion, and Growth

Abstract

We used laboratory and field experiments to investigate how spring and autumn zooplankton community changes at Pend Oreille Lake, Idaho, characterized by reduced cladoceran abundance, have affected the prey selection, growth, and digestion of age-0 kokanee Oncorhynchus nerka. In prey selection trials using newly emerged kokanee alevins (mean total length (TL) = 25 mm) with varied levels of stomach fullness and that were fed different ratios of cladocerans to copepods, cladocerans were selected except when their relative abundance was the lowest. Alevins ate more zooplankton when cladocerans were relatively more abundant and when their stomachs were empty, but stomach fullness did not influence the prey type selected. Alevins ingested prey that were larger than available prey at each level of stomach fullness, the greatest size selectivity occurring on empty stomachs. Underyearling kokanee (48–57 mm TL) that were fed cladoceran zooplankton ingested more biomass in 1 h than did those fed copepod zooplankton and digested their meal more quickly over time, though the differences were not statistically significant. Kokanee growth was not associated with prey selection, as a cladoceran diet did not produce the greater growth either for alevin kokanee in spring or for underyearlings in autumn (mean TL = 66 mm). Our work shows that changes in zooplankton species composition, relative densities, and size structure may interact to influence age-0 kokanee feeding both directly (by reducing prey numbers and biomass ingested) and indirectly (due to potentially important qualitative differences such as prey digestibility) in ways that were not apparent to us from earlier published Pend Oreille Lake studies, in which we hypothesized that total zooplankton densities would be most important for kokanee survival and growth.