Newly Emerged Kokanee Growth and Survival in an Oligotrophic Lake with Mysis relicta

Abstract

We used an in situ net-pen experiment and a feeding analysis of wild fry to investigate the survival and growth of newly emerged kokanee Oncorhynchus nerka in an oligotrophic lake containing an introduced opossum shrimp Mysis relicta. In the 21-d experiment, kokanee fry (average total length, 25.1 mm) were placed either in net-pens made of 1.59-mm mesh, allowing ambient zooplankton drift into the pens, or in pens made of 153-μm mesh that contained comparatively high, low, or very low zooplankton densities relative to net-pens with ambient zooplankton density. No deaths attributed to starvation occurred during the experiment. Kokanee in the ambient zooplankton (mostly copepods) densities grew 5.7%/d, whereas those fed the same diet at the lowest densities grew 2.42%/d and those fed high densities grew 7.05%/d. Final kokanee biomass was significantly different among treatment groups but not between the ambient and high treatments or the low and very low treatments. The copepod Cyclops bicuspidatus thomasi was the most abundant zooplankter in samples collected from the lake representing 91% of the prey items in the stomachs of wild fry. Cladocerans were present during sampling but did not appear in kokanee stomachs until the third sampling week. Our results suggest that newly emerged kokanee can survive and grow on a diet composed mostly of copepods, even when zooplankton densities are lower than those typically found in lakes after M. relicta become established.