A Net-Pen Experiment to Evaluate Kokanee Growth Rates in Autumn in an Oligotrophic Lake with Mysis relicta

Abstract

We used an in situ net-pen experiment to investigate autumn growth rates of kokanee Oncorhynchus nerka in a oligotrophic lake with introduced opossum shrimp Mysis relicta. In the 21-d experiment conducted in October, kokanee (53.9 mm average total length) were placed either in net-pens made of 1.59-mm mesh, allowing drift of ambient zooplankton into the pens, or in pens made of 153-μm mesh containing zooplankton at comparatively high, low, or very low densities relative to that of the ambient zooplankton density pens. Kokanee receiving ambient zooplankton densities grew 0.83%/d, whereas those fed the same diet at high densities grew 1.84%/d, and those fed the low and very low densities grew −0.69% and 0.17%/d, respectively. Final kokanee biomass differed significantly among treatment groups. Large growth differences also existed among treatment groups, but comparisons between the ambient and high treatments or between the ambient and very low treatments were not statistically significant. In samples collected from the lake, the copepods Cyclops bicuspidatus thomasi and Diaptomus ashlandi were the most abundant zooplankters, representing more than 70% of the samples. Kokanee growth in this study was consistent with that in other lakes with limited food resources and showed that growth of wild kokanee would have been greater in October in Lake Pend Oreille had zooplankton been more abundant.