Effects of the Establishment of Mysis relicta on the Macrozooplankton of a Large Lake

Abstract The effects of the introduction of opossum shrimp Mysis relicta on the macrozooplankton of Pend Oreille Lake, Idaho were examined from recent and historic zooplankton data. Establishment of M. relicta was associated with a temporal displacement in the seasonal occurrence of Daphnia and Bosmina species and a change in the species composition of Daphnia present. The amount of interaction between cladocerans and mysids appeared to be related to the vertical distribution of each. The cladocerans appeared and increased in abundance as the mysids became seasonally isolated from near-surface strata by increasing thermal stratification and transparency. A common negative response of cladocerans to mysid introduction is supported by the literature. Such introductions may result in the reduction of food available for kokanee Oncorhynchus nerka and other plankton-feeding fish.