Heavy Infections of Caligus orientalis (Copepoda : Caligidae) on Caged Rainbow Trout Oncorhynchus mykiss in Brackish Water

During the summer of 1988, heavy infections of the caligid copepod Caligus orientalis Gussev, 1951 occurred among pen-cultured rainbow trout (Oncorhynchus mykiss) in Lake Mokoto on the coast of the Okhotsk Sea, northeast Hokkaido. About 320 fish (mean body weight 597g) were held in a net pen (2.7•~1.8•~2.0m) when the parasite infection occurred in midAugust (surface water temperature, 16-23•Ž; maximum salinity, about 32%. The affected fish appeared to reduce their appetite, and most fish died within one month. The number of