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

Abstract

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 mid-August (surface water temperature, 16-23°C; maximum salinity, about 32%. The affected fish appeared to reduce their appetite, and most fish died within one month. The number of parasites was counted 128 and 188 for two rainbow trout of 402 g and 625 g in body weight, respectively. Most copepods including chalimus stage larvae were attached to the body surface and fins (Figs. 1 & 2). The infected fish showed whitish patches on the skin where the copepods attacked (Fig. 2). Kabatal) suggested that caligid copepods feed by pressing their mouth tube on the host's skin and scraping the tissues by a divided dentigerous bar located at the tip of the labium. Our histological observations indicated that skin epidermal tissues were de stroyed by the feeding activity of C. orientalis, and the damage was similar to that caused by salmon lice Lepeophtheirus salmonis2,3). © 1991, The Japanese Society of Fish Pathology. All rights reserved.