Why infestation by Lepeophtheirus salmonis (Copepoda : Caligidae) is not aproblem in the coho salmon farming industry in Japan

Coho salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss) are commercially cultured in net-pens in Onmae Bay, Onagawa, Miyagi Prefect ure, Japan, for eight months from November to the following July. Eight mon thly samples of five rainbow trout and three bimonthly samples of five coho salmon were taken from two adjacent net-pens in this bay to study the infe station patterns of salmon louse (Lepeophtheirus salmonis). The results ind icate that (1) rainbow trout was highly susceptible to salmon louse, (2) co ho salmon harbored only the adult and preadult stages of the salmon louse, (3) the parasite reproduced twice on rainbow trout during the study period, and (4) the prevalence and mean intensity of infestation increased steadil y into the harvest time (in July) on both species of salmonids. It is concl uded that, aside from the resistance of coho salmon to infestation by salmo n louse, the farmers' practice of rearing only the young fish and harvestin g the fish in less than a year of culture accounts for the mitigation of sa lmon louse problem in Japan.