Tetraonchus Infection of Masu Salmon, Oncorhynchus masou

Abstract

Tetraonchus infection of masu salmon, Oncorhynchus masou, was monitored monthly in the culture system at Okutama Branch, Tokyo Metropolitan Fisheries Experimental Station over a period of January 1978 to September 1979 from fry to adult of a single host fish stock artificially spawned there. Two species (T. awakurai and T. oncorhynchi) were always found from the host. It is assumed that the oncomiracidial invasion occurred from spring to autumn, when the water temperature was above 10°C. In T. awakurai infection of fry, both the prevalence and intensity of infection sharply increased in spring, and all the fish were infected in June to August. From September to April, the prevalence of infection stayed about 50% with decreasing mean intensity. It is probable that the lowered infection level in autumn and winter is attributed to the elimination of the parasite by the host. The effect of the elimination was different in degrees among the host fish, leading to over-dispersed distribution of the parasite within the host population. A new infection started in April next spring, but the prevalence of infection did not reach 100 %. The course of infection with T. oncorhynchi was much the same as that of the former species, although the infection level was usually much lower. No relationship was evident between the two tetraonchid species. The infection levels of the two parasites differed greatly from year to year. A possible method for parasite control is also discussed.