Seasonal changes in the infection of Acanthocephalus sp. (Acanthocephala) in juvenile masu salmon Oncorhynchus masou.

概要

論文の詳細を見る
The seasonal occurrence of an echinorhynchid acanthocephalan Acanthocephalus sp. in stocked juvenile masu salmon Oncorhynchus masou was examined in the Mena River, southwestern Hok-kaido. The prevalence of infection increased greatly soon after fish were stocked in May but de-creased sharply in summer and early autumn and increased again in late autumn and winter. The mean intensity of infection exhibited more typically seasonal changes than did the prevalence and highly increased in late autumn and winter. Gravid female worms first appeared in June and were the largest proportion between July and September with a peak in August. They decreased in autumn and winter, but immature females increased during these seasons. Isopods Asellus hilge ?? dorfi were infected with Acanthocephalus sp. larvae, and large-sized isopods (〉4mm) which had higher parasite prevalences than small-sized isopods, were heavily preyed on by juvenile masu salmon during October and January. Thus, the sharp increase in the infection level during late autumn and spring is in accordance with masu's heavy feeding on large-sized isopods in these seasons. The habitat changes of masu salmon from riffles to pools in winter appear to promote the masu's heavy predation on pool-dwelling isopods.