Abstract
Ecological factors influencing the infection levels of salmonids by Acanthocephalus opsariichthydis in Lake Yunoko, Japan, were analysed. The most important factor is the feeding habits of the definitive host. Brook trout, Salvelinus fontinalis, were heavily infected with this parasite as they fed on the intermediate host, Asellus hilgendorfi. By contrast, Biwa salmon, Oncorhynchus rhodurus, and kokanee, O. nerka, preyed mainly on fish and plankton, respectively, and hence, they were infected to a lesser extent with this parasite. The overlapping habitats of the definitive and intermediate hosts were noted. These are the factors accelerating greater contact between both the hosts. Most brook trout inhabited the littoral zone, to which A. hilgendorfi were restricted. However, Biwa salmon and kokanee were widely dispersed in the lake. A close distributional relationship between plant materials and A. hilgendorfi was also suggested.
