Some aspects of the life of planted masu salmon, Oncorhynchus masou, as determined by acanthocephalan infections
1990
Mayama, H. (Hokkaido Salmon Hatchery, Sapporo (Japan))
Hatchery-produced juvenile (0+) masu salmon, Oncorhynchus masou, were planted in the Mena River, a tributary of the Shiribetsu River, southwestern Hokkaido, in late May of 1984 and 1985, and as biological tags the infections of an echinorhynchid acanthocephalan, Acanthocephalus sp., in the intestines of masu salmon were monitored. In July, there was a marked difference in prevalence of infection in these fish between sampling sites, ranging from 0 to over 95%. This indicates that although planted juveniles disperse soon after the release, they thereafter do not migrate widely in the river in spring and early summer. However, since the prevalence increased at almost all sampling sites in May of the next year, it is suggested that juveniles move to the places where isopods, Asellus hilgendorfi, the intermediate host of the parasite, are abundant in late autumn and overwinter at those sites. The level of infection was dependent on juvenile size and was relatively higher in small-sized fish. This result indicates that juveniles use different feeding areas by size and particularly small-sized fish inhabit slower-current areas near banks. Hatchery-reared juvenile (1+) masu salmon were also planted in the same river in early May of 1984 and were examined for Acanthocephalus sp. infections. The prevalence increased rapidly soon after the release and markedly higher than that in wild fish, indicating that the swimming ability of planted fish is lower than that of wild fish and the former use slower-current areas as habitats, where they thus get infected more frequently. This was confirmed by stomach content analyses of planted and wild fish. There was no significant difference in condition factor between infected and uninfected fish
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