Abstract
Investigations for a continued period of five years were carried out at the Otsuchi River (Iwate Pref.) in order to know the freshwater mortality of released chum salmon fry during its seaward migration and the influence of predation upon it. The results of the initial three years, 1961-1963, are given in this report. Hatchery reared chum salmon fry were labelled with orally administered 65Zn (1961 and 1962) or 32P and released. Following the release the transfer of the radioactivity administered to the fry was traced by radiological surveys of fishes in the river and of birds and rats and moles in the surroundings. This survey, together with other investigations such as stomach contents examination, revealed the occurrence of predation of salmon fry by fish, mainly by Chaenogobius urotaenia, and by birds. However quantitative predation was not attained.
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