降海さけ稚魚の食害に関する研究-II. : 大槌川における調査1964, 1965

概要

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Succeeding the previous report, the results obtained in the 1964 and 1965 Otsuchi River Investigation to elucidate the predation of chum salmon fry in its river stage are presented. In these two years radioactive pins of 6OCo and 198Au (only in 1965) were inserted into the body cavity of the fry as tags. Subsequent to the release of the fry, intensive radiological surveys were conducted, either in water or on land of the experimental area and its surroundings, to trace the movement of the radioactive tags. Besides, all the descending fry were recovered by traps and this enabled to give an estimate of freshwater mortality with greater accuracy. The results tell that the mortality differs with the size of the fry and that large sized fry is subjected to less predation. Contrary to expectation no new predator species were found. Based on the five years experiences, the conditions to be fulfilled when performing these kinds of utilization of radioactive isotopes were pointed ou.