Co60腹腔内標識によるニジマス稚魚の天敵の追跡

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At about noon of May 1st, 1960, 191 individuals of rain-bow trout fry, in which 39 fish had a Co60 pin radiation source (about 100μc) inserted in the abdominal cavity of each, were liberated close to the midpoint of the same stream that was used in the last Zn65 injection experiment (Fig. 1). Since that afternoon, the vicinity around the stream was monitored by Crystal Scintillation Survey Meter (11/2" NaI Crystal) and by G-M Survey Meter at least once a day, and the locations where the radiation sources were detected, were recorded on the map. (Fig. 1) The recovery of these Co60 marked fry and the Co60 pins outside the liberated fry was done first from 9th to 11th of May and secondly from 26th to 29th of July. By these efforts, 36 pieces of Co60 pins were recovered in total. 1) 21 individuals of living fry carrying the Co60 pins inside their body (F in Fig. 1), 2) 10 pieces of the pins found on the bottom sand of the stream (S), 3) 5 pieces of the pins found in the holes under rocks (R). About the 10 pieces of Co60 pins found on the bottom sand, it was thought to be caused by the predation and the excretion of adult trouts. If it was really so, the adult trouts had taken about 1/4 of the liberated fry within 8 days. About the 5 pieces of the pins found in the holes under huge rocks, we could not think of any cause other than the result of the predation and the excretion of Japanese water shrew, Chimarrogale platycephala, nested in the experimental area (Fig. 2 and 3). The quantity of predation during 8 days by this mammal was estimated as about 1/8 of total fry liberated. However, the predation by a wild bird, Cinclus pallasii, the most powerful predator in the last years experiment, was not recognised, though a couple of this birds with two youngs were very often found flying in the experimental area. One of the reason why the bird did not predate the fry, was thought to be that the size of fry liberated in this year (average 8 gr.) was much larger than that of last year (average 1.2 gr. for small sized group and 2.7 gr. for large sized group). Moreover, it might be caused by that the distance from the point of liberation to the nest of this bird in this year was farther than that of last year. The nest was found just above the point of liberation in last year, but was not found in and near the experimental area this year.
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Co60腹腔内標識によるニジマス稚魚の天敵の追跡

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