放射性同位元素によるニジマスの天敵の迫跡

Abstract

Onodera and others1)-4) have investigated the reduction of number of trout fry after, liberation since 1957, but the cause of the reduction was not known yet. In 1959's investigation, HIYAMA and others joined to assure the role of predation by using radio-isotope. Among 2034 individuals of rain-bow trout fry liberated (small size group 1.2gr., large size group 2.7gr. average), just a half of them was injected at back muscle by 85ZnCl2 solution (1 μc/ 0.03cc per individual). No mortality rate, nor growth was affected by treatment during 2-3 month, while radio-activity remains distinctly above background, by external survey by G-M-survey meter. Water, soil, plants and animals in the vicinity were surveyed frequently after the liberation, but no radio-activity was found except adult trout survived since last year and the excrete of a bird (Cinclus pallasii). Feeding adult trout by radio-active trout fry powder in aquarium, ratio of up-take (fl=0.24) was measured. Getting Zn65 amount in adult trout body caught in the stream (x cpm) and that in fry at that time (ai), the number of trout fry taken by an adult trout (y) can be obtained by the formula; y=x/f1×2/ai. Substituting pigeon for Cinclus pallasii, the excretion ratio (amount of Zn65 in excrete in a day/amount of Zn65 in trout fry fed in a day), f2=0.84, was obtained. By getting the average amount of Zn65 in an excrete of a bird (-x=100 cpm), and getting information on frequency of excrete (N) (126 times a day by a couple of bird) from Mr. Tadakazu ISHIHARA, the number of trout fry taken by a couple of birds in a day (y) can be obtained by the formula; y=N-x/f2×2/ai=20. By these informations, at the day 57 days after liberation when the last adult trout was taken (11 ind. altogether), it was enumerated that 33.5 individuals of fry were taken by adult trout, and 1140 by a couple of birds nested near the liberation point. On the other hand, ONODERA and others enumerated that 782 ind. of trout fry survived at that day by test fishing. So, the reduction of trout fry since liberation was 2034-782=1252 individuals. Among them 94% was caused by predation by eleven adult trouts and a couple of birds, and 97% of predation was by the latter.