Succeeding the previous report, the settling season of some representative acorn barnacles was studies in relation to the warm water effluent from Takahama Nuclear Power Plants in Uchiura Bay from 1977 to May 1979. With respect to the most dominant species,
Balanus trigonus, the size of specimens settled on the celuloid plate suspended from May to December 1977 was also measured. According to the results of survey, the following points have been elucidated:
1) The longest settling season of
Balanus improvisus or
B. eburneus was obtained in the layers above 3 meters depth at station 1 which was only 500 meters apart from the discharge canal of the nuclear power plant (Figs. 1 and 2).
2) Judging from the diameter composition of the shell base of
B. trigonus, the presence of two populations with different breeding seasons was confirmed. The larger group might breed in spring and the smaller one in autumn. In the case of younger specimens settled at stations 1 and 2 in autumn, the average diameter of the animals in the layers above 2 meters depth was larger than that in the 5 meters layer (Fig. 3).
3) From the facts mentioned above, it will be safe to say that the warm water effluent has an influence on the barnacles, extending their settling season, promoting their reproduction and accelerating their growth.
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