今，アサリが危ない 有明海のアサリの幼生加入過程と漁場形成

Abstract

In 1987, the annual catch yields of the clam Ruditapes philippinarum in Japan reached their peak, ca. 140-160 thousand tons. The yields have drastically decreased since then, down to 25% of the peak after that period. This decrease was mainly due to decrease of the yields from Ariake Sound in Kumamoto Prefecture, southern Japan, which reached a peak (ca. 65 thousand tons) in 1977 and then drastically decreased, down to ca. 1% of the peak recently. In order to reveal the factors causing the catch depletion from 1977 onward in Kumamoto and from 1987 onward in Japan, we have studied patterns of larval recruitment of this species on tidal flats in Kumamoto. Frequent sampling was done for the different life stages of the clam (planktonic larvae, new settlers, juveniles, and adults), and cohort analyses were made using size frequency distributions. Our results indicate that larval supply may play critical role in structuring the age distribution and in either maintaining or inducing fluctuations in the benthic populations of the clam in Ariake Sound. This differs greatly from the our another study on tidal flats in Ise Bay along the Pacific coast of central Japan. Our studies in Ariake Sound strongly suggest that the drastic decrease in yields in the Sound may have been caused by a shortage of larval supply in addition to a decrease in size of the spawning stock and an increase in larval mortality resulting from environmental deterioration.