今，アサリが危ない 千葉県のアサリ漁業の現状

Abstract

The annual fisheries production of the manila clam Ruditapes philippinarum in Chiba Prefecture markedly declined to below 20, 000 metric tons (mt) in 1979, after reached 70, 000 mt at its maximum in the late 1960s. The decline was mainly due to the large-scale coastal development, e. g. the reclamation of the shallow tidal areas where the clam fisheries were intensively operated. However, clam fisheries production gradually decreased even after the cessation of a series of reclamations in 1979. In the Kisarazu area, clam production has been maintained between 5, 000 and 6, 000 mt since the late 1970s in spite of a de rease in the occurrence of wild clam seed. This is most probably due to 2, 000 to 3, 000 mt of seed clam transplanted from other areas and/or prefectures. On the contrary, in the northern Chiba area, where clam production relies only upon wild clam stocks without any transplantation, the clam population has been frequently affected by “Aoshio”, a bacterial bloom caused by upwelling of oxygen-deficient seawater, as well as by river floods of muddy freshwater and winter mortality. Despite these problems, the clam fishery in the northern region has continued during recent decades, because the clam stock has recovered rapidly after mass mortality in many cases. However, clam production sharply decreased from ca. 10, 000 mt in the late 1970s to 800 mt in 1999 due to the poor occurrence of wild juvenile clams. Although the causes of accidental mass mortality have been almost fully revealed, the reason for the decrease in wild juvenile clams is still not known.