2021 年 24 巻 p. 12-25
In Isahaya Bay, Kyushu, Japan, one of the edible ray species, red stingray, Hemitrygon akajei tends to acceleratedly accumulate total mercury (THg) that was released from Mt. Unzen beside the bay as it grows. In this study, we collected 20 individuals of the female ray with wide variety of body sizes (16 to 65 cm in disc size, 140 to 9,540 gww in body wet weight), and aimed to clarify its mechanism. The stomach content analysis revealed ontogenetic changes of feeding habits among the immature females, females in transitional maturity, and mature females in its long life of 10 to 15 years or more. Immature females favor to preying on epifaunal macro-benthic animals such as shrimps, which are classified as “Low THg content group” of the secondary consumers, while mature ones mainly feed on short-neck clam, Ruditapes philippinarum, and polychaete, Nectoneanthes ijimai, which are “High THg content group” of the primary consumers. Therefore, mature females with larger bodies tend to acceleratedly accumulate THg and reached a max. 1,370 ng g−1 dw, although the stable isotope analysis of carbon and nitrogen of the muscles of the mature females indicates the descent of the trophic position to the immature ones in the food web system.