クロダイ稚魚はアサリを直接殺さない : 瀬戸内海広島湾のアサリ漁場干潟におけるクロダイ稚魚の食性

瀬戸内海最大のアサリ漁獲量のある漁場である広島県廿日市市の前潟干潟において，2005～2006年に，クロダイ稚魚（7.5～13.4cm TL，n=26）の食性を調査した。胃内容物について，実体顕微鏡下にて定量的分析を行った。胃内容物重量指数は0～8.5%を示した。当才・満１才（7.5～10.9cm TL，n=25）では，生物群別の餌生物重要度指数の百分率（%IRI）は，多毛綱45.1%，藻類29.6%が高値を示した。種別の%IRIでは，大型藻類36.5%，ゴカイ科31.9%が高く，アサリは4.1%で低値を示した。アサリ，マテガイは水管のみが出現した。干潟のクロダイ稚魚の生息数は多い（釣りCPUE=4.7（個体/1.5時間/人，n=3））が，水管のみを捕食することから，アサリを直接死に至らせることはないことが示唆された。稚魚期の採餌場所利用の多様性と食性の柔軟性は，本種の資源維持の大きな要因と推察された。

Black porgy, Acanthopagrus schlegelii (Family: Sparidae), is an important commercial fish in Japan. This species in adults is known as a predator of consequence on Asari clam, Ruditapes philippinarum in tidal flats. The feeding habits of juvenile Black porgy (7.5-13.4 cm TL, n = 26) were investigated from 2005 to 2006 at the Maegata tidal flat in Hiroshima Bay, which has the most catches of Asari clam in the Seto Inland Sea. The stomach contents were quantitatively analyzed using a binocular stereomicroscope. The stomach content index was 0-8.5%. In the yearling fish (7.5-10.9 cm TL, n = 25), the index of relative importance percentage (%IRI) of the higher taxonomic groups was high at 45.1% for polychaetes and 29.6% for algae. The %IRI of the species was high at 36.5% for macroalgae and 31.9% for Nereidae, and low at 4.1% for Asari clam. Only siphons were detected in the Asari clam and the Japanese razor clam. Although many juvenile porgy inhabited the tidal flat (fishing CPUE = 4.7 (indiv. /1.5 hours/person, n = 3)), it was clarified that the juvenile porgy does not kill the Asari clam directly. It was considered that the variety of ways to use foraging habitats and the flexibility of feeding habits during the juvenile period were the major factors for maintaining the resources of the porgy.