広島湾江田島沖のカキ養殖筏の垂下構造中にみられた魚類

カキ筏には多様な付着生物が生育することが知られており，魚類にとっても寄り付きやすい環境であることが示唆されている。しかし，カキ筏に具体的にどのような魚類が出現するのかについての知見は乏しい。そこで，カキ筏が数多く係留されている広島湾北部の江田島沖合において，カキ筏に出現する魚類の潜水目視調査を2011年8月と10月に実施した。マガキの垂下された水深5-9mを筏に沿って約50m 遊泳しながら，水中構造内に存在する魚種と個体数を記録した。各月とも，マガキの成長した筏とマガキが垂下されて間もない筏について調査し，のべ400分の潜水観察により総計16科24種の魚類を確認した。そのほぼすべてが，沿岸岩礁域およびガラモ場にみられる魚種であった。クロダイ，アカオビシマハゼ，アミメハギ，カワハギの４種は，調査を実施した筏すべてにおいて出現が認められ，カキ筏環境への順応性の高さが伺われた。出現魚種数および出現個体数は，マガキの成長した筏が有意に高く，マガキの成長に伴い，より多様な魚が数多く寄り付きやすくなることが示唆された。

The oyster farming rafts occupy 200ha of the water surface in Hiroshima Bay. Each floating raft (ca.11m wide x 26m long) hangs a total of 600 wires (ca. 10m depth) each attaching 40 oysters’ collectors, providing a complex underwater structure. Though the raft structure has been suggested to provide nursery habitats for fishes, information of fishes found around the oyster raft has been very limited hitherto. In order to obtain data of the species composition and the abundance of fishes occurring on the raft structure, we conducted visual census at off Eta-shima Island, northern Hiroshima Bay, during the daytime on August 5 and October 3, 2011. The collecters were dropped in 0-15m depth from floating rafts. Five observers swam for 52m along the outer edge of the raft in 5-9m depth, with recording fish species and individual numbers occurring within the structure (ca. 100min observation for each raft). The underwater survey was conducted for two types of the rafts in each survey month; a raft with well-grown oysters after overwintering (called the late raft) and one with oyster spats (called the early raft). A total of 24 fish species (16 families) were confirmed and almost of them (23 species) have ever been known as occurring on rocky reefs in Hiroshima Bay. Four fishes, a sparid Acanthopagrus schlegelii, a gobiid Tridentiger trigonocephalus, monacanthids Rudarius ercodes and Stephanolepis cirrhifer, commonly occurred in all four surveyed rafts, suggesting their high abilities in fitting for the environmental conditions of the oyster rafts. The late rafts maintained significantly higher values both in mean fish species numbers and in mean fish abundance than the early rafts, implying that the environmental conditions would become attractive for fishes as a nursery habitat in accordance with the growth of oysters mooring for longer periods.