瀬戸内海の河口干潟域で確認されたトラフグ稚魚による刺毒魚アカエイの捕食

瀬戸内海の河口干潟域でトラフグTakifugu rubripes 稚魚による刺毒魚アカエイDasyaris akajei の捕食を初めて確認した。2012年9～11月と2014年10月に山口市椹野川河口で採集したトラフグ27個体（9.8～14.0cm SL, 11.9～17.5cm TL の食性を調査し2012年10月に採集した1個体（12.1cm SL, 15.1cm TL）の消化管内容物からアカエイ1個体を同定した。アカエイ被食部位は，尾部を含めた体後部で，計40個の断片として検出された。異なる2通りの復元方法により，体盤幅はそれぞれ11.1 ±2.4cm (±95% 信頼区間），12.8±3.7cm（同）と推定された。被食量は，前者ではアカエイ体重の18.6%，後者では11.8% に及ぶと考えられた。

Tiger pufferfish, Takifugu rubripes (Family: Tetraodontidae), is an important commercial fish in Japan. Landings of Tiger pufferfish at the Shimonoseki fish market, Yamaguchi Prefecture, where ca. 60% of the pufferfish caught in Japan is landed has decreased remarkably from 1,841 metric tons at the peak of catch in 1987 to 111 metric tons in 2013. This reduction in stock indicates the pufferfish population of Japan has reached a critical condition. As a result, the local population in Ariake Bay, East China Sea has been evaluated as an endangered local population by the Nagasaki Prefectural local government (Red List 2011, category: LP). Necessarily, it is desired that the resources should recover from the present state. It is known that juvenile tiger pufferfish grow in estuaries until late fall. They forage on small benthic animals there. Therefore, it is necessary to clarify the feeding habits of the juvenile pufferfish in the intertidal area in detail.

In this report, we clarified the predation on the Whip stingray, Dasyatis akajei by juvenile Tiger pufferfish in the estuary area. Because the stingray is well-known as a venomous species, it has been considered that no fish species could prey on stingrays in this area. In September-November 2012 and October 2014, 27 individuals of T. rubripes (9.8-14.0cm SL, 11.9-17.Scm TL) were collected in the Fushino River estuary in Yamaguchi Prefecture, western Seto Inland Sea to examine their feeding habits. A D. akajei was identified from the gut contents of the juvenile T. rubripes, 12.1 cm SL (15.1 cm TL), which was collected in October 2012. The prey, the stingray's body was detected as a total of 40 only partially digested pieces including 4 distinct pieces of the tail being the posterior portions of the stingray's body. By two different restoring methods, the disc width of the stingray was estimated to be 11 .1 ± 2.4cm (± 95% confidence interval) and as 12.8 ± 3.7cm. The amount of stingray body tissue consumed to total stingray body weight was estimated to be 18.6% of the former restoring method and 11.8% by the latter.

In the Seto Inland Sea of Japan, resources of benthotrophic fish species, such as flatfishes and pufferfishes, which use the estuary in their early life history, have decreased remarkably. For recovery and regeneration of these critical resources, it is necessary to clarify the relationships between organisms and the estuarine ecosystem.