瀬戸内海三原瀬戸におけるイカナゴの夏眠底質環境

瀬戸内海西部水域は過去に大規模な海砂利採取が為され，潜砂習性を持ち砂中で夏眠するイカナゴ資源の減少が明白な状況にあることから，イカナゴの生息底質の環境悪化が懸念されている。そこで，イカナゴの主要生息地の1つであった安芸灘三原瀬戸において，2014年と2015年の夏期にドレッジ調査を実施し，イカナゴの生息状況と底質性状を分析した。夏眠イカナゴは竹原沖から大三島南部の三原瀬戸西部エリア（3地点）と三原湾細島周辺の三原瀬戸東部エリア（2地点）で確認された。これら5 地点はいずれもイカナゴが好むとされる粒径0.25mm–4.0mmの砂粒子をおよそ80％の重量割合で含む砂底質であった。一方，イカナゴの出現が確認できなかった地点 の砂粒子成分の含有率は有意に低かった。三原瀬戸におけるイカナゴの夏眠生息に適した砂底質の分布は地理的に限られた状態にあり，イカナゴ資源回復を制約する要因になっている可能性が示唆された。

The sand lance Ammodytes japonicus in the Seto Inland Sea is known to estivate in fine sandy grounds when water temperatures reach above 19ºC, usually from late June to early December. Catches of the sand lance have decreased drastically since the mid-1980s, coinciding with large-scale quarrying for bottom sands, which strongly suggests that sand lance populations have been seriously impacted as a result of deterioration and disturbances to sandy-bottom habitats. Mihara Strait was once a major fishing ground for sand lance and has also suffered from quarrying. To evaluate the present condition of the sandy areas of Mihara Strait as potential estivation grounds of the sand lance, we sampled sediments at 11 survey points, using a square-shaped dredge towed by the training vessel Toyoshio-maru of Hiroshima University, in 2014 and 2015. We then analyzed the physical characteristics of the sediment samples. Sand lance individuals were observed in the sediments collected from five survey points: three on the western side and two on the eastern side of the strait. Sediments at these five survey points comprised sandy elements of 0.25–4.0 mm diameter, which constituted over 80% of the weight ratio of the sediment samples. Sediments from the other survey points included pebble elements and had significantly lower weight ratios of sandy elements, suggesting the unsuitability of the substrate at those points for burrowing by the sand lance. Thus, it is suggested that sandy grounds for estivation of the sand lance are geographically limited in Mihara Strait at present, a situation that may restrict population recovery of this species in these waters.