太田川河口域における小型曳き網のスズキ仔稚魚採集効率の推定

河口域における魚類群集の主要構成者であるスズキ仔稚魚に対する小型曳き網（幅2.3m，高さ1m）の採集効率を，地曳き網（幅16m，高さ1m）による採集結果と比較することにより推定した。2008年3月21日と4月7日に太田川放水路感潮域において合計15回の曳網を行い，合計1,502個体（標準体長14.5－24.0mm）のスズキ仔稚魚を採集した。地曳き網の網目からの逸出が生じる体長16mm未満と，個体密度が著しく低下する23mm以上については解析対象とせず，地曳き網による採集効率を100％と仮定した。体長1mmごとに区分した分布密度を採集具間で比較した結果，小型曳き網による体長16mm以上18mm未満のスズキ仔稚魚採集効率は約100％と推定された。体長18mm以上23mm未満では採集効率（C, ％）が体長（L, mm）に比例して直線的に減少した（C=－17.2*L+388.8，n=11，r2=0.77，p=0.0004）。

In order to estimate catch efficiency of a small seine (1.0 by 2.3 m, 2 mm mesh aperture) for larval and juvenile Japanese sea bass Lateolabrax japonicus, results from experimental hauls of the small seine were compared with those from a large seines (1.0 by 16 m, 2.5 mm mesh aperture) in the Ohta Diversion Channel on March 21 and April 7, 2008. A total of 1,502 Japanese sea bass, ranging between 14.5 and 24.0 mm in standard length were collected. Data of fish <15.9 mm was excluded from the analysis since extrusion of these fish through the mesh of the large seine was possible. Comparison of the Japanese sea bass abundance at lengths between the small and large seines revealed that the catch efficiency of the small seine for Japanese sea bass <18.0 mm was approximately 100%. The catch efficiency decreased with the increase in fish body length between 18.0 and 22.9 mm. A linear model was fitted to the relationship between the catch efficiency (C, %) and body length (L, mm): C=－17.2*L+388.8 (n=11, r2=0.77, p=0.0004). Catch efficiency was not estimated for Japanese sea bass ＞ 23 mm since their abundance was quite low in the surveyed area probably due to migration to downriver or sea areas, which are considered as habitats for the following life stages.