陸奥湾におけるイカナゴ属仔魚の摂餌，栄養状態と生残

To clarify the recruitment process of sand lance Ammodytes sp., we investigated larval condition factor, relative gut fullness (%GF), prey abundance and oceanographic structure in Mutsu Bay, Japan, during 1999–2001. Ammodytes sp. larvae, which were collected by horizontal hauls of Motoda nets and a ring net at depths of 1, 10, 20, 30 and 40 m, were mainly distributed at 10–30 m. Larvae at the first feeding time until 12 mm in body length (BL) fed predominantly on copepod nauplii, whereas large larvae with BL of 12.1–14.0 mm fed on a mixture of copepod nauplii, copepodites and appendicularians from late February to April. A path analysis showed that difference in water density between 35- and 5-m depths negatively affected naupliar abundance at 10–30-m depth (standardised path coefficient β = −0.71, p = 0.005 for 3.3–8.0-mm BL larvae and β = −0.78, p < 0.001 for 8.1–12.0-mm BL larvae). Naupliar abundance positively affected the %GF of Ammodytes sp. larvae (β = 0.75, p < 0.001 for 3.3–8.0-mm BL larvae and β = 0.66, p < 0.001 for 8.1–12.0-mm BL larvae), whereas it was negatively affected by water temperature (β = −0.45, p = 0.008 for 3.3–8.0-mm BL larvae and β = −0.56, p = 0.002 for 8.1–12.0-mm BL larvae), and the temperature effect was weak compared with that of naupliar abundance. In turn, %GF positively affected larval somatic weight (β = 0.91, p < 0.001 for 6.0-mm BL larvae and β = 0.70, p = 0.005 for 10.0-mm BL larvae). The recruitment failure in 1999 was likely caused by a reduced condition factor, which resulted from low naupliar abundance. In contrast, the abundance of nauplii and Oithona similis copepodites was high in 2000 and 2001. It is possible that the higher recruitment success in 2001 was because of the higher water temperatures in Mutsu Bay, sustaining faster growth of the larvae than in 2000 under the high-prey abundance conditions.1999–2001年2–4月に陸奥湾で採集されたイカナゴ属仔魚は，主に水深10–30 m層に分布し，主にかいあし類ノープリウスを捕食し，体長12 mmを超える大型仔魚はOithona similisと尾虫類も捕食していた。パス解析の結果，水深35 m層と5 m層の海水密度差が大きいほど環境中のノープリウスの豊度は低く，体長6 mmと10 mmの仔魚の体重は軽かった。1999年には，鉛直混合と栄養塩の供給が妨げられた結果，ノープリウスの生産が妨げられ，イカナゴは加入に失敗したものと推定された。一方，餌豊度が高かった2001年と2000年には，高水温だった2001年の方が成長が速く，生残率が高かったものと考えられた