北西太平洋におけるマイワシ稚魚の水平分布の変化と黒潮続流の動態

抄録

Distribution patterns of juveniles of Japanese sardine Sardinops melanostictus in the Kuroshio–Oyashio transition region were examined in relation to the dynamics of the Kuroshio Extension. The juveniles were collected using a pelagic trawl during 1996–2009. Index for juvenile abundance estimated in the current survey showed decreasing trends during 1996–2000, extremely low levels during 2001–2004, and fluctuating trends during 2005–2009. Mean standard length (SL) and age in days of juveniles collected in May ranged from 33.5–43.6 mm and from 46.2–56.6 days after hatching, respectively. The May-caught juveniles during 1996–2000 had significantly larger body size and older age in days than those during 2001–2004 and 2005–2009. Juveniles<40 mm SL occurred predominantly in waters west of 150°E during the decreasing (1996–2000) and fluctuating (2005–2009) periods, meanwhile the small juveniles appeared in waters east of 150°E during the extremely low period (2001–2004). Previous studies have demonstrated that the axis of the Kuroshio Extension showed a meandering path (unstable state) during 1996–2000 and 2005–2009 but a non-meandering path (stable state) during 2001–2004. This suggests that since smaller and younger juveniles were swiftly transported to the downstream waters east of 150°E, they could not stay in the favorable environment for their growth and survival during 2001–2005 resulting in extremely low recruitment.