Bull. Jpn. Soc. Fish. Oceanogr. 77(4), Page 290-298, 2013   Change in the horizontal distribution of juvenile Japanese sardine (Sardinops melanostictus) associated with the dynamics of the Kuroshio Extension in the western North Pacific Kaori Takagi1†, Motomitsu Takahashi2, Takahiro Kinoshita3*, Akihiko Yatsu2, Satoshi Honda1 and Hiroshi Nishida1** 1 National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan 2 Seikai National Fisheries Research Institute, 1551-8 Taira, Nagasaki, Nagasaki 851-2213, Japan 3 Japan Sea National Fisheries Research Institute, 1-5939-22 Suido, Chuou, Niigata, Niigata 951-8121, Japan * Present: Japan Sea National Fisheries Research Institute, Obama Station, 26 Tomari, Obama, Fukui 917-0117, Japan ** Present: Fisheries Agency, Ministry of Agriculture, Forestry and Fisheries, 1-2-1 Kasumigaseki, Chiyoda, Tokyo 100-8907, Japan † E-mail: takagik@affrc.go.jp 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. Key words: sardine, juvenile, horizontal distribution, recruitment, the Kuroshio Extension