Chlorophyll a variation in the Kuroshio Extension revealed with a mixed-layer tracking float: implication on the long-term change of Pacific saury (Cololabis saira)

Variation of chlorophyll a from March 2004 to July 2005 in the formation region of Subtropical Mode Water in the Kuroshio Extension was observed with a mixed‐layer tracking profiling float parking at around 40 m depth. Chlorophyll a concentration in the mixed‐layer is seasonally high from winter to early spring (January–April) even in the deep mixed layer; whereas during winter it tends to be lower for the corresponding deeper mixed layer. The chlorophyll a integrated over the winter mixed‐layer depth (MLD) increases with MLD for MLD <200 m and for MLD > 250 m the integrated chlorophyll a almost disappears, probably because of the large MLD exceeds in the critical depth with light limitation. These results suggest that the wintertime MLD influences the recruitment of Pacific saury (Cololabis saira) in the Kuroshio Extension because the wintertime food environment for larvae and juvenile of the Pacific saury could be better in the shallow wintertime MLD period of the 1950s and 1990s and worse in the deep MLD period of the 1970s and early 1980s.