Differing body size between the autumn and the winter–spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis TARO ICHII, 1, * KEDARNATH MAHAPATRA, 2 MITSUO SAKAI, 1 DENZO INAGAKE 1 AND YOSHIHIRO OKADA 2,3 1 National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu, Shizuoka 424-8633, Japan 2 Tokai University Frontier Ocean Research Center (T-FORCE), 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan 3 School of Marine Science and Technology, Tokai University, 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan ABSTRACT The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38– 46 cm) and the winter–spring cohorts (ML range: 16– 28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration hab- itats of the two different-sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter– spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food-rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food-rich habitat from winter through sum- mer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer. Key words: neon flying squid, North Pacific, Ommastrephes bartramii, satellite remote sensing, Subtropical Frontal Zone, Transition Zone Chlorophyll Front. INTRODUCTION The neon flying squid, Ommastrephes bartramii, is an oceanic squid occurring worldwide in subtropical and temperate waters (Roper et al., 1984). In the North Pacific, this species plays an important role in the pelagic ecosystem and is an international fish- eries resource with high commercial value (Seki, 1993). The neon flying squid migrates between its spawning grounds in subtropical waters and feeding grounds in Subarctic waters (Murata and Hayase, 1993). Recently, Polovina et al. (2000, 2001) reported that the Transition Zone Chlorophyll Front (TZCF) migrates seasonally between about 30°N in winter and 40°N in summer in the north Pacific. The TZCF marks the transition from waters with a low near-surface chlorophyll a (chl a) concentration (0.1 mg m )3 ) in the south to waters with higher chl a concentrations (‡0.2 mg m )3 ) in the north and can be readily mon- itored by ocean color remote sensing. Albacore tuna (Thunnus alalunga) and loggerhead sea turtles (Caretta caretta) have been shown to exploit the TZCF as a migration pathway and foraging habitat (Polovina et al., 2000, 2001), however, to date it is not clear if or how the life cycle of neon flying squid is associated with the TZCF. *Correspondence. e-mail: ichii@affrc.go.jp Received 27 March 2003 Revised version accepted 29 September 2003 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 13:5, 295–309, 2004 Ó 2004 Blackwell Publishing Ltd. 295