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

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 habitats of the two different-sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21-25degreesC) indicates that the spawning ground occurs farther north during autumn (28-34degreesN) than winter-spring (20-28degreesN). 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 summer, which, we hypothesize, causes its members to grow larger than those in the winter-spring cohort in summer.