Life history of the neon flying squid: effect of the oceanographic regime in the North Pacific Ocean

The North Pacific Ocean population of the neon flying squid Ommastrephes bartramii, which un- dertakes seasonal north-south migrations, consists of autumn and winter-spring spawning cohorts. We ex- amined life history differences between the 2 cohorts in relation to the oceanographic environment. The differ- ences could be explained by seasonal north-south movements of the following 2 oceanographic zones: (1) the optimum spawning zone defined by sea surface temperatures; and (2) the food-rich zone defined by the position of the transition zone chlorophyll front (TZCF). The 2 cohorts use the food-rich zone in different phases of their life cycles. The spawning grounds for the au- tumn cohort occur within the subtropical frontal zone (STFZ), characterized by enhanced productivity in win- ter due to its proximity to the TZCF, whereas the spawning grounds for the winter-spring cohort occur within the subtropical domain, which is less productive. As the TZCF shifts northward in spring, the autumn co- hort continues to occur in the productive area north of the TZCF, whereas the winter-spring cohort remains in the less productive area to the south until it migrates into productive waters north of the TZCF in the summer or autumn. Consequently, the autumn cohort grows faster than the winter-spring cohort during the first half of its life cycle, whereas the winter-spring cohort grows faster during the second half. This growth pat- tern may be responsible for differing migration pat- terns; males of the autumn cohort do not have to migrate given their early fast growth in the STFZ, whereas those of the winter-spring cohort must mi- grate to the food-rich subarctic frontal zone to compen- sate for their slow growth. These biological and eco- logical differences between the 2 cohorts suggest flexibility of their life history response to oceanographic environments. Hatchling of the neon flying squid Ommastrephes bartramii