Changes in abundance of the neon flying squid Ommastrephes bartramii in relation to climate change in the central North Pacific Ocean

Abstract

Interannual changes in abundance of the autumn cohort of neon flying squid Ommastraphes bartramii during 1979 to 2006 were examined in relation to the large-scale climate shift in the central North Pacific Ocean. Catch per unit of effort (CPUE) of driftnet surveys, which is assumed to reflect the squid stock level, was very low during 1999 to 2002, which was an anom- alous climate period. Given that the autumn cohort uses productive areas near and north of the transition zone chlorophyll front (TZCF) as favorable nursery and feeding grounds, we examined 3 covariates — (1) the winter position of the TZCF in the neon flying squid nursery ground, (2) the summer position of the TZCF in the feeding ground and (3) fishing mortality — to estimate which covariates were significantly correlated with low CPUE time periods. Multiple linear regression analysis suggested that only the effect of winter position of the TZCF in the nursery ground best predicted the change in the CPUE. The anomalous climate conditions caused a significant reduc- tion in net primary production over an extended area within the subtropical frontal zone (STFZ), which is the main nursery ground for the autumn cohort. Hence, we assume that food availability for juvenile neon flying squid could be critical for the level of stock recruitment of this cohort. Fur- thermore, the sea level anomaly (SLA), which is considered to be a useful index of variability in the upper ocean structure, significantly correlated to the CPUE during spawning and nursery periods in the STFZ. Our study supports the hypothesis that the bottom-up effect of the climate shift can be manifested rapidly in neon flying squid, indicating that squid abundance can act as a productivity integrator and a real-time ecosystem indicator that respond to large-scale environ- ment changes.