Preliminary stock assessment of the winter-spring cohort of neon flying squid using the stochastic surplus production model in continuous time (SPiCT) from 2005 to 2024

We applied the Stochastic Surplus Production Model in Continuous Time (SPiCT) to the winter–spring cohort of neon flying squid in the North Pacific, using annual total catch data reported by NPFC members, two fishery-dependent indices derived from Japanese and Chinese commercial fisheries, and a fishery-independent index obtained from the Japanese scientific driftnet survey. The assessment used data from the 20-year period of 2005–2024, during which annual catch data and all three indices were available, to estimate stock dynamics through 2025. For all input indices, the preliminary relationships with the annual catch data aligned with the general structural assumptions of the surplus production framework. The final parameter estimates were well constrained, with narrow confidence intervals; however, slight diagnostic issues remained in the retrospective analysis. The relative weights of the three indices estimated within the state-space model reflected difference in their reliability: the Chinese fishery CPUE exhibited the lowest estimated observation error, whereas the Japanese fishery CPUE showed the highest error, likely due to large annual fluctuations. The Japanese survey CPUE index occupied an intermediate position. The successful utilization of both fishery-dependent and fishery-independent indices within this state-space framework constitutes a valuable approach for future stock assessments of neon flying squid by the SSC NFS.