Preliminary application of the stochastic surplus production model in continuous time (SPiCT) to neon flying squid in the North Pacific using the Japanese survey index

We applied the stochastic surplus production model in continuous time (SPiCT) to the autumn and winter-spring cohorts of neon flying squid in the North Pacific using the total catch data from NPFC members, and two indices from the fishery and Japanese driftnet survey. The fishery index was estimated by standardizing the member’s CPUE using a generalized linear mixed model that included year and NPFC member as explanatory variables. We used the 13-year period from 2012 to 2024 for the autumn cohort and the 24-year period from 2001 to 2024 for the winter-spring cohort for the assessment periods, as data on total annual catch and the two indices were only available during these time frames. During the assessment periods, we confirmed a weak positive relationship (Pearson’s r = 0.27) between the fishery and survey indices for the autumn cohort and a strong positive relationship (r = 0.57) for the winter-spring cohort. For both cohorts, the relationship between the survey index and the catch data appeared to align with the general assumption of the surplus production model. The parameter estimates derived from the autumn cohort assessment using SPiCT were nearly satisfactory with narrow confidence intervals, however, some issues from model diagnostics and retrospective analysis were found. The estimated values in the winter-spring cohort model appeared acceptable, as the model diagnostics were more satisfactory and there was no retrospective bias for either relative biomass or fishing mortality. The utilization of both fishery and survey indices in the surplus production models constitutes a valuable approach for future stock assessments of neon flying squid by the SSC NFS.