Climate-driven changes in the timing of spawning and the availability of walleye pollock (Gadus chalcogrammus) to assessment surveys in the Gulf of Alaska

Climate-driven changes in the timing of spawning or migration can affect the availability of fish to surveys designed to monitor their abundance, complicating efforts to assess stock status and sustainably manage fisheries. From 2017 to 2019, trends in biomass estimates from four surveys used to monitor Gulf of Alaska pollock diverged. These conflicting trends increased uncertainty in the stock assessment and occurred during a time of rapid environmental change. We hypothesized that changes in spawn timing affected availability of pollock to a winter survey that targets pre-spawning aggregations. To test this, we reconstructed relative spawn timing using two independent data sources: spring larval surveys and observations of spawning state in mature female pollock. We found that changes in spawn timing relative to survey timing explained a significant portion of recent and historical discrepancies between survey and model estimates of biomass. We then incorporated measures of spawn timing/survey timing mismatch as catchability covariates in an enhanced state-space stock assessment model. Including spawn timing-based catchability covariates significantly improved the model fit to survey data and provided a mechanistic explanation for recent survey discrepancies.