Environmental forcing of winter spawning grounds on the abundance trend of Pacific saury (Cololabis saira) in the Northwest Pacific.

Pacific saury (Cololabis saira) is a highly migratory pelagic fish with a short lifespan, whose recruitment is highly sensitive to environmental fluctuations. This study investigated the drivers of recruitment by establishing a statistical relationship between winter spawning conditions and fishery-dependent abundance indices from 2013 to 2023. Specifically, we hypothesize that winter spawning habitat conditions pre-determine year-class strength by jointly regulating parental spawning success and the survival of eggs and larvae throughout their early life history. Utilizing a geostatistical spatio-temporal framework (sdmTMB), we standardized the catch-per-unit-effort (StdCPUE) to derive a robust abundance index. Concurrently, an index for the SST-suitability-weighted size of spawning grounds (WSSG) was developed to quantify habitat quality during the critical winter phase. Results revealed that winter sea surface temperature (WSST) exerted a persistent negative pressure on recruitment, while WSSG exhibited a robust positive correlation (r = 0.68) with StdCPUE, serving as a proxy for the environmental carrying capacity of nursery grounds. Generalized Additive Models (GAMs) identified a significant non-linear effect of WSST_lag (p < 0.001), identifying a thermal optimum at approximately 19.1 °C, beyond which recruitment potential declines precipitously. This robust one-year lag effect demonstrates that winter oceanographic conditions pre-determine the strength of the age-1 cohort before it enters the commercial fishery. Consequently, these winter indices serve as reliable early-warning tools, allowing managers to anticipate recruitment failures and adjust harvest strategies a year in advance. Integrating these findings into stock assessment frameworks is essential for the proactive management of Pacific saury in a rapidly warming ocean.