Linking temperature-dependent survival of Pacific cod (Gadus macrocephalus) early life stages to habitat suitability and catch variability

Korean coastal waters mark the southern distribution limit of Pacific cod (Gadus macrocephalus) in the northwestern Pacific, where climate regime shifts (CRSs) since the late 1980s have driven regional variability in cod catches. Despite the species' ecological and commercial importance, limited research has explored how temperature-dependent physiological traits during early life stages (ELSs; eggs and larvae) interact with local oceanographic conditions to influence recruitment success. This study experimentally assessed cod ELS survival and growth across temperatures representative of Korean spawning seasons and applied these results to estimate a habitat suitability index (HSI: 0-1) across three regions, namely the eastern, western, and southern coasts of Korea, over multiple decades. Hatching rates declined sharply below 3 °C and above 10 °C (HSI < 0.6), while larval survival increased at lower temperatures. Along the eastern coast of Korea, persistent stratification and bottom cooling maintained favourable ELS conditions (HSI ≥ 0.6), coinciding with steadily rising catches. In the western coast of Korea, catches declined in the late 1980s despite favourable spawning conditions, likely due to unfavourable larval-stage environments (HSI < 0.6), which improved following the re-expansion of the Yellow Sea Bottom Cold Water. In the southern coast of Korea, broader warming rendered most areas unsuitable for ELS survival, but Jinhae Bay maintained favourable conditions (HSI ≥ 0.6) due to cold-bottom inflow via the Korea Strait. These findings advance our understanding of climate-driven shifts in cod ELS habitats and provide a theoretical basis for predicting regional recruitment dynamics and informing adaptive fishery management.