Climate Extremes and Reproductive Phenology: Regional Variability of Hatch Timing in the Subarctic North Pacific

ABSTRACT

Climate‐related shifts in reproductive phenology can reshape the seasonal dynamics of marine ecosystems, yet spatiotemporal variation in hatch timing has the potential to buffer populations against these changes. We investigated spatiotemporal variation in Pacific cod ( Gadus macrocephalus ) hatch timing in the Gulf of Alaska, a subarctic ecosystem experiencing recurrent marine heatwaves (MHWs). First, we compared hatch timing before (2007, 2009, 2010, 2012–2014), during (2015–2017, 2019), adjacent to (2018, 2020), and after (2021–2023) MHWs based on long‐term collections along Eastern Kodiak. Second, we compared hatch timing at Eastern Kodiak with three other regions (Kaiugnak Bay, Agripina Bay, and the Shumagin Islands) that were sampled from 2018 to 2023. Third, we estimated spatiotemporal variation in the probability of hatch success using regional temperature data and temperature‐dependent embryonic survival rates. Along Eastern Kodiak, hatching shifted 2–4 weeks earlier during and adjacent to MHWs and remained 1–2 weeks earlier after MHWs despite a return to cooler temperatures, suggesting persistent phenological shifts. Across regions, we observed a strong spatial gradient in phenology with progressively later hatching toward the southwest, Shumagin Islands. The temperature‐dependent predicted probabilities of hatching displayed a similar spatial gradient ranging from below 75% at Eastern Kodiak to above 80% toward the southwest, indicating better thermal conditions for spawning. Observed phenological shifts during the MHWs (2014–2016 and 2019) matched predictions. In 2019, there were smaller phenological shifts in the cooler southwest region than in Eastern Kodiak, potentially indicating a muted response to the 2019 MHW. Notably, a previously unobserved winter‐hatching cohort appeared at Eastern Kodiak in 2021. This cohort expanded the hatching window and potentially reflects a combination of phenotypic plasticity in spawning strategies and selection. These results highlight how spatiotemporal variation in phenological responses could potentially partially buffer populations against ocean warming.