Abstract/Summary

Climate-driven ocean warming is profoundly reshaping marine ecosystems, with cascading effects on biodiversity and trophic interactions. For migratory marine predators such as seabirds, demographic responses to warming depend on when and where populations are exposed, across the annual cycle. Therefore, integrating demographic monitoring and tracking data, across broad geographic and temporal scales is essential, given the spatial and seasonal variability in ocean warming. Here, we integrated long-term demographic data, seasonal distributions and sea surface temperatures, for 26 populations of five seabird species in the North-East Atlantic to assess the effects of sea surface temperatures (SSTs) on reproduction, survival and population growth trajectories. Demographic responses varied widely among populations and seasons, but negative effects were most consistently associated with warming during the post-breeding autumn period, particularly in the Barents and East Greenland Seas. Winter warming also corresponded to reduced survival, while breeding-season SSTs showed fewer effects on reproductive rates. Populations with dual responses to warming in both the breeding and non48 breeding seasons had the lowest projected population growth rates under SSTs given a high emissions scenario. These results demonstrate that population vulnerability reflects the interaction between annual-cycle phenology and regional ocean warming, underlining the importance of integrating year-round tracking and long-term monitoring to inform climate-resilient conservation strategies and marine spatial planning.