The coastal pulse of fish spawning – a nationwide Atlas of marine fish egg distribution

Spawning locations of marine fish set the initial conditions for processes that influence survival, abundance, distribution, and the structure of the adult populations. The prevailing paradigm suggests that fish spawning is aggregated in few and spatially well defined geographic areas. While corroborated, this view is biased by the sampling strategy. Here, we challenge the main paradigm and analyze an extensive and unique 16-year dataset of egg catches along the entire Norwegian coast to examine spawning patterns of Atlantic cod ( Gadus morhua ). Our results reveal that cod populations exhibit persistent spawning aggregations throughout the entire coastline, spanning a large range of oceanographic conditions. Distinct regional patterns were identified, linking egg abundance to habitat characteristics, including temperature, salinity, distance from fjord mouth, and advective potential of the spawning site, pointing to a high degree for local adaptations. We found that regional differences are driven by the cod spawning strategies that are directed more toward fresher, warmer, and less advective body of waters, going from North-Norway to the Skagerrak area. Collectively our results demonstrate that spawning in marine fish can comprise a series of events at multiple locations and times, each uniquely associated with oceanographic variables. This research provides a foundation for developing a new framework to characterize marine fish life cycle closure and population structure, offering a critical baseline for conserving coastal marine habitats.