Meeting the challenge of latitude: a climatic gradient shapes the reproductive strategy of a nest-brooding marine fish

High-latitude environments represent climate-related challenges, imposing a limit on species distributions. By elucidating how marine species adjust their reproductive strategy to cope with climatic variation, we can better predict the resilience of populations to environmental change. We addressed this question with a large-scale field study of a marine fish with a central role in the northeast Atlantic coastal ecosystem: the two-spotted goby Pomatoschistus flavescens . We developed a multi-faceted approach to examine how the species’ reproductive strategy and life history change along a climatic gradient. Six populations were surveyed along a gradient of 10° latitude, nearly reaching the northern limit of the species’ range, combining environmental, physiological, phenotypic and demographic data with a genomics investigation of population connectivity. Northern fish experienced colder environments for growth and reproduction than their southern counterparts, occurred at lower adult density and had lower body condition. At high latitudes, a different life history strategy emerged, whereby adults reproduced as 2 yr olds rather than 1 yr olds. Genomics analyses unveiled a clear genetic structure, with a break in connectivity between Skagerrak and the Norwegian coast further north, consistent with findings in other marine species. However, our genomic analyses did not provide clear evidence of adaptation to ecological variables, leaving open questions about the roles of adaptation and plasticity in latitudinal shifts in reproductive strategy. The study highlights the complex life history adaptations to high latitudes in a marine species with high gene flow and underscores the importance of sea temperature on fish reproductive strategy.