Are Biological Responses Consistent With Ocean Warming Across Levels of Organisation in the Bay of Biscay?

ABSTRACT Global change has altered the climate and biogeochemistry of oceans, which has cascading consequences across all levels of biological organisation. Under warmer conditions, most fish species develop faster during early life stages but reach smaller adult sizes. At the population level, fish respond to warming by altering the phenology of key events and/or shifting spatial distribution. The interwoven action of these responses cascades up to entire ecological communities. Here, we analysed changes in fish communities over the last three decades in the Bay of Biscay (BoB). We examined changes in multiple indicators across levels of organisation, from individual species physiology to community composition, to test whether trends were consistent with those expected from ocean warming. At the individual and population levels, we assessed changes in species body size, reproductive investment, spawning phenology and spatial distribution on four pelagic fish. Mean adult body size significantly decreased for anchovy, sardine and Atlantic mackerel, but increased for horse mackerel. Anchovy increased reproductive investment and advanced spawning. Atlantic mackerel and horse mackerel spawned earlier and polewards. We also analysed bottom trawl data to highlight changes in life‐history and ecological traits across three fish communities from the BoB with 249, 252 and 235 species. Small, fast‐growing and warm‐affinity species that feed at a slightly low trophic level were favoured. Our analyses revealed that 24 of the 41 developed indicators (59%) significantly aligned with the expected impact of warming on marine fish. Observed trends reveal a clear signature of broad‐scale processes impacting marine communities.