Seasonal and interannual variation of common dolphin's densities in Portuguese waters

Modelling a species' ecology and abundance provides important insights into its habitat preferences, population trends and distribution. Here, we studied how environmental factors relate to common dolphin’s (Delphinus delphis) densities in the waters off mainland Portugal. We analyzed an opportunistic dataset collected using European Seabirds at Sea (ESAS) methodology with distance sampling, spanning from 2004 to 2020. We fitted habitat-based Density Surface Models (DSMs) relating common dolphin counts, corrected for detectability, to environmental factors. We then predicted and mapped common dolphin densities seasonally (winter, spring, summer and autumn) throughout the sampling period. In general, common dolphins had higher densities in coastal environments, especially during the summer. In the other seasons, the range expands to further offshore waters, still with higher densities closer to the coast. Common dolphin densities seem related to productivity, with expected peaks in waters with higher chlorophyll-a and zooplankton concentrations, which are likely important for the dolphin's prey. The interannual variation of this species’ densities is mostly explained by the annual sardine biomass. These results highlight the spatiotemporal distribution of common dolphins and demonstrate the critical importance of integrated sardine fisheries management for marine ecosystem conservation and the sustainability of both dolphin populations and fishing communities.