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 waters off mainland Portugal. We analyzed an opportunistic dataset collected using European Seabirds at Sea (ESAS) methodology within a distance sampling framework, spanning from 2004 to 2020. We fitted habitat-based Density Surface Models (DSMs) relating common dolphin counts, corrected for detectability, to environmental factors. We found significant effects of distance to the coast – varying seasonally - and a selection of dynamic covariates, on dolphin densities. We then predicted and mapped common dolphin densities throughout the sampling period. Predictions were restricted to coastal regions in spring and winter, due to a lack of offshore effort. Coastal abundance was highest during the summer. When accounting for the entire EEZ, estimated abundance was greatest during autumn, although with considerably large uncertainty measures. Distribution along the coast was patchy, across all seasons. Offshore densities were higher in the autumn than in the summer. These results highlight the spatiotemporal distribution and abundance fluctuations of common dolphins, demonstrating how, when treated carefully, opportunistic data can fill in the blanks on species ecology and abundance.