Seasonal and interannual variation of common dolphin densities in Portuguese waters

Modelling a species ecology and abundance provides important insights into its habitat preferences, population trends and distribution. This can be used to inform conservation efforts, highlighting populations at risk of extinction and regions in need of protection. Here, we studied how environmental factors relate to common dolphins (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. There were 1151 sightings of common dolphins, 737 of which were during effort. Detection functions were fitted to model the dolphins detectability as a function of the distance to the track line. We used smooths of static (depth, slope, and distance to the coast, 200m and 1000m isobaths) and dynamic (sea surface temperature, salinity, chlorophyll-a concentration with and without a time lag, zooplankton, and sardine biomass) environmental factors, relating them to counts corrected for detectability and survey effort, to fit habitat-based Density Surface Models. We then predicted and mapped common dolphin densities seasonally (winter, spring, summer autumn) throughout the sample years. The results show how different environment variables relate to dynamic common dolphin densities. In general, common dolphins had higher densities in coastal environments, especially during the summer due to high productivity and lower sea surface temperature in shelf waters, associated with seasonal upwelling. During the summer, there were predicted local peaks in common dolphin densities, in areas with high zooplankton and chlorophyll-a concentration. In the other seasons, its range expands to more offshore waters, with less local density peaks. The interannual variation of this species densities is mostly explained by the annual sardine biomass. Overall, this brings important insights into this species conservation and potential impacts from anthropogenic activities, such as climate change and overfishing.