When Sardines Disappear: Tracking Common Dolphin, Delphinus delphis, Distribution Responses Along the Western Iberian Coast

The common dolphin, Delphinus delphis, is the most abundant cetacean species along the Western Iberian Peninsula, facing many anthropogenic threats, with bycatch being the most impacting. Its favoured prey, sardine (Sardina pilchardus), has shown abundance fluctuations over the past decade, potentially influencing dolphin distribution. This study provides the first insights into common dolphin distribution along the Portuguese main-land coast, using sighting data from vessel research surveys (2005-2020) to identify hotspot areas, accounting for monthly and seasonal distributions overlapping with sar-dine abundance. Common dolphin hotspots were located along the central-western and southern Portuguese mainland coasts, coinciding with important fishing ports, oceano-geographic features and sardine juvenile habitats. Furthermore, during 2013-2016, com-mon dolphins were observed significantly farther from the coast, coinciding with a period of particularly low sardine biomass in coastal areas. However, GAM analysis indicated that the relationship between sardine biomass and the distance of common dolphins was not significant. This study highlights the major common dolphin hotspots and the most comprehensive temporal and distribution maps of the species available for the Western Iberia in response to the abundance and management plans of their favoured prey (sar-dines). These results can be used by managers to inform conservation measures and for the sustainable management of the Portuguese sardine purse seine fishery fleet, which in-teracts the most with the species.