Occurrence and relative abundance of common dolphins in three sites of the Portuguese shore

Throughout the years some researchers have dedicated their efforts to the study of cetaceans' occurrence off Portugal mainland. However, it is still missing a systemic scientific methodology for studying the presence of coastal small cetaceans. This work is a recent approach to the occurrence and relative abundance estimative of cetaceans and especially common dolphins off the west central coast of Portugal. Boat-based visual surveys were conducted in three different geographic locations, resulting in the sighting of several species. In Nazaré we have only observed Delphinus delphis while in Peniche and Sesimbra a great diversity of species was recorded, including Tursiops truncatus, Stenella coeruleoalba, Phocoena phocoena and Balaenoptera acutorostrata. Most frequently sighted was common dolphin (62%). Occurrence of large groups of this species seems to take place along main ocean topographic features, like the Portuguese submarine canyons and we consider that great depths near shore are suitable habitats for more pelagic species of dolphins such as common dolphins. Total index of common dolphin abundance was of 2.198 individuals per surveyed hour. A much smaller APUE was found for Peniche than for the other two regions, where common dolphins show a similar relative abundance. Future studies using linear transects and continuous long-term approaches will give further insight to small cetaceans' occurrence along Portugal mainland and its relation with different oceanographic features, particularly depths and distances to shore. On-going efforts in coming years added to this first approach will be required to obtain knowledge for the conservation of cetaceans.