Feeding habits of two sympatric dolphin species off North Patagonia, Argentina

Abstract

Among marine mammals, small cetaceans have received more attention due to operational interactions with fisheries and the subsequent incidental catch of many species around the world. Even though marine mammals can be considered top predators, their role as consumers and the effects of this consumption on species interactions and community structure are little known. Since small cetaceans can feed at lower trophic levels or take smaller sizes of prey than fisheries, competitive effects have been considered less important. However many species, and especially those with pelagic habits, can be consuming “forage fish” species, and then interactions with fisheries and their effects may be indirect but important, instead of negligible (Crespo and Hall 2001). In the western South Atlantic, short-beaked common dolphins, Delphinus delphis, and dusky dolphins, Lagenorhynchus obscurus, are the most abundant small cetaceans (Crespo and Dans 2008). The common dolphin is a widely distributed species that inhabits temperate, subtropical, and tropical habitats (Evans 1994, Perrin 2002), while dusky dolphins are restricted to cold temperate waters in the southern hemisphere (Van Waerebeek 1992, Crespo et al. 1997). However, in North Patagonian waters (35°–43°S) both species occur together. In this area of sympatry, aggregations of two dolphin species are frequently observed together with sea lions, Otaria byronia, and several species of seabirds, such as shearwaters (Puffinus griseus, Puffinus gravis), penguins (Spheniscus magellanicus), gulls (Larus dominicanus, Larus atlanticus), and terns (Sterna hirundinacea), composing an assemblage feeding on pelagic schooling fishes (Crespo and Dans 2008). Information about common dolphins feeding ecology in the southwest Atlantic is lacking. On the other hand, dusky dolphins cooperative foraging behavior was well studied in a closed bay off the Patagonian coast (Würsig and Würsig 1980), and more detailed studies of feeding habits were done through the analysis of stomach contents of animals incidentally caught in fishing nets (Koen Alonso et al. 1998). Both species feed in herds from few tens to several hundred individuals mainly on schooling fish and squid (Koen Alonso et al. 1998, Perrin 2002). Although sympatric associations between these dolphin species have been reported, feeding habits of these interacting species have not been studied in great detail.