Seasonal distribution of bottlenose (Tursiops truncatus) and pan-tropical spotted (Stenella attenuata) dolphins (Cetacea : Delphinidae) in Golfo Dulce, Costa Rica

There are no detailed studies on the sympatric dolphin species of Golfo Dul ce. Studies in other areas have indicated that the presence of sympatric do lphins is maintained by each species using different habitats. This study d escribes the distribution of bottlenose (Tursiops truncatus) and pan-tropic al spotted (Stenella attenuata) dolphins in Golfo Dulce and its relation to habitat. A total of 428 boat surveys were conducted from September 1991 th rough December 1992. There were 529 bottlenose dolphin sightings and 200 sp otted dolphin sightings. The two species were only once observed in mixed a ggregations. There were differences in relative abundance and group size be tween both dolphin species. Bottlenose dolphins were found mostly in shallo w waters, close to shore, near rivers, and along steep marine slopes. Spott ed dolphins were associated with deeper waters and increased distance from shore. Despite apparent differences in habitat preferences, the two dolphin species were commonly observed in the same areas, albeit at different time s, due in part to seasonal variations in dolphin distribution and habitat p reference. Most notably, during the Late Wet season of 1991 bottlenose dolp hins were found in the middle and spotted dolphins at both ends of the Gulf . This pattern was almost reversed during the Late Wet season of 1992. The observed seasonal variation patterns are inadequately explained by habitat- preference arguments alone.