Abundance, Ranging Patterns, and Social Behavior of Bottlenose Dolphins (Tursiops truncatus) in an Estuarine Terminus

Bottlenose dolphins (Tursiops truncatus) exhibit intricate social and ranging behaviors, which can subdivide stock boundaries. This study examined the abundance, ranging patterns, and social structure of estuarine bottlenose dolphins within the North Inlet-Winyah Bay (NIWB) estuarine system in South Carolina. This system supports a small group of dolphins that we hypothesize may have a unique social structure given the semi-isolated nature of the habitat at the northern terminus of an extensive series of interconnected estuaries. Mark-recapture photo-identification surveys were conducted from spring of 2011 through winter of 2012. Using the closed capture and open population models in the programs MARK, CAPTURE, and RELEASE, the bottlenose dolphin abundance in the NIWB system was estimated to be 117 individuals (95% CI 92 to 142) during the warm season (May through October) and 74 individuals (95% CI 48 to 100) during the cold season (December through February), with 11 dolphins sighted in both seasons. Eighty-three percent of the sightings with known males were mixed-sex groups. Dolphins associated nonrandomly with conspecifics, based on the analyses using the program SocProg, and four social communities were identified within the NIWB estuary. Kernel density methods in ArcMap were used to describe the distribution of each social community, revealing overlapping ranges but distinct core use areas for each. The membership and distribution of these communities was consistent with historical surveys in the NIWB system since 1997. This population falls toward one end of the continuum of bottlenose dolphin communities, with a small resident population and distinct yet overlapping social communities, smaller than normal core use areas, and a general lack of sexual segregation among adults. It is possible that other species could exhibit similar patterns along the continuum of coastal and estuarine habitats, and this study could serve as a guide for future research in these systems.