Abundance, distribution, and social behavior of estuarine bottlenose dolphins (Tursiops truncatus) in northern South Carolina

Background/Question/Methods

Accurate and updated stock assessments are imperative for management of protected species.  Bottlenose dolphins (Tursiops truncatus) exhibit intricate social and ranging behavior, which can obscure stock boundaries.  Eight estuarine system stocks, extending from Central Florida through North Carolina, have been defined along the Atlantic coast.  However, these stocks continue to be reviewed, as knowledge of estuarine dolphins is incomplete.  We conducted this study to determine the abundance, ranging behavior, and social structure of estuarine bottlenose dolphins within the North Inlet-Winyah Bay (NIWB) estuarine system in northern South Carolina, which became part of a newly defined estuarine stock in 2013.  We performed mark-recapture analyses via photo-identification and through the use of the programs MARK and CAPTURE to determine the seasonal abundance and used the program SOCPROG to identify associations and assess the presence of social communities.  Additionally, we used ArcMap and Hawth’s Tools to determine the home range of each community using the minimum convex polygon and kernel density methods.

Results/Conclusions

The estimated bottlenose dolphin abundance in the NIWB system was 117 individuals (95% CI 92−142) during the warm season (May through October) and 63 individuals (95% CI 49−77) during the cold season (December through February).  Bottlenose dolphins sighted in the NIWB estuarine system associated non-randomly with conspecifics, and we identified four social communities within the NIWB estuary.  We also determined that dolphins preferentially used specific areas of the NIWB estuary.  Further, these heavily used areas differed between the warm and cold seasons, which likely indicates dolphin movement out of the NIWB estuary during the cold season.  Seasonal changes in dolphin abundance and ranging behavior likely indicate movement out of the NIWB estuarine system with possible impacts on the identification and differentiation of bottlenose dolphin stocks.  Baseline data is necessary for a better assessment of the Northern South Carolina Estuarine System Stock and habitat management/protection, and this study provides abundance, ranging behavior, and social community estimates for such purposes.