Estimating Abundance of Bottlenose Dolphins in North Carolina

In previous research supported by the NC Sea Grant FRG program, we estimated that 1,033 (95% CI=860-1,266) dolphins were present in the estuaries of North Carolina during July 2000 (Read et al. 2003). This was the first estimate for bottlenose dolphins in the inshore waters of North Carolina and was used in the formal NOAA Fisheries stock assessment process.

We conducted a mark-recapture survey of bottlenose dolphins in estuarine and coastal waters of North Carolina during July 2006 using photographic identification techniques. Specifically, we wanted to update our previous (2000) estimate of dolphin abundance in estuarine waters and to expand these estimates into coastal waters. We conducted two surveys: in the first survey (July 6 th to July 11 th ), we ‘marked' individual dolphins by taking photographs of their dorsal fins, and in the second survey (July 19 th to July 26 th ), we ‘recaptured' a sample of these marked individuals by taking their photographs for a second time. The two surveys were conducted a week apart, to minimize the possibility of dispersal, mortality or births between the two sampling periods. Our recent field effort will provide an updated estimate of abundance after a period of six years; recent changes to North Carolina gill net fisheries should have resulted in an increase in the abundance of dolphins during this period.

We used three vessels, operated by Duke and UNCW, simultaneously for each survey: one in the southern portion of the state (Southport, Cape Fear River, New River, Bogue Sound), a second in the central portion (Newport River, Core Sound, Neuse River, Pamlico River and southern Pamlico Sound), and a third in the northern portion (northern Pamlico Sound, Roanoke Sound, Albemarle Sound and Currituck Sound).

We attempted to take photographs of the dorsal fin of each dolphin using digital Nikon cameras. Survey effort, the number of dolphins observed, and the number of photographs taken were all comparable in the two week periods. We have begun collating and editing the nearly 11,000 digital images taken during our surveys, and will estimate abundance from the proportion of dolphins photographed in the second sample that were also observed in the first sample, using statistical techniques designed specifically for this application (see Read et al. 2003).

Supported by the North Carolina Sea Grant Fishery Resource Grant Program

For additional information contact: Kim Urian (kurian@ec.rr.com)