Historical Abundance and Spatial Distribution of the Atlantic Bottlenose Dolphin (Tursiops truncatus) Along the Southeast Coast of the United States

The Atlantic coastal bottlenose dolphin (Tursiops truncatus) has a complex distribution along the southeast coast of the United States. The current population estimates for the 15 stocks of these dolphins are based primarily on a series of surveys, with the incorporation of stranding data and trends observed during a series of mass strandings caused by a morbillivirus epizootic in 1987-1988. Currently, there are no density or abundance estimates for areas south of Cape Hatteras, North Carolina, before these stranding events. From April 1982 to August 1984, aerial sightings of dolphins were recorded seasonally along the continental shelf from Cape Hatteras to Key West, Florida. These data were collected in conjunction with the Southeast Turtle Surveys (SETS) conducted by the National Marine Fisheries Service. We analyzed these datasets to determine population density and abundance across locations, seasons, and years prior to the 1987-1988 event. The average abundance estimate from 1982 to 1984 was 10,931 dolphins (CV = 0.06). Abundance estimates for 1982 (11,720, CV = 0.14), 1983 (11,393, CV = 0.09), and 1984 (14,408, CV = 0.12) were not significantly different (p = 0.05). Nor were the summer (10,324, CV = 0.14), spring (13,312, CV = 0.09), and fall (15,900, CV = 0.19) estimates. Results demonstrate that dolphins in the early 1980s followed similar migratory patterns as currently observed, including a shift of dolphins north in the summer and south in the winter. The estimated abundance in the winter of 1983 was less than estimates from replicate studies in the winter of 1992 and 1995, while estimated abundance of dolphins in the summers of the early 1980s was approximately half that estimated in a summer 2002 survey. These data present a baseline of density and abundance estimates before the mass stranding events of 1987-1988, and preliminary comparisons indicate that the mortality rate may have been low for the overall population of dolphins between Cape Hatteras and Key West.