DISTRIBUTION AND ABUNDANCE OF THE BOTTLENOSE DOLPHIN, 

 TURSIOPS TRUNCATUS (MONTAGU, 1821), IN VIRGINIA^ 



Robert A. Blaylock^ 



ABSTRACT 



The distribution and abundance of the bottlenose dolphin, Tursiops truncatus, was examined by conduct- 

 ing aerial surveys of the Chesapeake Bay mouth and nearshore coastal waters of Virginia in 1980 and 

 1981. Bottlenose dolphin density was estimated using line transect methods and a 4-term Hermite 

 polynomial was chosen to model the detection function. Six surveys in the Chesapeake Bay mouth resulted 

 in an average density estimate of 0.159 dolphins/km". Ten surveys along the southern Virginia coast 

 produced an average density estimate of 3.446 bottlenose dolphins/km' within 2 km of shore. Average 

 bottlenose dolphin abundance in the Chesapeake Bay mouth and along the southern Virginia coast was 

 estimated at 340 dolphins (±104, 95% C.I.). An estimate of 0.208 bottlenose dolphins/km" along the 

 northern Virginia coast is tenuous because only one survey was conducted there. Dolphin sightings were 

 distributed uniformly along the southern Virginia coast with the exception of some clustering of herd 

 sightings at the capes bordering the Chesapeake Bay mouth. The percentage of calves per herd aver- 

 aged 7.5% in Chesapeake Bay mouth, 4.3% in the southern coastal area, 9.0% in the northern coastal 

 area, and peaked in June. Five of seven bottlenose dolphins identified by unique dorsal fin shapes in 

 1980 were resighted in 1981, suggesting seasonal residency of individuals. 



Of the 23 cetacean species occurring along the 

 Virginia coast (Leatherwood et al. 1976; Blaylock 

 1985) the bottlenose dolphin, Tursiops truncatus 

 (Montagu, 1821), is the only cetacean found near 

 shore regularly and in large numbers. However, 

 there are few quantitative data available to assess 

 the abundance, distribution, and seasonal occur- 

 rence of Tursiops truncatus (hereafter referred as 

 Tursiops) in Virginia coastal waters. 



Those bottlenose dolphins occurring seasonally in 

 Virginia are believed to form part of a population 

 distributed from northern North Carolina to New 

 Jersey during the summer. This population was the 

 focus of a sporadic fishery along Hatteras Island, 

 NC from circa 1797 to 1929, the primary products 

 of the fishery being hides and oil (True 1891; Town- 

 send 1914; Mead 1975). From cumulative catch 

 records, Mitchell (1975) estimated a historical popu- 

 lation size of 13,748-17,000 dolphins and inferred 

 annual migration from biannual peaks in catches 

 during the fall and spring. True (1891) earlier sug- 

 gested a north-south migration, reporting on fish- 

 ermen's observations that bottlenose dolphins were 

 usually seen traveling south in the fall and north in 



the spring, with only a few remaining near Hatteras 

 during the summer. 



Analysis of large-scale aerial surveys along the 

 northern and mid-Atlantic U.S. coast revealed a 

 bimodal longitudinal Tursiops distribution, inter- 

 preted as separate nearshore and offshore areas of 

 abundance (CETAP 1982). These areas represent 

 the habitats of two distinct morphological types of 

 T. truncatus. The offshore type is slightly larger at 

 the onset of physical and sexual maturity than the 

 nearshore types and ultimately attains a greater 

 size^. 



An important finding of the CETAP surveys was 

 the presence of multiple latitudinal peaks in coastal 

 sightings indicating discontinuities in the north- 

 south distribution of nearshore Tursiops (CETAP 

 1982). These observations indicate either an uneven 

 distribution of nearshore Tursiops or the presence 

 of multiple coastal populations or subpopulations. 

 However, a recent epidemic suggests that the 

 U.S. east coast Tursiops may represent a single 

 stock. 



Tursiops mortalities south of North Carolina 

 during autumn of 1987 increased sharply with the 

 apparent emigration of Tursiops from Virginia 



'Contribution No. 1464 from the Virginia Institute of Marine 

 Science. 



^Virginia Institute of Marine Science, Gloucester Point, VA 

 23062. 



'J. G. Mead, Division of Mammals, Smithsonian Institution, 

 Washington, DC, pers. commun. June 1978. 



Manuscript accepted June 1988. 



FISHERY BULLETIN; VOL. 86, NO. 4, 1988. 



797 



