FISHERY BULLETIN: VOL. 79, NO. 4 



study area suggested the presence of several 

 "subherds" (Figure 5). The bulk of the tag sight- 

 ings in the southern part of the study area were of 

 the same subadult male groups, whereas cow-calf 

 pairs were more commonly sighted in the north 

 (Wells et al. 1980). However, some identifiable 

 bottlenose dolphins were sighted in all parts of the 

 study area and were associated with as many as 20 

 other tagged animals. Overall, we interpret the 

 resightings of tagged animals to indicate 

 that different age and sex classes may have 

 favored different areas, but that social relation- 

 ships were still maintained among members of the 

 entire herd. 



Bottlenose dolphins from adjacent areas that 

 occasionally approached animals from the study 

 herd remained for only a few minutes, and social 

 interactions between the different groups 

 were not observed. Various species of macropods, 

 primates, and ungulates have similar social 

 organizations; subgroups join to form discrete 

 social units ("mobs," "troops," or "herds") that 

 exhibit spatial fidelity and have little interaction 

 with conspecifics outside the social unit (see re- 

 view by Wilson 1975). 



The size of the herd within the study area was 

 difficult to determine. Boat survey results were 

 variable, and information on bottlenose dolphin 

 migration was unavailable. However, the lack 

 of sightings from outside the study area, the 

 observed movements of visually and radio-tagged 

 dolphins, and an increase in tag sightings as the 

 number of tags installed increased (Table 1) all 

 suggested that the captures involved a discrete 

 population of bottlenose dolphins. Assuming a 

 constant population size with no emigration 

 or immigration, we estimated that the local pop- 

 ulation contained 102 bottlenose dolphins (95% 

 confidence limits = 90-117), using a Lincoln Index 

 (Overton 1971) and a basis of 35 survey days 

 (165 h), from 9 May through 9 July 1976 (Table 1). 

 Until more data are available about this assump- 



tion, however, our population estimate must be 

 viewed with caution. 



Assuming that the group home range was 

 85 km^ (Wells et al. 1980), the estimated popula- 

 tion size suggests a density of 1.3 bottlenose 

 dolphins /km^. Aerial surveys indicate densities of 

 0.23-0.68 bottlenose dolphin/km^ in other coastal 

 areas of the southeastern United States (see 

 review by Leatherwood 1979). Monthly mean 

 bottlenose dolphin densities derived from surface 

 survey data of Shane ( 1980) were 1.5-5.1 bottlenose 

 dolphins/km^ near Port Aransas, Tex., whereas 

 aerial density estimates at the same area were 2.6 

 bottlenose dolphins /km^ (Barhametal. 1980). The 

 reasons for the large discrepancies between aerial 

 and surface survey density estimates are unclear. 

 Some animals may be counted more than 

 once from boats, or perhaps observers in rapidly 

 moving aircraft do not see all bottlenose dolphin 

 groups. In any case, the differences in density 

 estimates suggest that population estimates of 

 bottlenose dolphins in Florida based on aerial and 

 surface surveys may not be directly comparable. 



It is not known whether the study herd re- 

 mained intact throughout the year or changed 

 composition seasonally. Tagged bottlenose dol- 

 phins that were not sighted for long periods 

 (see Figure 4) may have lost their identifying 

 tags, or may have left the study area, as did some 

 bottlenose dolphins in Argentina (Wiirsig and 

 Wiirsig 1977). 



Fewer than 15'7f of the field sightings were 

 of solitary bottlenose dolphins, which is an indica- 

 tion of the high degree of gregariousness of free- 

 ranging bottlenose dolphins (Figure 8). Average 

 group size (n = 688 groups) varied from 2 to 6 

 about an overall mean of 4.8 bottlenose dolphins/ 

 group (SE = 0.16; Figures 3A, 8). During summer 

 1975 and early summer 1976, groups of >40 

 unmarked bottlenose dolphins, probably from 

 adjacent herds, were observed <1 km offshore in 

 the Gulf of Mexico and within 1 km of the northern 



Table l. — Sightings of marked and unmarked dolphins and population size estimates, during periods from December 



1975 to July 1976. 



Tagged dolphins were found dead on 5 March and 3 May 1976: population estimates are adjusted to account for survey days after these 

 animals were dead. 

 ^Includes 95 unmarked dolphins (in 5 groups) sighted on the periphery of the study area. 



680 



