IRVINE ET AL.: MOVEMENTS AND ACTIVITIES OF ATLANTIC BOTTLENOSE DOLPHIN 



(1979); however, Shane (1980) reported more 

 movements against than with the tidal current, 

 and Leather wood <1979) reported seeing no rela- 

 tion between the bottlenose dolphin movements 

 and tide. As indicated in Figure 7, considerably 

 more bottlenose dolphins in our study were mov- 

 ing with than against the tidal currents, although 

 the numbers of groups moving with and against 

 the currents were almost equal. This observation 

 suggests that larger groups of animals more often 

 moved with the tide. The data were not analyzed 

 statistically because a large number of animals 

 seen were in the "milling" category, moving across 

 the current or in irregular patterns. 



The tidal current in the study area varied 

 with physiography, but was strongest in narrow 

 channels and passes, at times exceding 5 km/h. In 

 Palma Sola Bay, a shallow bay with one access 

 channel, bottlenose dolphins more often moved 

 against than with a sometimes strong current. 

 The animals rarely reversed direction in the 

 Palma Sola Bay channel but often swam near the 

 sides of the channel possibly because current 

 velocity was reduced there. 



Movement and activity patterns were not influ- 

 enced by other environmental conditions in any 

 recognizable way. Possible sun orientation, 

 as reported for the common dolphin, Delphinus 

 delphis, by Pilleri and Knuckey (1968) and Evans 

 (1971), was not observed, although the restric- 



tions of movements dictated by area physiography 

 may have masked such effects. We did not detect 

 the distinctive day-night dive interval patterns 

 noted for other dolphin species by Evans (1971, 

 1974, 1975), Leatherwood and Evans (1979), 

 Leatherwood and Ljungblad (1979i, Norris 

 and Dohl (1980b), and Wursig (in press, see 

 footnote 11). 



Social Structure 



Available evidence suggests that the study area 

 was occupied by a single discrete social unit 

 or "herd." Groups containing naturally marked 

 bottlenose dolphins that were seemingly not a 

 part of this herd were repeatedly observed north, 

 west, and south of the study area. These observa- 

 tions suggest that the bottlenose dolphin popula- 

 tion on Florida's west coast may be composed of 

 a number of distinct herds inhabiting limited 

 geographical areas. Overlapping home ranges 

 have also been proposed for coastal bottlenose 

 dolphins off southern California (Leatherwood 

 and Reeves 1978). 



The uneven dispersal of sightings of bottlenose 

 dolphins of different age and sex classes within the 



"Wiirsig, B. 1976. Radio tracking of dusky porpoises 

 I Lagenorhynchus obscurus) in the South Atlantic, a preHminary 

 analysis. ACMRR Scientific Consultation on Marine Mammals, 

 Bergen, Norway, 21 p. 



1 oa 



May- 

 July 



1975 



Augus t- 

 Oc tobe r 



No vemb e r - 

 January 



1976 



February- 

 April 



May- 

 July 



TOTAL NUMBER 

 OF DOLPHINS: 

 W I TH AGAI NST MILLING 



180 B 



71 C 



127 7't 69 ,136 103 If*?, 65 '•6 87 1 75 29 181, 120 97 96 



 1 1 ' • • * ! I ' i • ' ' •   



TOTAL NUMBER OF DOLPHINS IN EACH COLUMN 



215 D 



FIGURE 7.— Seasonal relationship of bottlenose dolphin move- 

 ments relative to tidal flow in selected areas. A) Palma Sola 

 Bay, Bl Longboat Pass, C) Sarasota Pass, and D) North Sarasota 

 Bay. Histograms show percent of dolphins sighted m each area 

 that were swimming with the current, against the current, or 

 judged to be milling: the numbers in each category and area 

 indicate total number of dolphins seen. 



679 



