FISHERY BULLETIN: VOL. 78, NO. 3 



ered for almost as many flood tide collections as 

 ebb tide." (Copeland 1965). Thus, dolphins may 

 have developed a special technique for taking ad- 

 vantage of the concentration of food which appar- 

 ently occurs in Aransas Pass during ebb tide. A 

 second possibility is that dolphins which maintain 

 a stationary position against a strong tidal cur- 

 rent, as dolphins were frequently observed to do, 

 were resting. Resting captive dolphins reportedly 

 face against the current in the tank and use slow 

 beats of the fluke to maintain position (McBride 

 and Hebb 1948). 



The relationship between time of day and dol- 

 phin movements was stronger in Morris and 

 Cummings Cut than in the lower sections of the 

 study area (Table 3). The observation that most 

 dolphins moved northward early, all directions at 

 midday, and southward late in the day was sub- 

 stantiated statistically (Table 3). The higher 

 Cramer's V" values for the time of day-direction 

 relationship as compared with the tide-direction 

 relationship indicated that time of day influenced 

 direction of movement more than tide did in Mor- 

 ris and Cummings Cut. 



The agreement between individual and group 

 movement data in the present study showed that 

 group size was not a significant variable. In con- 

 trast, Irvine et al. (footnote 3) found that more 

 dolphins moved with the tide than against it, be- 

 cause group sizes were larger for dolphins moving 

 with the current. They found approximately the 

 same number of groups moving against the tide as 

 with it. 



In summary, dolphin movements were signifi- 

 cantly related to tide and time of day in some 

 sections of the study area. The relationship with 

 tide was strongest in Aransas Pass where tidal 

 effects were most pronounced and resulted in a net 

 outflow. The relationship with time was strongest 

 in Morris and Cummings Cut where tidal effects 

 were diluted. 



Individual Distribution Patterns 



The concentration of sightings of individual 

 dolphins in portions of the study area (Figure 3, 

 Table 4) indicated that some individuals included 

 parts of the study area in their home ranges (as 

 defined by Burt 1943). Caldwell (1955) provided 

 the first evidence for T. truncatus having a home 

 range. Later, Caldwell and Caldwell (1972) pro- 

 posed a dumbbell-shaped home range for T. trun- 

 catus which included two home ranges connected 



by a traveling range. At least two interpretations 

 of my data are possible: each dolphin had one home 

 range that extended outside of the study area, and 

 some dolphins used different portions of their 

 home ranges on a seasonal basis; or each dolphin 

 had two or more home ranges connected by travel- 

 ing ranges, and one home range partially or com- 

 pletely coincided with the study area and was used 

 seasonally. Irvine and Wells (1972) and Saayman 

 et al. (1973) indicated that the bottlenose dolphins 

 which they studied exhibited localized move- 

 ments, but did not specify home ranges. Wells et al. 

 (in press) defined a home range of 85 km^ for the 

 dolphin herd which they studied, and they stated 

 that it was inhabited year-round by at least some 

 of the dolphins. 



Eight of the 10 dolphins whose distributions 

 were considered in Table 4 apparently recognized 

 a boundary between Regions A and B. The bound- 

 ary might have been a physical feature (e.g., deep 

 channels versus large, shallow bays), or it might 

 have been a social barrier separating social 

 groups. Two dolphins. Teaser and Cloud, passed 

 over the apparent boundary between Regions A 

 and B on a regular basis. Segregation according to 

 sex in the two regions is unlikely because Short 

 Triangle, Raggedy Ann, and Teaser were females 

 and used Region A, Region B, and Regions A and 

 B, respectively. 



Southpaw and Half Fin and the other dolphins 

 with unique dorsal fins that were observed only in 

 the Gulf of Mexico may be part of a population of 

 T. truncatus that is socially or otherwise segre- 

 gated from inshore T. truncatus. The existence of 

 offshore T. truncatus has been discussed (Mitchell 

 1975; Odell et al.^). However, it is not known 

 whether offshore and inshore T. truncatus are 

 reproductively isolated and, if they are, why 

 this is so. 



The February 1977 sightings of Short Triangle 

 and the October 1976 sighting of Thick Fin indi- 

 cated that they may have spent the winter in the 

 Gulf of Mexico or in other bay systems reached by 

 traveling through the gulf. The June and July 

 1979 sightings of Thick Fin at Port Aransas and 

 Port O'Connor showed that T. truncatus does move 

 long distances in fairly short periods of time. Wiir- 

 sig and Wiirsig (1977) documented a round-trip 

 movement of 600 km for T. truncatus, but this 



>'Odell, D. K., D. B. Siniff, and G. H. Waring (edi- 

 tors). 1975. Final report: Tursiops truncatus assessment 

 workshop. Rosenstiel School of Marine and Atmospheric Sci- 

 ences, University of Miami, Miami, Fla., 141 p. 



600 



