FISHERY BULLETIN: VOL. 77, NO 2 



spent 92''6 of its time in water < 10 m deep, and 

 was never seen in water -39 m during the 21 -mo 

 study. "Coastal dolphins" is therefore truly an ap- 

 propriate label. Various investigators have men- 

 tioned the presence of bottlenose dolphins farther 

 from shore and in deeper water populations dis- 

 tinct from the nearshore populations seen in the 

 same general geographic area (e.g., Norris and 

 Prescott 1961 ). We never saw Tursiopn in offshore 

 waters >3 km from land despite over 100 attempts 

 to find them lay boat in deeper water. Instead, a 

 different animal, the dusky dolphin. Lagenorhyn- 

 chus obacurua. was seen farther offshore during 

 the entire year ( Wiirsig in press; Wiirsig and Wiir- 

 sig see footnote 4i. 



The bottlenose dolphins studied here were al- 

 most always found in water  10 m deep; however, 

 in autumn, winter, and spring they moved into 

 deeper water for brief periods during midday. At 

 that time, they sped up and moved as fast as 24 

 km h. Because terns were seen feeding near such 

 movement, and because of the wide swath of sea 

 covered by the dolphins while rapidly advancing 

 in this manner, we believe that during these times 

 they were searching for and at times feeding on 

 aggregations of schooling fish. Although adult 

 southern anchovy are apparently not abundant in 

 the area in winter (Brandhorst et al. 1971), juve- 

 niles are present near shore in small schools at 

 this time (Brandhorst and Castello 19711. Similar 

 group feeding by bottlenose dolphins has been re- 

 ported on numerous occasions (Morozov 1970; 

 Hoese 1971; Tayler and Saayman 1972; Saayman 

 and Tayler 1973; Busnel 1973; Leatherwood 1975; 

 Hamilton and Nishimoto 1977). We could not de- 

 termine, however, whether the dolphins were ac- 

 tively herding fish into a tight unit against the 

 surface of the water, as dusky dolphins are 

 thought to do (Wiirsig and Wiirsig see footnote 4). 

 The daily periodicity of deepwater movement dur- 

 ing nonsummer indicates that the schooling prey 

 of bottlenose dolphins may be more abundant in 

 those waters during midday. However, this is at 

 present only conjecture. At any rate, no such mid- 

 day increase in speed and depth of water was evi- 

 dent in summer. During summer, southern an- 

 chovy are not found very often in coastal waters 

 less than approximately 40 m deep (Ciechomski 

 1965; pers. obs. ). Instead, these schooling fish are 

 found in deeper (and cooler) offshore waters, 

 where dusky dolphins feed on them. The present 

 population, however, does not go into these deep 

 waters in the southeast portion of (}olfo San Jose, 



and thus does not appear to have this resource 

 available in this area during summer. Whether or 

 not there is active competitive e.xclusion between 

 bottlenose dolphins and dusky dolphins is not 

 known. 



During summer afternoons, Tursiops moved 

 more rapidly and in slightly deeper water than at 

 other times of day. We do not know whether they 

 were cooperatively hunting and feeding on school- 

 ing fish as in the manner described above. It is 

 possible that the animals were moving into some- 

 what deeper water to avoid very warm water ( up to 

 25' C, pers. obs. ) present in 0-4 m depth during hot 

 summer weather, but as bottlenose dolphins live 

 in warmer water elsewhere, it is more probable 

 that their daily movement pattern was food- 

 related. 



Average speed of travel by bottlenose dolphins 

 was 6.1 km h. This represents one of the first times 

 that such a speed has been reported for an undis- 

 turbed group of wild dolphins. It is similar to speed 

 estimates made for the Indian Ocean bottlenose 

 dolphin, Tiirsiops (ultincus (9.9 km h) and for 

 Soiisa sp. (4.8 km h) during normal progression 

 (Saayman et al. 1972; Saayman and Tayler in 

 press). As an interesting sidelight, which may be 

 recognized as having general significance as more 

 population studies of dolphins unfold, bottlenose 

 dolphins studied by Saayman et al. 1 1972) were 

 usually found in deeper water while humpback 

 dolphins were almost always Ibund in shallow wa- 

 ters. While moving near shore, .south Atlantic bot- 

 tlenose dolphins moved roughly as fast as did 

 humpback dolphins, and while farther from shore, 

 they moved roughly as fast or faster than the In- 

 dian Ocean bottlenose dolphin. The same trend is 

 true for dusky dolphins in the south Atlantic wa- 

 ters ( Wiirsig and Wiirsig see footnote 4 ). A possible 

 explanation may be that nearshore searching for 

 food and feeding are more often functions of indi- 

 viduals, while deeper water prey search appears 

 often to utilize the combined sensory abilities of 

 the entire group as it actively echolocates for 

 whole schools of fish. A similar pattern of dis- 

 persed individual feeding near shore and group 

 feeding offshore has been found by Irvine, et al. 

 (see footnote 2). The nearshore search for food re- 

 quires looking in detail at the prey possibilities 

 near rocks, plants, and on the bottom, while mo.st 

 efficient search in open water is likely to be facili- 

 tated by covering as large an area as possible 

 within a small space of time. Possibly more impor- 

 tant intraspecifically is a recent suggestion that 



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