FISHERY BULLETIN: VOL 79. NO. 4 



free-ranging bottlenose dolphins may be inter- 

 individual distances and ease of access to the 

 surface for breathing. The size of the herd 

 home range of Tursiops truncatus, the frequently 

 changing group compositions, and the number of 

 bottlenose dolphins apparently residing in the 

 study area suggest that the social organization is 

 very complex. 



Food Resources and Feeding Behavior 



Striped mullet, Mugil cephalus, one of the four 

 most common fish species in the Gulf of Mexico 

 (Gunter 1941), is thought to be the mainstay of the 

 diet of bottlenose dolphins (Gunter 1942; D. K. 

 Caldwell and M. C. Caldwell 1972). Seasonal 

 movements and ranges of tagged striped mullet 

 have been determined in several areas of the gulf 

 coast (Idyll and Sutton 1952; Broadhead and 

 Mefford 1956; de Sylva et al. 1956; Ingle et al. 

 1962). Usually, the fish remained within 32 km of 

 the capture location, but there is little documenta- 

 tion of daily movements. Local commercial fisher- 

 men reported that striped mullet spawn in the 

 Gulf of Mexico in November and remain there 

 until spring. Bottlenose dolphin movements from 

 inshore to gulf waters in November thus appear to 

 be similar to those of their primary prey. 



Reports by Futch (1966) and local commercial 

 fishermen indicated that the fish movements, and 

 therefore bottlenose dolphin feeding activities, 

 may also be influenced by the tides. Apparently 

 striped mullet are often found in small groups on 

 the shallow banks of bays and estuaries during the 

 flood tide, and gather into larger schools in deeper 

 water as the tide begins to ebb. Dolphin move- 

 ments and feeding activities cannot be directly 

 correlated with fish distributions in our study 

 area, but such correlations have been reported for 

 nearshore groups of bottlenose dolphins (Wilrsig 

 and Wiirsig 1979) and humpback dolphins (Saay- 

 man and Tayler 1979). 



We surveyed potential food resources of 

 the bottlenose dolphin by interviewing and occa- 

 sionally accompanying commercial fishermen in 

 the study area. Although striped mullet were most 

 commonly caught, significant numbers of pinfish, 

 Lagodon rhomboides; sheepshead, Archosargus 

 probatocephalus; and crevalle jack, Caranx 

 hippos, were also taken in the same areas. Accord- 

 ing to fishermen, local dolphins prefer striped 

 mullet, but when striped mullet are not plentiful 

 will eat any available fish, including the hardhead 



catfish, Arius felis, which they swallow after 

 detaching the head. Opportunistic feeding by 

 bottlenose dolphins has also been noted in other 

 areas (D. K. Caldwell and M. C. Caldwell 1972; 

 Leatherwood 1975). 



The use of radio tracking data to indicate 

 feeding behavior has been proposed for the harbor 

 porpoise, Phocoena phocoena (Gaskin et al. 

 1975), and small pelagic cetaceans (see reviews by 

 Leatherwood and Evans 1979). Observations of 

 apparent feeding by tagged and untagged bottle- 

 nose dolphins in our study area, however, sug- 

 gested that respiratory intervals interpreted from 

 breaks in transmitter signals were not a valid 

 criterion to indicate foraging for this species. 

 We believe that the long dives associated with 

 foraging for pelagic species are not typical in the 

 shallow habitat of our study area, and therefore 

 transmitted dive times were relatively uniform. 

 Dive intervals ranged from a few seconds to 4 min 

 25 s, but no relations between dive intervals and 

 time of day were detectable. Lengths of hourly 

 dives averaged 30-40 s, but varied with location 

 and individual bottlenose dolphin. 



Feeding strategies of bottlenose dolphins 

 appear to vary with prey abundance and depth. 

 Large compact groups of feeding bottlenose dol- 

 phins were seen in the Gulf of Mexico, although 

 the dispersed foraging pattern reported for 

 common dolphins ( Evans 1971, 1974, 1975) and the 

 spinner dolphin, S. longirostris (Norris and Dohl 

 1980b), was also evident. When foraging through 

 shallow bays and grass flats, bottlenose dolphins 

 typically formed slow-moving, dispersed groups. 

 Humpback dolphins off South Africa (Saayman 

 and Tayler 1973, 1979) and bottlenose dolphins off 

 Argentina (Wiirsig and Wiirsig 1979) also forage 

 close to shore in small groups. Dispersed feeding 

 would be especially effective if the dolphins stayed 

 in acoustic contact, then responded to certain 

 signals by converging on a concentration of fish 

 discovered by one or more individuals. This type of 

 convergence on food sources has been proposed for 

 dusky dolphins (Wiirsig and Wiirsig 1980). We did 

 not observe dolphins rapidly converging on fish 

 schools in shallow areas, but group members did 

 occasionally move to an area where a single 

 dolphin had paused to feed. 



Shallow-water feeding was often characterized 

 by a rapid erratic chase that ended in a sudden 

 tight spin or pinwheel — the process lasting 1-5 s 

 and covering 5-20 m. Fish sometimes leaped 

 ahead of the approaching bottlenose dolphin and 



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