DOLPHIN HABITATS IN THE EASTERN TROPICAL PACIFIC 



David W. K. Au and Wayne L. Perryman^ 



ABSTRACT 



Research-ship surveys by the Southwest Fisheries Center provided information on the distributions of 

 spotted, spinner, striped, and common dolphins in the eastern tropical Pacific The main surveys were 

 conducted from January to March during 1976, 1977, 1979, and 1980. Two ships were used per survey, 

 and together they overlapped most areas in the eastern Pacific where dolphins and yellowfin tuna are 

 jointly fished by purse seiners. 



The spatial distribution of sightings and of sighting rate of these species show a complementarity to 

 their patterns, although there is a broad overlap. Spotted and spinner dolphins occurred primarily in tropical 

 waters north of the Equator, but also in the seasonal tropical waters south of the Galapagos Islands. These 

 dolphins were relatively infrequent along the Equator, off Costa Rica, and northern South America. Com- 

 mon and striped dolphins tended to be more frequent in these same areas of less frequent spotted and 

 spinner dolphins. 



The differences in habitats of these two species pairs can be described in oceanographic terms. Spotted 

 and spinner dolphins are primarily in Tropical Surface Water, centered off southern Mexico and extending 

 westward along lat. 10°N, where thermocline "ridging" and relatively small annual variations in surface 

 temperature are features. Common and striped dolphins appear to perfer equatorial and subtropical waters 

 with relatively large seasonal changes in surface temperature and thermocline depth and with seasonal 

 upwelling. 



The species composition of various areas in the eastern tropical Pacific supports the contention of two 

 major communities. South of where spotted and spinner dolphin schools predominate (along with Risso's, 

 bottlenose, and rough-toothed dolphins), striped and common dolphins and also pilot whales become in- 

 creasingly important. Observations along the Equator also suggest a fauna different from that of the 

 Tropical Surface Water that is most characterized by spotted and spinner dolphins. 



A trophic basis to these faunal differences is suggested by the interactions with fish and birds. Assum- 

 ing the birds indicate co-occurring tuna, only the spotted and spinner dolphins are commonly found with 

 fish. The distribution of these dolphins as they co-occur with bird flocks and tuna indicates that this inter- 

 specific association is confined primarily to the TVopical Surface Water and is a characteristic feature 

 of its epipelagic community. 



The eastern tropical Pacific Ocean supports produc- 

 tive tuna fisheries as well as an abundant and diverse 

 cetacean fauna. Ibna fishermen there take advan- 

 tage of the fact that tuna and dolphins frequently 

 swim together. In the "porpoise-tuna" fishery for 

 yellowfin tuna, Thunnus albacares, spotted and spin- 

 ner dolphins, Stenella attenuata and S. longirostris, 

 are temporarily caught by purse seiners in order to 

 take the associated tuna. Striped and common 

 dolphins, S. coeruleoalba and Delphinus delphis, are 

 caught to a lesser extent for the same reason. These 

 dolphins suffer incidental mortality in the fishery 

 and, because of the resulting concern, the Southwest 

 Fisheries Center has been studying their populations 

 to better advise on their management (Smith 1983). 

 Learning about their habitats is one aspect of these 

 studies. .<■ 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92038. 



Perrin (1975a), using the information collected 

 mainly aboard tuna seiners, first showed that the 

 geographic distributions of spotted, spinner, and 

 striped dolphins in the eastern Pacific are extensive, 

 stretching westward from the American coasts past 

 long. 145°W at about lat. 10°N and also dipping 

 south and southwest of the Galapagos Islands. Evans 

 (1975) showed that the common dolphin occurs off- 

 shore of Central America to about long. 112°W and 

 also along the Equator, westward past the Galapagos 

 Islands. Recent summaries of available information 

 (Au et al. 19792; gcott 1981; Perrin et al. 1983) have 

 shown that the distributions of these dolphins are 

 even more extensive than originally perceived. In- 

 deed the species have been reported from localities 

 across the entire Pacific (Alverson 1981). 



Manuscript accepted January 1985. 

 FISHERY BULLETIN: VOL. 83. NO. 4, 1985. 



2Au, D. W. K., W. L. Perryman, and W. F Perrin. 1979. Dolphin 

 distribution and the relationship to environmental features in the 

 eastern tropical Pacific Admin. Rep. LJ-79-43, 59 p. Southwest 

 Fisheries Center La Jolla Laboratory, National Marine Fisheries 

 Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



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