AU and FERRYMAN: DOLPHIN HABITATS 



Table 4. — Percent composition of species as encountered in two areas' during the January- 

 March research cruises. 



'Areas are shown in Figure 10. 



^Species in mixed schools were tabulated separately. 



^Normal approximation to binomial distribution. 



east of the Galapagos Islands, i.a, off Peru. Unlike 

 during the January-March cruises, physical ocean- 

 ography was the primary task on most of these 

 transects, hence the ships did not usually divert 

 course toward the schools, and many schools could 

 not be identified. Nevertheless some idea of the 

 species compositions can be obtained. The observa- 

 tions (Ihble 5) showed that pilot whales and Risso's 

 and bottlenose dolphins were frequently encountered 

 species during October- December off Peru, and com- 

 mon dolphins were often seen near the coast. On the 

 equatorial transects, between long. 85°W and 

 110°W, striped and common dolphins were the 

 characteristic species. The common dolphin was seen 

 most often near the Galapagos Islands. Pilot whales 

 were relatively abundant during May- July 1981 in 

 this equatorial section. West of long. 110°W along 

 the Equator, pilot whales again were the most fre- 

 quently encountered species. Interestingly, sightings 

 of Eraser's dolphins, Lagenodelphis hosei, and 

 "blackfish" (probably Peponocephala electra) were 

 also relatively frequent, especially between long. 

 110°W and 145°W. These two species often school 

 together and appear to prefer equatorial waters 

 (Perryman et al.'*). In the next section, another 



distinctive feature of equatorial waters will be 

 brought out. 



Dolphins, Birds, and Tuna 



A conspicuous feature distinguishing the dolphin 

 communities is the difference in the species-specific 

 association with tunas. In the eastern tropical 

 Pacific, spinner dolphins and especially spotted 

 dolphins are found associated with "surface" 

 yellowfin tuna. It is these two species, therefore, that 

 are mainly affected by the porpoise-tuna fishery 

 (Smith 1983). Surface tunas occur at the sea surface 

 and can be caught by purse seine, trolling, and pole- 

 and-line gear. "Deep tunas" of the same species are 

 caught by longline gear, generally in and below the 

 thermocline Since these surface tunas drive food to 

 the surface, making it available to certain seabirds 

 (Ashmole and Ashmole 1957; Murphy and Shomura 

 1972), a reliable indication that tuna are accompany- 

 ing a dolphin school is the presence of a bird flock. 

 Birds are the most important cue used by fishermen 

 to locate dolphin-tuna schools. 



Birds do not occur equally among the different 

 dolphin species. During the 1977, 1979, and 1980 

 January-March cruises (when the best bird observa- 



■•Perryman, W. F., D. W. K. Au, and S. Leatherwood. Manuscr. 

 prep. Melon-headed whale, Peponocephala electra (Gray, 1946) 

 (with notes on the pygmy killer whale Feresa attenuata). South- 



west Fisheries Center La Jolla Laboratory, National Marine 

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



635 



