AU and FERRYMAN: DOLPHIN HABITATS 



tions were taken), 70.7% of 167 spotted, spinner, or 

 mixed spotted and spinner dolphin schools, sighted 

 between lat. 5°N and 30°N (where most dolphin-tuna 

 associations occur), had 10 or more associated birds 

 (Tkble 6). We assume this minimum flock size in- 

 dicates associated tuna, although we did not often 

 actually see the tuna. For the same period and area, 

 30.6% of 49 common dolphin schools and only 1.6% 

 of 64 striped dolphin schools were with 10 or more 

 birds. The different percent occurrences indicate 

 that tuna are most frequently associated with spot- 

 ted and spinner dolphins and very seldom with the 

 striped dolphin. 



The bird species most numerous in offshore waters 

 with dolphin schools are boobies {Sula spp.); wedge- 

 tailed shearwaters, Puffinus pacificus; and sooty 

 terns. Sterna fuscata. Frigate birds {Fregata spp.) 

 are also closely associated with these dolphins, 

 though their average flock size is only seven (Au and 

 Pitman^). These bird species are all strongly depen- 

 dent upon tunas in their feeding. Our observations 

 are that the birds feed primarly in association with 

 the fish, not the dolphins. 



The dolphin-tuna-bird association appears to be 

 area- as well as species-specific Assuming flocks of 

 ^10 birds indicate the presence of yellowfin tuna, 

 this association seems to occur in all areas with 

 higher relative abundance of spotted and spinner 

 dolphins (Fig. 11). The association seldom occurs 

 along the Equator, or in areas outside the traditional 

 porpoise-tuna fishing grounds (roughly these are 

 waters within the triangular-shaped area whose base 

 is formed by the American coasts between lat. 25°N 

 and 15°S, and whose apex is at lat. 10°N, long. 

 150°W; see Calkins 1975 and lATTC 1979-81), even 

 though the required species of dolphins, tuna, and 



^Au, D. W. K., and R. L. Pitman. Manuscr. prep. Seabird 

 interactions with dolphins and tuna in the eastern tropical 

 Pacific Southwest Fisheries Center La Jolla Laboratory, Na- 

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

 92038. 



birds may be present there Relatively few of the 

 spotted or spinner dolphin schools sighted near the 

 Equator were with bird flocks. South of the Equator 

 and in the Central Pacific, there are abundant flocks 

 of sooty terns with fish schools, but dolphins do not 

 usually co-occur (Au and Pitman fn. 5). The most im- 

 portant area of the dolphin-tuna-bird association is 

 centered about the divergence zone near lat. 10°N, 

 an important porpoise-tuna fishing ground for 

 yellowfin tuna (primarily Area I of Figure 10). 



These areal changes in the interactions among 

 species are usually clearly apparent during north- 

 south transects across the eastern Pacific during any 

 season. An example is the July-September 1980, 

 NOAA ship Researcher transits between Manzanillo, 

 Mexico, and lat. 3°S, long. 100°W, via Clipperton 

 Island. At that time Tropical Surface Water (T > 

 25°C) extended to the Equator with transition 

 toward Equatorial Water marked by temperature 

 and salinity fronts at lat. 5°N-6°N and at the 

 Equator (Fig. 12). In the tropical waters north of the 

 front at lat. 5°N there was a conspicuous increase 

 in abundance of flocks of sooty terns, boobies, and 

 shearwaters {Puffinus spp.), all broadly centered 

 about the thermocline ridge at lat. 10°N. The larger 

 gadfly petrels (Pterodroma spp.) were also abundant. 

 All of these birds frequently flock over fish and 

 dolphins. South of this boundary was another avian 

 group, with Pterodroma leucoptera, a small petrel 

 from the Southern Hemisphere, and planktivorous 

 storm petrels (Oceanodroma spp.) predominating, 

 and peak abundances at the Equator. These latter 

 birds usually feed independently of fish and seldom 

 flock over fish and dolphins. 



Correlations among water masses, seabirds, and 

 the different cetaceans along the transect were dif- 

 ficult to quantify because of the small sample size 

 of the latter. However, the observations are suppor- 

 tive of such relationships. There were 39 dolphin 

 schools, of which 15 were unidentified, and 21 whale 

 schools along this transect. Spotted and spinner 



Table 6. — Dolphin schools associated with seabirds. 



637 



