FISHERY BULLETIN: VOL. 83, NO. 4 



Figure 11.— Distribution of bird-associated spotted and spinner dolphin schools during the January-March research cruises. Schools with 



10 or more birds are assumed to be with tuna. 



dolphns did occur coincidentally with bird flocks (and 

 probably with tuna) in the warmer tropical waters 

 centered about lat. 10°N, but also were seen at the 

 Equator, though without associated bird flocks. 

 Striped and common dolphins occurred in coastal 

 and equatorial waters, without bird flocks. These lat- 

 ter dolphins were the characteristic species on the 

 equatorial transect of this same cruise (see Ikble 5, 

 col. 4), where only one of the schools seen, a mixed 

 spotted and spinner dolphin school, had an 

 associated bird flock. The distinct change in the bird 

 fauna south of lat. 5°N to species that do not forage 

 commensally with fish suggests there are changes 

 with water masses in the nature of epipelagic prey 

 and how the top predators forag& 



DISCUSSION 



It should not be surprising that the two major 

 divergence zones of the eastern Pacific, near lat. 

 10°N and along the Equator, are important features 

 of the oceanic habitats of eastern Pacific cetaceans. 

 Enriched by the effects of wind and the major zonal 

 currents (Brandhorst 1958; Cromwell 1958; Reid 



1962; Wyrtki 1966), the zones are evident areas of 

 enhanced biological production (Blackburn 1966, 

 1976; Blackburn et al. 1970; King 1974; Parsons et 

 al. 1977; Brinton 1979). that are important to tunas 

 (Calkins 1975; Blackburn 1965; Blackburn and Laurs 

 1972; Blackburn and Williams 1975; Sund 1981) and 

 cetaceans as discussed above These two zones are 

 not qualitatively the same, however; the strong, 

 shallow thermoclines that have been related to suc- 

 cessful porpoise-tuna fishing (Green 1967; Miller et 

 al.^) and to aggregations of dolphins and baleen 

 whales (Rovnin 1969; Volkov and Moroz 1976) are 

 characteristic of the lat. 10°N zone, but not of the 

 Equator. Similarly the oxygen minimum layer, noted 

 by Perrin et al. (1976) to be correlated with the 

 distribution of the spotted dolphin, occurs only north 

 and south of the Equator in the eastern Pacific 

 Equatorial waters are characterized by shallow, weak 

 (<2°C/10 m) thermoclines, due to upwelling and the 

 Equatorial Undercurrent, and cool surface temper- 



«Miller, F. R., C. J. Orange, R. H. Evans, and K. A. Bliss. 

 Manuscr. prep. Analysis of environment related to tuna fishing 

 in ETR Inter-American Tropical TUna Commission, La Jolla, CA 

 92038. 



638 



