AU and FERRYMAN: DOLPHIN HABITATS 



The dolphin-tuna-bird association is one manifesta- 

 tion of community difference that is both striking 

 and of ecological interest. The distribution of this 

 association is notable in that it seems coincident with 

 both the main habitats of spotted and spinner 

 dolphins and the distribution of "surface" yellowfin 

 tuna in the eastern Pacific (see Figure 11, Shingu 

 et al. 1974, and Suzuki et al. 1978). Since it is 

 primarily these dolphins that are associated with 

 birds and with yellowfin tuna, the geography of the 

 dolphin-bird association also defines that of the 

 dolphin-yellowfin tuna association. This association 

 of birds and fish with dolphins occurs in all tropical 

 waters, including the southern Subtropical Water 

 during the southern summer. It is apparently rare 

 in equatorial waters of the eastern Pacific, in the cen- 

 tral and western Pacific (Myazaki and Wada 1978; 

 Au et al. fn. 2), and in the eastern tropical Atlantic 

 (Levenez et al. 1980). In the central Pacific the same 

 bird species found with spotted and spinner dolphins 

 in the eastern tropical Pacific can be abundant 

 (Gould 1974), and sooty terns especially, are fre- 

 quently associated with small tunas, but these are 

 most likely skipjack tuna (Murphy and Ikehara 1955; 

 Waldrom 1964; Hida 1970; Blackburn and William 

 1975). Apparently those flocks seldom accompany 

 dolphins or schools of larger yellowfin tuna. Yellowfin 

 tuna and dolphins seldom seem to associate outside 

 the eastern tropical Pacific 



The obvious feeding activity often seen in these 

 joint aggregations of birds, spotted and spinner 

 dolphins, and tuna suggests that these species have 

 similar food and foraging requirements. Our obser- 

 vations indicate that the mammals and fish are not 

 tightly associated in the aggregations (see Au and 

 Perryman 1982) and probably feed independently 

 (see also Norris and Dohl 1980a). However the tuna, 

 birds, and spotted dolphin (at least) do appear to be 

 feeding at the same time. Both the tuna and spot- 

 ted dolphins feed on epipelagic fish and on squids 

 (Perrin et al. 1973; Olson 1982), but the spinner 

 dolphins feed differently and may forage more at 

 night (Perrin et al. 1973; Norris and Dohl 1980b); 

 though they are active in these feeding aggregations, 

 they may not be directly associated with the tuna. 

 Judging from the associations of bird and dolphin 

 species, only spotted and spinner dolphins frequently 

 find it advantageous to feed with yellowfin tuna. Fur- 

 thermore the distribution of this association suggests 

 that the necessary kind and behavior of prey that 

 is likely the basis of the association appears charac- 

 teristic of tropical, but not equatorial, waters. It oc- 

 curs especially where a shallow thermocline may con- 

 strain the yellowfin tuna to the surface layer with 



the dolphins, a complex interaction between environ- 

 ment and physiology (Sharp 1978) that may cause 

 the phenomenon known as "surface tuna". Finally 

 the distribution of the bird-dolphin association in- 

 dicates that the dolphin-tuna association is 

 characteristic of areas of higher school density of 

 spotted and spinner dolphins. High population den- 

 sities of both dolphins and yellowfin tuna, and 

 suitable prey are therefore likely requisites for joint 

 dolphin-tuna schools. The dolphin-tuna association 

 is a feature of the most productive tuna fishing zones 

 of these tropical seas. In such rich areas, feeding tac- 

 tics to exploit clumped prey could lead to multi- 

 species aggregations of predators, as explained by 

 Schoener (1982). 



ACKNOWLEDGMENTS 



This paper was reviewed by the following persons 

 whose help is much appreciated: David E. Gaskin, 

 Kenneth S. Norris, Carleton Ray, Gunter R. Seckel, 

 Paul N. Sund, James G. Mead, Frank G. Alverson, 

 Richard Pimentel, John A. McGowan, Gary T. 

 Sakagawa, Jay Barlow, Andy Dizon, Albert C. 

 Myrick, William Perrin, Steve Reilly Tim Smith, Paul 

 E. Smith, Rennie S. Holt, Eric D. Forsebergh, and 

 Michael D. Scott. 



We thank the officers and crews of the NOAA 

 ships involved in this study, especially those of the 

 Townsend Cromwell and David Starr Jordan, for 

 their cooperation and help. We particularly thank the 

 biological technicians who manned the watches 

 through high-powered binoculars, for their keen 

 observations and provoking discussions at sea. 

 Among them we especially appreciate the work of 

 Gary Friedrichsen, Phillip Unitt, James Lambert, 

 Dale Powers, Robert Pitman, James Cotton, and 

 Scott Sinclair. 



We sincerely thank Lorraine Prescott and her 

 staff for careful typing of this manuscript and Ken 

 Raymond, Roy Allen, and Henry Orr for drafting the 

 illustrations. 



LITERATURE CITED 



Alverson, F. G. 



1981. Comments on the distribution of spotted, spinner, com- 

 mon and striped dolphin in the tropical Pacific Ocean. In 

 P. S. Hammond (editor), Report on the Workshop on lUna- 

 Dolphin Interactions, p. 109-124. Inter-Am. Trop. Ibna 

 Comm., Spec Rep. 4, App. 5. 



ASHMOLE, N. P., AND M. J. ASHMOLE. 



1957. Comparative feeding ecology of seabirds of a tropical 

 oceanic island. Yale Univ. Peabody Mus. Nat. Hist. Bull. 

 24:1-131. 



641 



