Au: Polyspecific nature of Thunnus albacares in eastern tropical Pacific 



353 



Polyspecific tuna and dolphins 



The tuna-dolphin association of the ETP is the associa- 

 tion most similar to the polyspecific associations studied 

 among primates and terrestrial birds, in that specific 

 species appear to forage together without strong or ob- 

 vious interactions. However, virtually all porpoise-fish 

 sets are attended by birds that appear to be closely 

 following the feeding tuna (Au and Pitman 1986). That 

 and the high 80-90% successful set ratio of these sets 

 compared with the other set types (Table 1) suggest 

 that dolphins do not commonly feed with birds that are 

 not also with tuna. These specific dolphins that feed 

 with tuna are ecologically successful (i.e., abundant, 

 especially relative to other non-tuna-associated dolphin 

 species; Au and Pitman 1988). And since these dolphins 

 are largely found within the habitat of surface- 

 schooling yellowfin in the ETP (see Allen 1985 for 

 distributions), and not the converse, it could be that 

 dolphins exploit the feeding behavior of tuna more 

 often than the reverse. If so, this would agree with 

 what seems typical in polyspecific associations, that it 

 is the behaviorally more-adaptive species that takes 

 advantage. Considering further the opportunistic, 

 casual nature typical of polyspecific associations, one 

 should not be surprised that tuna and dolphins are not 

 intimately associated in many other seas and even in 

 certain areas within the ETP. Moreover, if either the 

 tuna or the dolphins were to be overexploited where 

 they are associated, dire (or propitious) consequences 

 to the other of the pair need not be expected. 



Acknowledgments 



I thank M. Hall, P. Arenas, S.B. Reilly, N.W. Bartoo, 

 J.L. Squire, G.T. Sakagawa, S. Smith, and two 

 anonymous reviewers for their helpful critiques, and 

 0. Escamilla for assisting in extracting data from the 

 original logsheets. 



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