Abstract- Yellowfin tuna Thun- 

 nus albacares are commonly found 

 associated with other species, espe- 

 cially sharks, birds, and dolphins, in 

 the eastern tropical Pacific. Data 

 from the purse seine fishery there, 

 collected during 1974 and 1975, indi- 

 cate that sharks occurred with tuna 

 in 40% or more of the purse seine 

 sets made around floating logs. Most 

 other species, including rays, billfish, 

 and small tunas, usually occurred in 

 less than 10% of the sets. The asso- 

 ciation rate of these bycatch species 

 declined progressively from log- to 

 school- to porpoise-associated sets. 

 This, together with their behavior as 

 understood, suggests that at least 

 some of these species stay with the 

 tuna as much as they can. Such be- 

 havior would be like that of poly- 

 specific associations in which two or 

 more species travel together for 

 foraging and protective advantages. 



Polyspeciflc Nature of 



Tuna Schools: Shark, Dolphin, 



and Seabird Associates 



David W. Au 



La Jolla Laboratory, Southwest Fisheries Science Center 

 National Marine Fisheries Service. NOAA 

 P.O. Box 271. La Jolla. California 92038 



Manuscript accepted 25 February 1991. 

 Fishery Bulletin, U.S. 89:343-354 (1991). 



Tuna are often found associated with 

 other species, a behavior that is also 

 seen among other schooling, herding, 

 or flocking animals. While it is often 

 convenient to think of any of these 

 species as being effectively mono- 

 specific (single species) in behavior, 

 there is growing awareness that poly- 

 specific, enduring assemblages are 

 common among higher animals. Such 

 associations, comprised of several 

 species that feed, interact, and travel 

 together for periods of time, are not 

 merely aggregations of animals along 

 routes or points of common attrac- 

 tion. The specific interactions in- 

 volved are not well understood, even 

 though field observations have been 

 intensive in some cases. 



Mixed-species schools of fishes are 

 frequently observed and caught. 

 While usually seen as chance associa- 

 tions (e.g., Itzkowitz 1977), some 

 species in these schools may obtain 

 enhanced protective and foraging 

 benefits (Ehrlich and Ehrlich 1972, 

 Barlow 1974, Wolf 1987). Many such 

 schools may therefore be polyspe- 

 cific, and represent purposeful inter- 

 actions. 



Polyspecific bird flocks are com- 

 mon (Moynihan 1962; Morse 1970, 

 1977; Munn and Terborgh 1979). In 

 some cases, over a dozen species 

 move together through different en- 

 vironments with the composition of 

 individuals and species apparently 

 changing little (Hutto 1987). Dia- 

 mond (1981) pointed out that poly- 

 specific flocks often consist of similar 

 core species that stay together, per- 



haps for years, maintaining flock 

 characteristics and holding group 

 territories. There are leader species, 

 which are usually conspicuous, and 

 follower species (Caldwell 1981, 

 Greig-Smith 1978). 



Mammals, with even more complex 

 behaviors, abound with examples of 

 polyspecific associations. Such asso- 

 ciations have been noted in bats, 

 cetaceans, often in ungulates, and 

 particularly in the behaviorally ver- 

 satile primates. Among the latter, 

 species pairs of certain cercopithecid 

 monkeys may be together 50% of the 

 time, and it often appears that one 

 species initiates and terminates the 

 association, and uses the other to 

 better discover food and detect 

 danger (Struhsaker 1981, Cords 

 1987). Locally, these polyspecific 

 associations are species-specific; 

 other sympatric monkeys are ap- 

 parently ignored and always by 

 themselves (Gautier-Hion et al. 

 1983). This suggests that benefactor 

 species are actively searched for by 

 follower species. These associations, 

 which vary in species composition by 

 area, may last for days or months. 

 Group territories may be defended. 

 Similar behaviors have been ob- 

 served in cebid monkeys, some of 

 whose polyspecific associations ap- 

 pear to be permanent (Terborgh 

 1983). Interestingly, monkey species 

 that appear to initiate and benefit 

 from the polyspecific associations 

 can be observed sometimes in front, 

 sometimes at the rear, of traveling 

 groups. 



343 



