Edwards Associated tunas and dolphins in eastern tropical Pacific 



687 



WHEN SHOULD TUNA ASSOCIATE WITH DOLPHIN? 



Figure 7 



Decision table predicting conditions under which yellowfin 

 tuna Thunnus albacares and spotted dolphins Stenella attenu- 

 ata should (or should not) associate. 



Avoiding competition for food 



As is characteristic of pelagic ocean systems, both 

 predators and prey in the ETP occur in clumped 

 distributions. Individuals occur in schools or aggrega- 

 tions separated by (often vast) distances devoid of other 

 individuals. The prey, like the yellowfin tuna, will tend 

 to occur in schools of like-sized individuals with similar 

 swimming speeds. Aggregations of tuna and dolphins 

 will typically consist of dolphins of assorted sizes ac- 

 companied by tuna of approximately one size. The 

 feeding strategy of the predators will involve searching 

 for a clump of prey, simultaneous (or nearly so) arrival 

 at the prey patch by both tuna and dolphins, and 

 repeated incursions by individuals of both predator 

 groups into the clump of prey wherein prey are seized 

 and swallowed whole individually. 



Associated yellowfin tuna may be able to mitigate 

 this direct competition with dolphins for food on the 

 basis of the difference in size between the fish and the 

 feeding adult dolphins (~100cm vs. 200cmTL). Be- 

 cause the tuna are smaller, they have smaller maximum 

 stomach capacity (~400g wet wt for age-2 yellowfin, 

 ~1100g wet wt for age-3 yellowfin; Olson and Boggs 

 1986) compared with spotted dolphins (~2000g wet wt 

 in adults; Bernard and Hohn 1989). Even if the smaller, 

 presumably more-agile tuna could seize individual prey 

 only as fast as the dolphins and no faster, they would 

 satiate more quickly than the dolphins. 



As both groups would begin feeding at the same time, 

 when the prey concentration was maximum, the tuna 

 at any time would be relatively closer than the dolphins 

 to satiation, given the observed (average) relative pro- 

 portions of tunas and dolphins in a typical association. 



The tuna would be filling their stomachs while the prey 

 were still relatively dense. Depending on the size of the 

 prey patch, dolphins might never succeed in satiating, 

 even though the tuna had their fill. Even if the prey 

 patch was sufficiently limited that neither group 

 achieved satiation, the tuna would always be relative- 

 ly more full at any given time. Thus, although the 

 dolphins require more prey overall, the tuna could suc- 

 ceed competitively by satiating sooner (being relative- 

 ly more successful) during any given prey encounter. 

 However, it may not always be to the tuna's ad- 

 vantage to associate with dolphins, even given this 

 scenario. The benefit (or not) can be assessed by 

 evaluating the relative advantages of associating or 

 not, given the range of possibilities for prey spatial 

 distribution and abundance. 



WInen should the association occur? 



The possibilities can be summarized in a simple deci- 

 sion table (Figure 7). At the extremes, prey abundance 

 may be either low or high and any given abundance 

 may be either homogeneously distributed (frequent) or 

 clumped (rare). The possibilities for locating prey are 

 that (1) dolphins are more adept than tuna, (2) both are 

 equally adept, or (3) dolphins are less adept than tuna. 

 The advantages for tuna to associate with dolphins can 

 be assessed for each cell in the table. 



Consideration of each cell in the table suggests that 

 tuna may benefit from associating with dolphins only 

 when (1) prey are distributed in rare patches and (2) 

 dolphins are more adept than the tuna in finding these 

 patches. This would be true regardless of prey concen- 

 tration within the patches, because whenever tuna and 

 dolphin associate they will compete for food. If tuna 

 are more adept than dolphins at finding food, then 

 there will be no foraging-related advantage for the tima 

 to associate with their competitors. The tuna would be 

 able to find food more easily on their own than by 

 following dolphins, and would not have to risk sharing 

 these resources once located. If the tuna and dolphins 

 are equally adept, there is still no advantage, for the 

 same reason. 



If the prey are distributed in relatively small but 

 numerous patches, there is still no advantage for tuna 

 to associate with dolphins, again because the spatial 

 frequency of schools would produce a relatively high 

 probability of tuna encountering the food on their own 

 without risk of sharing with their competitor. In addi- 

 tion, when patches are small, the tuna would be espe- 

 cially disadvantaged by having to compete with 

 dolphins because the presence of dolphins could pre- 

 vent the tuna from satiating, despite the fact that the 

 tuna would still be relatively more full than the dolphins 

 when the patch had been exhausted. 



