Edwards. Associated tunas and dolphins in eastern tropical Pacific 



685 



WL,p = (F, + Ua)*Csp, 



where ^sp^ = 0.20 and 24Ua = 0.07 for yellowfin tuna; 

 Fa = 0.125 and Ua = 0.07 for spotted dolphins^s. 



Growth Specific rate of growth (calories available for 

 growth • calories of animal" ^ • day " ^ ) was estimated as 



Discussion: IVIodel impiications 



The strict "result" of exercising the models is estima- 

 tion of food consumption by yellowfin tuna and spotted 

 dolphins of various sizes. This information alone is not 

 particularly helpful in furthering our understanding of 

 the tuna-dolphin bond. However, the process of model 



Gsp = C,p-(R,p + SDA,p+WL3p). 



Total calories available for growth (Gcai) is 



Goal = Gsp*CALan. 



Total grams wet- weight biomass available for growth 

 (Gwwg) is then 



Gwwg = Gca|/CD. 



The formulas and parameter values presented above 

 produce reasonable model estimates of the various 

 energy fluxes for both yellowfin tuna and spotted 

 dolphins (Edwards 1992). 



Resuits: Estimated consumption 



Despite the apparent similarity between yellowfin tuna 

 and spotted dolphins in food composition (prey type 

 and size-''), estimated food requirements for tuna and 

 dolphins differ considerably. Estimated food require- 

 ments for individual tuna and dolphins imply that each 

 dolphin requires 5-10 times more food per day than 

 each yellowfin tuna, depending on the sizes of the tuna 

 and dolphin being compared (Fig. 5). In a "tj^pical" 

 association of 200 dolphins and 500 tuna, total dolphin 

 requirements are still 2-5 times higher than total tuna 

 requirements per time-period (Fig. 6), despite the 

 greater number of tuna than dolphins. 



^^ Based on the relative assumed nondigestible portions of tuna diet 

 items by analogy to similar items (Cummins and Wuycheck 1971). 



-''Based on measurement of non-fecal excretion by carnivorous fish 

 (Brett and Groves 1979). 



-^Together these processes probably account for 15-20% of ingested 

 food energy in spotted dolphins, as found for other small marine 

 mammals eating fish (Shapunov 1973, Ronald et al. 1984, Ashwell- 

 Erickson and Eisner 1981, Lavigne et al. 1982, and references 

 therein.) 



-"Diet is undoubtedly an important factor in the tuna-dolphin associa- 

 tion, as associated yellowfin tuna and spotted dolphins apparent- 

 ly have nearly identical feeding preferences (Perrin et al. 1973). 

 Stomach contents of co-occurring tuna and spotted dolphins con- 

 sisted primarily of small pelagic schooling fish (e.g., mackerel Auxis 

 thazard) and squid of similar types and sizes. 



4 6 8 10 12 14 



AGE (year since birth) 



16 



Figure 5 



Estimated annual ration for individual spotted dolphins 

 Stenella attenuata ages birth-18 yr, and yellowfin tuna Thun- 

 nus albacares ages II-III, occurring in mixed associations in 

 the eastern tropical Pacific Ocean. 



CONSUMPTION vs. 

 SCHOOL SIZE 



400 

 NUMBER IN SCHOOL 



800 



Figure 6 



Estimated annual ration for schools of yellowfin tuna Thun- 

 nns albacares and spotted dolphins Stenella attenuata occur- 

 ring in mixed associations in the eastern tropical Pacific Ocean 

 (ETP). Solid circles indicate number of individual dolphins and 

 individual tuna in a typical mixed association. Ration estimates 

 for schools were based on average observed size-frequency 

 distributions of tuna and dolphin in the ETP. 



