Abstract. - The basis for the 

 curious association between yellow- 

 fin tuna Thunnus albacares and 

 spotted dolphin Stenella attenuata in 

 the eastern tropical Pacific Ocean 

 has never been explained. Considera- 

 tion of the bioenergetics of the 

 associated tuna and dolphins sug- 

 gests that the association may be 

 based on the combined effects of a 

 shallow thermocline, overlapping 

 size (length) ranges of associated 

 yellowfin and young dolphins, con- 

 gruent diets, hydrodynamic con- 

 straints on swimming speeds of 

 dolphin schools, and social (care- 

 giving) behavior of dolphins. Insights 

 developed during construction and 

 exercise of comparative bioenerget- 

 ics models for the tuna and dolphin 

 suggest that tunas are more likely to 

 follow dolphins than dolphins to 

 follow tunas, and that the strength 

 of the association in a given area may 

 be related to oceanographic condi- 

 tions affecting prey distribution and 

 abundance. 



Energetics of associated tunas 

 and dolphins in tlie eastern tropical 

 Pacific Ocean: A basis for the bond 



Elizabeth F. Edwards 



Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA 

 P,0 Box 271, La Joila, California 92038 



Manuscript accepted 29 July 1992. 

 Fishery Bulletin, U.S. 90:678-690 (1992). 



In the eastern tropical Pacific Ocean, 

 yellowfin tuna Thunnus albacares 

 and spotted dolphin Stenella attenu- 

 ata form an association strong 

 enough that the fish can be captured 

 by capturing the associated dolphins 

 (e.g., Orbach 1977). The dolphins, 

 easier to locate than the tuna, form 

 the sighting cue for locating tuna 

 schools. Despite chases lasting on 

 average about half an hour (and occa- 

 sionally as long as 2-3 hours) the fish 

 tend to remain with the dolphins 

 throughout. Eventually the dolphins 

 tire and can be encircled, along with 

 the associated tunas, with a purse- 

 seine net. 



Although the subject of substantial 

 conjecture (e.g., Perrin 1969, Orbach 

 1977, Au and Pitman 1986, Au 1991), 

 no definitive explanation exists for 

 the association, perhaps in part 

 because conjectures to date have 

 been qualitative rather than explicitly 

 quantitative. Quantifying the advan- 

 tages or disadvantages of the associa- 

 tion in terms of the energetics of its 

 component groups holds promise for 

 helping understand the bond, be- 

 cause such quantification can more 

 readily expose conceptual errors, 

 lead to unexpected insights, and form 

 the basis for testable hypotheses. Ex- 

 pressing relationships in terms of 

 energy flow (e.g., cost of finding 

 food, cost of reproduction, feeding re- 

 quirements, etc.) has often proved a 

 useful format for developing under- 

 standing of biological phenomena. 

 Following this precedent, I present 

 here bioenergetics models for both 

 tunas and dolphins in a "typical" 



association in the eastern tropical 

 Pacific Ocean. I use these models to 

 estimate feeding rates of tuna and 

 dolphins, and discuss implications 

 concerning the ecological advantage 

 to tuna (or dolphins) when associated 

 with dolphins (or tuna). 



Estimates of forage requirements 

 predict that tuna and dolphins should 

 experience severe competition under 

 some circumstances of prey distribu- 

 tion and abundance, but perhaps not 

 under others. Observations of over- 

 laps in sizes between associated tuna 

 and dolphins and of morphological 

 similarities between the animals have 

 implications for the importance of 

 swimming energetics to the associa- 

 tion. 



These estimated forage require- 

 ments and considerations about 

 swimming energetics are discussed in 

 terms of their implications for deter- 

 mining which component (tuna or 

 dolphins) controls the association, 

 how the competition might be miti- 

 gated, when the association might be 

 more likely to occur, and how these 

 factors might be used to locate large 

 yellowfin tuna unassociated with 

 dolphins. The last point is important 

 in relation to current interest in 

 eliminating the practice of "dolphin- 

 fishing" in the eastern tropical 

 Pacific Ocean. "Dolphin-fishing" in- 

 volves location and capture of tuna 

 schools by locating and capturing 

 associated dolphin schools; as air- 

 breathers, the dolphins are more 

 easily sighted than the tuna due to 

 the dolphin's surface activity. Other 

 explanations for the bond, and poten- 



678 



