Edwards; Associated tunas and dolphins in eastern tropical Pacific 



679 



tial conflicting evidence, are discussed briefly as they 

 relate to the energetics models and results presented 

 here. 



Methods: Model development 

 and description 



The tuna-dolphin association 



The tuna-dolphin association occurs in the eastern 

 tropical Pacific Ocean (ETP) in a triangular region 

 roughly the size of the continental United States (~10 

 million km^), extending along the western coast of the 

 Americas from the tip of Baja California (~20°N) south 

 to Peru (~20°S) and seaward to ~140° W (Fig. 1). Total 

 productivity in this area tends to be low relative to all 

 other oceans, but high relative to other tropical oceans. 

 Ocean currents and winds generate a typical pelagic 

 environment in which areas of high productivity are 

 distributed in dynamic, nonrandom, complex patterns 

 (Fiedler et al. 1990, Fiedler 1992). 



The ETP is characterized by an exceptionally shallow 

 surface mixed layer. In contrast to other areas of the 

 equatorial Pacific where the thermocline is generally 

 150-200 m deep (Kessler 1990), the depth of the ther- 

 mocline layer throughout much of the ETP extends 

 only 50-lOOm below the surface (Fig. 1). Water tem- 

 peratures in this wind-mixed layer are quite warm 

 (25-30 °C) and oxygen concentrations are high (Wyrtki 

 1966 and 1967, Fiedler et al. 1990, Fiedler 1992). Below 

 this layer, water temperatures fall relatively rapidly 

 (from ~27 to ~15°C) through the thermocline (usual- 

 ly 5-25 m vertical extent), stabilizing again below the 

 thermocline (Fiedler et al. 1990). Oxygen concentra- 

 tions also decrease relatively rapidly through the 

 thermocline, increasing again in cold water at greater 

 depths. 



Strong dependence on warm water and on high con- 

 centrations of oxygen apparently force both tuna and 

 dolphins into this unusually shallow mixed layer. Tuna 

 must swim more or less constantly both to provide an 

 adequate flow of sufficiently-oxygenated water over 

 their gills and to locate adequate food supplies (e.g., 

 Magnuson 1978, Olson and Boggs 1986). Yellowfin 

 tuna would likely have difficulty maintaining an ade- 

 quate energy balance swimming in the colder waters 

 below the mixed layer, nor can they afford being caught 

 for long in the oxygen minima characteristic of the 

 thermocline. 



Dolphins are constrained to reside near the ocean sur- 

 face in order to breathe. Only temporary excursions 

 below the mixed layer are tolerable, both because of 

 this requirement for gaseous oxygen and because the 

 blubber layer of the tropical dolphins involved in the 

 tuna-dolphin association is too thin to maintain thermo- 



100"E 120"' 140* 



20'^ 100° SO^W 



Figure 1 



Depth of mixed layer in the area of the eastern tropical Pacific 

 Ocean characterized by associations between yellowfin tuna 

 Thunnus albacares and spotted dolphins Stenella attenuata. 

 Tuna-dolphin fishery occurs roughly in area delimited by the 

 300 m isocline. 



nuetrality in waters much colder than that in the mixed 

 layer (unpubl. estimates). This is not necessarily a 

 disadvantage, as the major prey for associated tuna and 

 dolphins (small fish and squid; Perrin et al. 1973) also 

 tend to concentrate in this upper mixed layer, at least 

 periodically throughout a 24-hour day. 



Although any individual tuna-dolphin association is 

 doubtless dynamic in the detaOs of its spatial configura- 

 tions and component individuals, the association in 

 general can be envisioned as a loose aggregation of 

 animals characterized by dolphins swimming relative- 

 ly near the ocean surface, separated vertically from the 

 tuna swimming below by only a few meters (Fig. 2). 



Although several species of dolphins and two species 

 of tuna have been found to associate in the ETP, one 

 species of dolphin (spotted dolphin Stenella attenuata) 

 and one species of tuna (yellowfin Thunnus albacares) 

 comprise the majority (> 80%) of the associations (e.g., 

 Orbach 1977, lATTC 1989). The remainder of this 

 paper assumes the "tuna-dolphin association" includes 

 only these two groups. 



Energetics models 



Both models followed the same format, using the stan- 

 dard bioenergetics approach of balancing food require- 

 ments against estimated energy costs for metabolism 

 and energy savings as growth in biomass (University 

 of Wisconsin Sea Grant 1989). The Wisconsin bioener- 

 getics model derives estimates of consumption by 



