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Fishery Bulletin 90(4), 1992 



development and comparisons of similar energy 

 fluxes in the completed models generated several in- 

 teresting observations with potentially significant 

 implications. 



ticipate. The similarity in feeding preferences and 

 probable similarity in feeding behaviors provides one 

 explanation and suggests that tuna are more likely to 

 follow dolphins than the reverse. 



Hydrodynamics and body length 



Length frequencies of the tuna and dolphins in a typ- 

 ical association show a surprisingly strong overlap 

 between age-III yellowfin and neonate- 1st yr dol- 

 phins. Both animals begin their respective years at 

 ~85cmTL, and complete the year at ~125cmTL 

 (Fig. 4). This is significant for two reasons. First, this 

 size range comprises the majority of the yellowfin 

 tuna found associated with dolphins (Fig. 3F. Second, 

 both animals have relatively stiff torpedo-shaped bodies 

 with stiff fins and carangiform swimming behavior. 

 Because theory predicts that optimum swimming 

 speeds (the speed at which the least energy is consumed 

 for a given distance covered) of geometrically-similar 

 swimmers will be comparable (Weihs 1973, Webb 

 1975), the similar body forms and swimming behaviors 

 of the tuna and the dolphins imply that optimum swim- 

 ming speeds will also be similar for either animal of 

 a given length. 



Swimming speeds of sonic-tagged yellowfin tuna 

 measured in situ show that individual undisturbed 

 yellowfin, of the size most often found associated with 

 dolphins, choose in their natural environment to swim 

 on average at their predicted optimum cruising speed 

 (e.g., yellowfin 90-lOOcmFL swim at 100-130cm/sec; 

 Holland et al. 1990). Because yellowfin tuna tend to 

 associate in schools of like-sized individuals, the 

 expected speed of the tuna group is similar to the 

 expected speed of the individuals involved. 



In contrast, tracking studies (Perrin et al. 1979) of 

 spotted dolphins in the ETP indicate that dolphin 

 schools swim on average not the speed most efficient 

 for the majority of the individuals in the school (i.e., 

 ~160-170cm/sec for large adults) but the speed most 

 efficient for the neonate-lst yr animals (~120 cm/sec; 

 Fig. 4). 



These observations imply that yellowfin associating 

 with dolphin schools may do so at little or no added 

 hydrodynamic cost. The associated fish, unlike larger 

 or smaller sizes of yellowfin, need swim neither faster 

 nor slower than their apparently preferred optimum 

 in order to maintain an association with dolphins. 



The observation that associating with dolphins may 

 cost tuna little does not explain why the tuna par- 



' Figure 3 includes fish from all areas of the fishery, not just the 

 offshore areas where most dolphin fishing occurred during the years 

 these data were collected, causing dolphin-fish distribution to be 

 skewed to left. 



Who follows whom 



The higher forage requirements of dolphins both in- 

 dividually and as an association imply that dolphins 

 following tuna, particularly single dolphin schools 

 following single tuna schools, would fall far short of 

 meeting their daily energy requirements. Dolphin 

 schools might avoid this energy deficit by switching 

 from one tuna school to another, but they would have 

 to switch consistently from recently-successful to 

 soon-to-be-successful schools of foraging tuna. This 

 frequent switching could be difficult because it would 

 likely involve periods of searching at speeds greater 

 than sustainable by the young dolphins, in order to find 

 new tuna schools (and new patches of forage) faster 

 than the patches could be found by the current tuna 

 school. 



Measurements of muscle mass and estimates of 

 power-time curves for various sizes of spotted dolphins 

 imply that the relatively small muscle mass of neonate- 

 lst yr dolphins probably cannot sustain speeds much 

 faster than their predicted optimum for any extended 

 length of time (unpubl. data). If searching for new 

 schools of tuna requires sustained accelerated swim- 

 ming, the young dolphins could have trouble keeping 

 up with the rest of the school. Because it is unlikely 

 that dolphins, as nursing mammals and highly social 

 animals, would simply leave their young behind, switch- 

 ing frequently from one tuna school to another may not 

 be a practical option. 



The disparity in feeding requirements implies that, 

 while dolphins would probably be disadvantaged by 

 having to rely upon tuna to locate sufficient prey, the 

 tuna could recognize an advantage by following dol- 

 phins. The fish would then be associating with another 

 predator that is searching for the same prey, but which 

 must encounter that prey either more often or in con- 

 siderably larger patches than required by the tuna, per 

 time period. 



However, the greater need of the dolphins for food 

 implies concomitantly that competition for resources, 

 if those resources are limited, could be fierce. The 

 schooling characteristics of the predators and prey, 

 coupled with feeding behaviors and differing sizes of 

 the predators, provide one possible explanation for the 

 ability of the smaller yellowfin tuna under some cir- 

 cumstances to persist in this potentially competitive 

 association despite the dolphin's greater size, and need 

 for food. 



