680 



Fishery Bulletin 90(4). 1992 



Figure 2 



Idealized representation of a typical association of yellowfin tuna Thunnus albacares and spotted dolphins 

 Stenella attenuata. Sizes and size-frequencies of tuna and dolphins are representative. Overlap in sizes be- 

 tween age-Ill yellowfin and neonate-lst yr dolphins (85-125cmTL) is emphasized. 



iteratively fitting an energetics equation for growth in 

 body weight over time, to observed growth-rate curves 

 derived from field samples of the organism in question. 

 When the model growth curve simulates well the 

 observed growth curve, the other fluxes estimated by 

 the model are presumed to be reasonably accurate. 



Specific rates (calories of flux ■ calories of animal " ' 

 • day" ' ) of energy flux were estimated based on data 

 derived from various sources for individual tunas and 

 dolphins as a function of size. Rates of energy flux for 

 schools of dolphins and tuna were estimated as the sum 

 of weight-specific estimates for individuals in each 

 group. 



Costs of reproduction were ignored for both yellowfin 

 and spotted dolphins; in the yellowfin model because 

 the model focuses on the sizes of yellowfin associated 

 with dolphins, which tend to be relatively immature 

 fish. Spawning activity in yellowfin does not occur in 

 fish much smaller than 80cm, and increases slowly to 

 the maximum activity in fish larger than ~ 150 cm 

 (Joseph 1963). Energy costs of reproduction for spotted 

 dolphins were omitted because the fraction of preg- 

 nant, lactating, or pregnant and lactating females in 

 a typical school at any time is relatively small (~25%; 

 see School composition). 



Some of the energetics parameters reported here for 

 spotted dolphins are based on morphological measure- 



ments from 4 dolphin specimens from the ETP; 3 

 spotted dolphins measuring 81-189cm total length 

 (TL), plus 1 spinner dolphin Stenella longirostris 114 

 cm in length. The 81 cm individual was a very late-term 

 fetus carried by the 189cm animal. Although this sam- 

 ple is very small, all morphological measurements from 

 these 4 animals fall well within the bounds of size- 

 related regressions of morphological characteristics 

 derived subsequently for a sample of 34 spotted 

 dolphins measuring 74-215cmTL (tip of rostrum to 

 fluke notch) (unpubl. data). 



School composition The yellowfin model addresses 

 only those sizes of yellowfin found associated with 

 dolphins (relatively large age-II and age-Ill fish, 55-125 

 cmTL; Fig. 3). Based on catch records from the fisheiy, 

 an "average" association was assumed to include 500 

 yellowfin with an age composition of 65% age-II and 

 35% age-Ill fish per school (Ashley Mullin, lATTC, c/o 

 Scripps Inst. Oceanogr., La Jolla; unpubl. data from 

 commercial fishery). 



Dolphin school composition was assumed to reflect 

 the apparent age distribution of the spotted dolphin 

 population, which in turn was assumed to appear as 

 the length (age) distribution of dolphins collected dur- 

 ing purse-seining operations in the ETP (Smith 1979, 

 Barlow and Hohn 1984; A. Hohn, NMFS Southwest 



