FISHERY BULLETIN: VOL. 74, NO. 1 



When estimates of the true stasis energy rela- 

 tions are finally available, they can be easily 

 incorporated into the model. 



Probably the most difficult process to define, 

 estimate, and measure is that of growth. The 

 energy requisite to growth (Eg) can be esti- 

 mated minimally as the biomass gain per time 

 period as converted to calories. This is a highly 

 unsatisfactory method because of the many 

 energy requiring steps between ingestion of a 

 food organism and the consequential deposition 

 of the materials assimilated into the living bio- 

 mass of the growing organism (Phillips 1969). 



One slight change in the accepted method- 

 ology of bioenergetic accounting which we will 

 make is in our definition of specific dynamic 

 action (SDA). If one is willing to accept that the 

 SDA contributed little other than heat to the feed- 

 ing organism, then it can be defined as the loss 

 of energy due to the inefficiency of the digestive 

 processes, including cost of transport, deamina- 

 tion, biosynthesis, and related processes. The 

 rate of inefficiency (percent of SDA energy with 

 respect to total ingested energy) is variable in 

 most animals studied as a function of feeding 

 level (Warren and Davis 1967) and environmental 

 conditions (Warren 1971). In our definition of 

 SDA we do not include the unavailable portion 

 of foodstuffs. 



For our purposes we will assume that growth 

 of yellowfin tuna in the CYRA is relatively con- 

 tinuous with respect to season or environmental 

 state. There are several assumptions involved in 

 this basic tenet which require some discussion. 

 Tunas are highly endothermic animals, and 

 Carey and Teal (1966) have shown the presence 

 of a relatively high efficiency heat exchange 

 (conservation) mechanism in tunas. This sug- 

 gests that tunas are likely to be somewhat inde- 

 pendent of ambient temperatures in that the 

 temperature variability encountered within the 

 core of these fishes is likely less than the ambient 

 variability. Their large mass (>1 kg) would con- 

 tribute to thermal stability over a wide ambient 

 change (Neill and Stevens 1974). 



Observations of temperature dependent activ- 

 ity indicate a lower activity as temperature de- 

 creases in small yellowfin tuna (<50 cm, <2.5 kg) 

 at a Qio of near 2 (Neill, pers. commun.). This size 

 of yellowfin tuna is rarely encountered in the 

 CYRA at temperatures below 23°C and is found 

 aggregated on the warm side of the north-south 

 surface temperature cline including this tempera- 



ture, indicating some preference for tempera- 

 tures near 23°C. Preliminary studies of effects of 

 the environmental characteristics on the abun- 

 dance and availability of 40- to 70-cm yellowfin 

 tuna in the CYRA indicate a direct relationship 

 between the 23°C isotherm depth of the av- 

 erage number of fish per school, and the overall 

 availability of these fish to surface fishing gear 

 (Inter-American Tropical Tuna Commission 

 1975). 



All this is emphasized to indicate the limited 

 range of temperatures likely to be affecting the 

 metabolic rates of yellowfin tuna as compared 

 to that affecting smaller species without the 

 complex stabilization mechanisms (heat ex- 

 changers, etc.) as is the typical situation in fishes. 



The relative activity, mobility, and distribution 

 with respect to temperature of yellowfin tuna 

 can be used as supportive background for as- 

 suming a relatively stable growth energy avail- 

 ability as they developed, bringing us to the con- 

 clusion that a first approximation of the SDA 

 can be made with respect to the energy equiva- 

 lent to the biomass change on a daily basis. 

 From studies discussed by Paloheimo and Dickie 

 (1966) and Warren and Davis (1967) on several 

 species and estimates by Kitchel et al.^ for K. 

 pelamis, it appears that SDA probably accounts 

 for 30-40% of the total consumed calories which 

 could be part of the growth process. We have, 

 therefore, assumed that Eg is going to equal the 

 equivalent caloric value of the tissues plus the 

 SDA which will be given by the relation 



SDA 



(Biomass change in grams per day) 



where, if 1 g is calorically equivalent to 1.46 kcal 

 (Kitchell et al. see footnote 2) then 



3 

 Eg = — Biomass change (grams) (1.46 kcal/g) 



= 2,190 kcal /kg growth. 



Smit (1965) has provided the mathematical 

 basis for our determinations of energy output 

 and caloric requirements due to swimming. He 

 shows that: 



(Meg S) (143 X 103) gcm2 (1) 



Power 



3,600 



^Kitchell, J. F., W. H. Neill, and J. J. Magnuson. Bio- 

 energetics of skipjack tuna, Katsuwonus pelamis. Manuscr. 



40 



