3. Thrust power must be equal to drag force 

 multiplied by velocity 



Cd  10-' 



iplied by velocity 



Pi = 0.5  p  S  U^ 



where Pi = power (watts), 



p = water density (1.0234), 



S = surface area (0.4 L^) where L = 



length (centimeters), 

 U = velocity (centimeters/second), 

 Cd = drag coefficient. 



4. The drag coefficient is estimated using 

 Webb's (1975) formula, as 



Cd = 10 



( 



pLuy 



M / 



where /j. = water viscosity (0.0096). 



5. Assuming an oxycaloric equivalent of 3.4 

 cal/mg O2 , power in watts can be converted into 

 oxygen uptake in milligrams 02/hour by multi- 

 plying watts by 253. 



The simple model of energy consumption pre- 

 sented here makes no pretention of precision 

 because no attempt was made to accurately deter- 

 mine either the coefficient of drag or the surface 

 area of the fish. Magnuson and Weininger (1978) 

 and Magnuson (1978) did do that. We have in- 

 cluded their estimates for power consumption of a 

 40 cm, 1,003 g skipjack tuna in Figure 10. The five 

 triangles are estimates of power consumption 

 based on Magnuson's (1978) determination of drag 

 forces, the points based upon Lighthill's (1969) 

 model of thrust forces (data from Magnuson 1978: 

 table XI). Whether a sophisticated estimate of 

 power consumption or a simple one is employed, 

 the correspondence between the theoretically 

 expected and the empirically derived power con- 

 sumption is good. We take this as additional 

 evidence that our experimental values are rea- 

 sonable estimates of oxygen uptake of skipjack 

 tuna swimming straight courses at sea. 



Skipjack and other tunas are warm bodied, 

 owing to their high metabolic rates coupled with 

 large thermal inertia (Neill et al. 1976; Stevens 

 and Neill 1978). Thus, our fish undoubtedly were 

 warmer than the water in which they swam. 

 Skipjack tuna used in the laboratory experiments 

 probably had core-temperature excesses on the 

 order of 2°-4° C (cf. Stevens and Fry 1971; Neill et 



FISHERY BULLETIN: VOL. 79, NO. 1 



al. 1976); the just-caught fish, being more active, 

 may have had core temperatures as much as 10° 

 C above ambient water temperature (cf. Stevens 

 and Fry 1971). Interpretation of our results has 

 not been complicated by consideration of the 

 difference between tissue and environmental 

 temperatures, because metabolism of skipjack 

 tuna has been shown to be virtually independent 

 of temperature (Gordon 1968; Chang et al. foot- 

 note 4). 



CONCLUSION 



Our findings emphasize the unique evolution- 

 ary position of the skipjack tuna (and, by exten- 

 sion, other tunas) among fishes. The skipjack 

 tuna epitomizes what Stevens and Neill (1978) 

 have termed "energy speculators": forms that 

 "operate to maximize energy gain by gambling 

 large energy expenditures ... on the expectation 

 of proportionately large energy returns." The 

 skipjack tuna's "standard" metabolic rate is two 

 to five times that of typical fishes of similar size. 

 Moreover, the skipjack tuna is relatively ineffi- 

 cient in its use of oxygen and food-energy for 

 swimming (at least at low speeds) and it dies 

 at O2 levels still well above those lethal to 

 other fishes. Clearly, the skipjack tuna's ability 

 to sustain high levels of activity has not been 

 achieved without substantial physiological cost. 



ACKNOWLEDGMENTS 



This study was initiated by Reuben Lasker and 

 John Hunter, both of the Southwest Fisheries 

 Center La JoUa Laboratory, National Marine 

 Fisheries Service, NOAA. We are grateful for 

 their early involvement and support. 



We thank J. R. Brett, Fisheries Research Board 

 of Canada, Nanaimo, British Columbia; E. D. 

 Stevens, University of Guelph, Ontario; and 

 D. Weihs, Technion, Haifa, Israel for reviewing 

 the manuscript. 



LITERATURE CITED 



Anonymous. 



1965. Oxygen studies in relation to catching tuna. Com- 

 mer. Fish. Rev. 27(12):28-29. 



Baldwin, W. J. 



1970. Oxygenating device for live-bait wells. J. Fish. 

 Res. Board Can. 27:1172-1174. 

 BARKLEY, R. A., W. H. NEILL, AND R. M. GOODING. 



1978. Skipjack tuna, Katsuwonus pelamis, habitat based 



46 



