FISHERY BULLETIN: VOL. 85, NO. 1 



when ambient temperature increases to keep active 

 metabolic rate temperature independent. 



The effect of water temperature (20°-25°C) on 

 heart rate was variable (Qio's ranged from 6.71 to 

 1.82). The mean values (±95% confidence intervals) 

 of 3.74 (±1.9), 2.56 (±0.35), 2.14 (±0.17) for skip- 

 jack tuna, yellowfin tuna, and kawakawa, respec- 

 tively, are not significantly different from each other 

 and are close to the Q^q (2-3) found for the effect 

 of temperature on the heart rate of lingcod, Ophio- 

 don elongatus, (Stevens et al. 1972). 



Why Are The SMR's of Tunas So High? 



Also shown in Figure 1 is the SMR-body weight 

 relationship for sockeye salmon at 20°C, taken from 

 Brett and Glass (1973). Even with the differences 

 in the slopes of the lines, it is still apparent that 

 tunas have remarkably high SMR's. In the follow- 

 ing paragraphs, I argue (as did Stevens and Neill 

 1978; Stevens and Dizon 1982) that tunas are 

 "energy speculators", gambling high rates of energy 

 expenditure against high rates of energy return. I 

 also hypothesize that tunas' physiology and anatomy 

 have evolved to increase maximum sustainable (i.e., 

 aerobic) metabolic rates (MMR's) and that high 

 SMR's are an inevitable consequence of this ability. 

 In other words, high SMR's are a result of anatom- 

 ical and physiological adaptations (primarily large 

 gill surface areas) associated with high MMR's. 

 Tunas have high MMR's and high SMR's, whereas 

 sluggish bottom-dwelling flatfish (e.g., Platichthys 

 Jleusus) have low MMR's and low SMR's (Duthie 

 1982). Active fish like salmon have MMR's and 

 SMR's intermediate between these two extremes 

 (Brett 1972). 



Advantages of High Maximum 

 Metabolic Rates 



Tunas live in the open ocean, an environment 

 which provides no shelter and where patches of 

 forage are widely scattered (Sund et al. 1981). In 

 this environment, high sustainable swimming speeds 

 (i.e., high MMR's) enable tunas to travel quickly be- 

 tween food patches and to search large volumes of 

 water in the least amount of time. Also, tunas have 

 been shown to have very high rates of digestion 

 (Magnuson 1969), which is advantageous for species 

 that must be able to fully exploit a food patch when- 

 ever one is found. Since digestion is an energy con- 

 suming process, high rates of oxygen delivery and 

 blood flow are required for high rates of digestion. 



Because the pelagic environment provides tuna 



no place to hide and rest while repaying an oxygen 

 debt, the ability to quickly metabolize lactate is also 

 advantageous. High MMR's therefore allow tuna to 

 rapidly repay an oxygen debt when one is accum- 

 ulated. Tuna's only defense against predators such 

 as blue marlin, Makaira nigricans, is presumably 

 a burst of maximum (i.e., anaerobic) swimming. 

 Prey capture by tunas also must involve some high 

 speed swimming. Coulson (1979) has argued that the 

 ability to achieve high rates of anaerobic glycolysis 

 allows vertebrate ectotherms to successfully com- 

 pete with vertebrate endotherms, which are capable 

 of much higher rates of aerobic metabolism. How- 

 ever, most vertebrate ectotherms, whether terres- 

 trial or aquatic, must spend long quiescent periods 

 to metabolize lactate (Coulson et al. 1977). Yet tunas 

 have the ability to metabolize some of the highest 

 muscle lactate levels ever recorded in vertebrates 

 in only a few hours (Barrett and Connor 1964; 

 Hochachka et al. 1978). Other teleosts may take as 

 long as 24 h to recover from severe exercise even 

 though they accumulate lower white muscle lactate 

 concentrations (Black et al. 1961; Wardle 1978). 

 Tunas' vascular heat exchangers appear to also aid 

 the rapid movement of lactate from the white muscle 

 where it is produced to the red muscle where it is 

 presumably metabolized (Stevens and Carey 1981). 

 Although using different terminology, McNab 

 (1980) citing terrestrial vertebrates and Pauly (1981) 

 citing fishes, both argue that given certain con- 

 straints, high MMR's are advantageous because 

 rates of somatic and gonadal growth are dependent 

 upon rates of delivery of oxygen and substrate to 

 the tissues. Indeed, Pauly (1981) has shown that the 

 growth rates of fishes are proportional to, and per- 

 haps controlled by, gill surface area. Furthermore, 

 he suggests that it is maximum rate of oxygen 

 delivery to the tissues, rather than food supply, that 

 limits growth rates and that species like tunas, 

 which have the largest gill surface areas, have the 

 highest growth rates. Koch and Wieser (1983) have 

 shown that fish reduce activity levels during periods 

 of gonadal growth. Tunas cannot make this trade 

 off. For tunas, it is probably necessary to maintain 

 a high rate of activity during gonadal synthesis 

 which, in turn, requires respiratory and cardiovas- 

 cular systems capable of delivering oxygen and 

 metabolic substrates to the tissues at high rates. 



Adaptations of Tunas For Achieving 

 High Maximum Metabolic Rates 



In a series of studies on the MMR's in land mam- 

 mals (see Taylor and Weibel 1981, and the papers 



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