BRILL; STANDARD METABOLIC RATES OF TROPICAL TUNAS 



rates observed in paralyzed tunas may be due to the 

 vagolytic action of Flaxedil (Grollman and Grollman 

 1970). However, as the data from aholehole and rain- 

 bow trout show, estimating SMR using animals 

 paralyzed with Flaxedil and by extrapolation of 

 swimming speed-metabolic rate curves back to zero 

 swimming speed yield similar results. 



Effect of Body Size and 

 Acute Temperature Change on SMR 



In Figure 1, it appears that the SMR's of yellow- 

 fin tuna are lower than those of skipjack tuna and 

 kawakawa. However, based on the 95% confidence 

 intervals, the heights of the regression lines (at 

 mean body weights) are not significantly different 

 from each other. Based on the 95% confidence inter- 

 vals, the weight exponents of the regression equa- 

 tions for kawakawa, yellowfin tuna, and skipjack 

 tuna also are not significantly different over the size 

 ranges tested (Fig. 1). In other words, the effect of 

 body weight on the SMR is not significantly differ- 

 ent among the three tuna species. The exponent in 

 the allometric equation describing the effect of body 

 size on the SMR of other teleosts ranges from ap- 

 proximately 0.65 to >1 (Winberg 1956; Fry 1957; 

 Beamish 1964; Beamish and Mookherjii 1964; Glass 

 1969; Brett 1972). The lower values of the exponents 

 for tunas indicate that the weight specific SMR^ 

 (i.e., mg 02/(g-h)) of tunas decreases more rapidly 

 as body size increases than it does for other teleosts. 



Gooding et al. (1981) also estimated the SMR of 

 skipjack tuna. When converted to the same units 

 used in this study (SMR in mg 02/h and W in kg), 

 the relationship they found for the effect of body 

 weight on SMR was 



SMR = 234 1^119. 



The exponent greater than one means that they 

 predict the weight specific SMR to increase with in- 

 creasing body size. As shown in Figure 1, Gooding 

 et al.'s predicted SMR's are lower than mine for 

 small fish, but exceed my estimates above approx- 

 imately 2.5 kg body weight because of the large 

 weight exponent. 



To estimate SMR, Gooding et al. (1981) used a 

 multiple linear regression equation of the logarithm 



^If the allometric equation to describe the effect of body size on 

 whole body standard metabolic rate (SMR) is SMR = aW^, then 

 the corresponding equation to describe weight-specific SMR ver- 

 sus body weight is SMRIW = aW^-IW or SMR' = aW'-i; where 

 SMR' = weight-specific SMR, W = body weight, and a and 6 are 

 fitted parameters. 



of metabolic rate versus swimming speed and the 

 logarithm of body weight, and then extrapolated 

 back to zero swimming speed. Their data and extra- 

 polations were based on several groups of different- 

 sized fish swimming at voluntary speeds in a tank 

 respirometer. This methodology is not equivalent to 

 the more conventional one of estimating SMR based 

 on swimming speed-metabolic rate curves that are 

 constructed by forcing one fish, swimming in a tun- 

 nel respirometer, to undergo stepwise increases in 

 swimming speed during which the fish remains for 

 at least 1 h at each speed (Brett 1972). Furthermore, 

 Gooding et al. (1981) expressed swimming speeds 

 in body lengths per second. Boggs (1984) has shown 

 that this will cause appreciable bias when fitting 

 multiple linear regression equations because the 

 effect of the body size on active metabolic rate is 

 different at different swimming speeds. 



The Effect of Acute Temperature 

 Change on SMR and Heart Rate 



As shown in Table 1, the Qio's (effect of tempera- 

 ture) for the SMR's of skipjack tuna, yellowfin tuna, 

 and kawakawa are the same. They are also close to 

 the Qio's for SMR's of other teleost species sub- 

 jected to acute temperature change (Qio = 2.16, 

 Moffitt and Crawshaw 1983; Qio = 2.10, Boehlert 

 1978), and for the effect of temperature on SMR 

 where fish were acclimated to each test tempera- 

 ture (Qio = 2.48, Ott et al. 1980; Qio = 1.82-2.83, 

 Duthie 1982). 



This result was not expected since studies on the 

 effect of temperature change on the metabolic rate 

 of isolated red and white muscle samples (Gordon 

 1968, 1972a, 1972b), volitional swimming speed 

 (Dizon et al. 1978), and prehminary work on active 

 metabolic rate of skipjack tuna showed all three to 

 be unaffected by temperature. 



Comparing the metabolic rate (1,052 mg 02/h, 

 from Gooding et al. 1981) of a 2.0 kg skipjack tuna 

 at its minimum swimming speed (1.4 body lengths/ 

 s) to its directly measured SMR (608 mg 02/h, from 

 Brill 1979), shows that the SMR constitutes 58% of 

 the minimum swimming metabolic rate. Because 

 skipjack tuna's SMR constitutes a large fraction of 

 their metabolic rate at minimum swimming speeds 

 and increases as temperature increases, whereas 

 swimming metabolic rate and volitional swimming 

 speed do not, increases in muscle efficiency (i.e., in- 

 creases in thrust developed by the caudal propeller 

 per unit of O2 uptake), reductions in hydrodynamic 

 drag (perhaps due to reduction in water viscosity), 

 or unknown physiological adjustments must occur 



31 



