GOODING ET AL.: RESPIRATION RATES AND LOW-OXYGEN TOLERANCE IN SKIPJACK TUNA 



but still may have underestimated the skipjack 

 tuna's maximum or "active" (Fry 1971; Brett 1972) 

 rate of oxygen consumption. Wild skipjack tuna 

 similar in size to our experimental fish can swim 

 at sustained speeds probably exceeding 10 L/s 

 (Yuen 1970); our just-caught fish swam at speeds 

 ^5 L/s during the intervals when oxygen-uptake 

 rates were measured. However, the experimental 

 fish may have been repaying an oxygen debt 

 incurred during the feeding frenzy preceding cap- 

 ture or during the early minutes of captivity; 

 recovery from oxygen debt could have heightened 

 oxygen-uptake rates to levels above those com- 

 mensurate with sustained swimming at the ob- 

 served speeds (Brett 1972). 



"Standard" Metabolism 



Even though tunas never lie stationary in the 

 water, it is of interest from the bioenergetic and 

 comparative standpoints to separate the routine 

 metabolic rate into standard and activity-related 

 components. From the equation on p. 38, with 

 swimming speed set equal to 0.0, 



log Vo, - -1.20 + 0.19 logW, 



where Vba = oxygen-uptake rate (milligrams O2/ 

 gram per hour), 

 W= mean fish weight (grams). 



Solutions of this equation at our experimental 

 extremes for W are Vbj = 0-21 mg 02/g per h 

 at W = 632 g and Vo^ = 0.30 mg Oj/g per h 

 at W = 3,834 g. These values are extraordinary 

 for two reasons: 1) They are at the extreme upper 

 limit for nontuna (cf. fig. 4 of Brett 1972), a fact 

 that becomes even more remarkable when one 

 considers that other teleost values are almost all 

 for small (10-100 g) individuals, and 2) the weight 

 exponent is a positive 0.19, not a negative value 

 in the neighborhood of -0.2 characteristic of 

 typical fishes (Fry 1957, 1971; Winberg 1960). 

 While weight exponents for active metabolic rate 

 in salmonids may frequently approach 0.0 (Job 

 1955; Brett 1965; Rao 1968), we know of no data to 

 suggest weight exponents as large as -1-0.2 for 

 metabolic rate in nonscombrid fishes. The valid- 

 ity of a large, positive value for the weight ex- 

 ponent of "standard" metabolic rate in skipjack 

 tuna is supported by independent data, via direct 

 calorimetry, on heat production rates; the red 

 muscle of sedated skipjack tuna (maintained by 



gill perfusion) metabolized at a rate proportional 

 to W^3(Neilletal.l976). 



In marked contrast with our estimate of skip- 

 jack tuna's weight exponent for standard metab- 

 olism is that reported by Brill (1979) — negative 

 0.44, a value at the other extreme for fishes. 

 Considering that Brill's and our groups of fish 

 were similar in size range and preexperimental 

 history, we must deduce that the large discrep- 

 ancy between estimates relates principally to 

 the difference in experimental methodologies: 

 Brill took, as the standard metabolic rate, the 

 stabilized minimum Vq.^ of perfused skipjack 

 tuna that had been first injected with the neuro- 

 muscular blocking agent gallamine triethiodide, 

 then spinalectomized. 



Activity-Related Metabolism 



Our respiration experiments estimated only 

 rates of oxygen uptake, not rates of instantaneous 

 metabolic demand for oxygen. Neill et al. (1976) 

 estimated that the oxygen demand of red muscle 

 in highly active (chased) skipjack tuna can reach 

 7 mg 02/g per h for periods on the order of 1-2 

 min. For even shorter periods, involving only true 

 burst swimming, the rate of oxygen demand must 

 be even higher. Brett (1972) has estimated that 

 burst-swimming fishes' instantaneous rate of 

 oxygen demand (on a whole-body basis) exceeds 

 the maximum rate of supply by a factor of 10. Any 

 excess of demand over supply accumulates as an 

 oxygen debt that ultimately must be repaid. 



Our observations on just-caught fish provided 

 (probably conservative) estimates of the maxi- 

 mum rate at which skipjack tuna can supply oxy- 

 gen to meet their metabolic demands. Like the 

 skipjack tuna's "standard" metabolic rate, its 

 maximum (active) rate of oxygen uptake must be 

 substantially beyond that typical of fishes. Just- 

 caught skipjack tuna respired at a median rate of 

 1.3 mg 02/g per h; the highest five values (those 

 obtained during the fish's first hour of captivity) 

 were between 1.5 and 2.5 mg 02/g per h. Brett 

 (1972), in reviewing his own and others' work, 

 reported that fishes' maximum rates of oxygen 

 consumption reach a "probable ceiling" near 1.0 

 ±0.2 mg 02/g per h. 



The activity-respiration relationship obtained 

 at sea for just-caught skipjack tuna was reason- 

 ably consistent with that extrapolated from the 

 laboratory experiments (Figure 8). However, data 

 of the two kinds may have agreed less well had 



41 



