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



RESULTS 



Oxygen-Uptake Experiments in Laboratory 



Condition Factor 



It has been our experience that skipjack tuna do 

 not do well in captivity for extended periods. 

 Obviously, valid behavioral and physiological 

 data require that the experimental animals be in 

 good health. As mentioned above, our fish were 

 actively feeding and had been captive for less 

 than a month. Some additional evidence relative 

 to their general condition is provided by com- 

 paring the length-weight relationship of the ex- 

 perimental fish with the relationship obtained 

 by Nakamura and Uchiyama (1966) for freshly 

 caught skipjack tuna. For our captive fish, logW 

 (grams) -= -2.657 + 3.532 /logL (centimeters); 

 for wild skipjack tuna, logW (grams) = -2.317 + 

 3.368 /logL (centimeters) (log = logio). This com- 

 parison indicates that our experimental fish were, 

 on average, about 14% lighter at a given length 

 than wild fish of the same mean length (ca. 48 cm), 

 the difference in weight-at-length decreased with 

 increasing fork length. Part of the weight dis- 

 crepancy resulted from the near emptiness of the 

 guts of the experimental fish. Raju (1964) reported 

 that pole-and-line caught skipjack tuna weighing 

 about 1.4 kg had stomach contents comprising 

 about 1.5% of body weight on the average and 

 6.3% of body weight at maximum. 



General Behavior 



The behavior of a group or pair of fish over the 

 four experimental runs changed very little, and 

 behavioral VcU"iation among the 10 series of ex- 

 periments was slight. 



The fish, usually in close company, continuous- 

 ly circled the respirometer 20-30 cm from the 

 sides. Their course in the smaller respirometer 

 was usually elongate but quite frequently shifted 

 to a circle with a radius of 60-70 cm. Direction 

 reversal and figure of eight patterns were not 

 unusual. Rarely one fish would break away and 

 swim separately, sometimes in a direction oppo- 

 site the other fish, but such divergent patterns 

 never persisted for long. 



Swimming Speed 

 Swimming speed of skipjack tuna, averaged 



over all experiments, was 1.4 L/s. Speed was 

 independent of O2 over the experimental range 

 (5.0-7.0 mg O2/I), but stepwise multiple regres- 

 sion indicated significant (P^0.05) effects of fish 

 weight, length of time the fish had been in 

 the respirometer, and the time-order of experi- 

 mental series: 



S = 3.55 - 0.53 nogW - 0.02t - 0.04 • k 



where S = swimming speed (lengths/second), 

 W = mean fish weight (grams), 

 t = time (hours) the fish had been in the 



respirometer, 

 k = time-order (1, ... , 10) of the experi- 

 mental series. 



Thus, length-specific swimming speed decreased 

 with increasing fish size and with increasing 

 values of both time-related variables. Solution 

 of the regression equation at mean values of 

 t (9^8 h) and k (5.5) yielded S = 3.14 - 0.53 

 /logW (Figure 2). 



Respiration Rate 



Mean respiration rate over all experiments was 

 0.52 mg 02/g per h (Table 1). The data suggested 



22 



20 



1.8 



•= 1.6 



a. 

 to 



1.4 



1 12 



? 1.0 



MINIMUM SPEED (lengths- sec"') 

 MAGNUS0N(I973) 



SPEED(length9.sec"') = 3. 14-0.53 log W 



(C)"' 



SPEED(cm.sec-') = l2 08ll + l6 0594logW i jq 



100 



90 



UJ 



a. 



80 2 



z 

 s 



70 



60 



O 

 (O 

 CD 



< 



I I I I I I 



J L 



200 



500 1000 



WEIGHT (g) 



5000 



FIGURE 2. — Relation between voluntary swimming speed and 

 body weight in skipjack tuna. Line A connects point-solutions 

 from Magnuson's minimum speed function (Magnuson 1973). 

 Line B is the relation between relative swimming speed and 

 weight observed in the present study. Line C is the relation 

 between absolute speed and weight observed in the present 

 study. Length measure is fork length. 



37 



