FISHERY BULLETIN: VOL. 79, NO. 1 



a slight but statistically nonsignificant decrease 

 in respiration rate with decreasing oxygen con- 

 centration. Thus, our assumption of respiratory 

 independence at oxygen concentrations down to 

 5.0 mg O2/I seems valid; however, our experi- 

 mental design did not permit proper analysis of 

 the relation between respiration rate and O2 . 



Of the other variables included in a step- 

 wise regression analysis (fish weight, swimming 

 speed, total time in respirometer, time of day, 

 experimental order), only fish weight and swim- 

 ming speed significantly (P^0.05) affected res- 

 piration rate: 



log Vo, = -1.20 + 0.19 logW + 0.21s 



body weight (Figure 2): S = 3.14 - 0.53 /\ogW. 

 When this and the previous equation are com- 

 bined to express the relation between oxygen- 

 uptake rate and body weight for skipjack tuna 

 swimming at their characteristic speeds, we are 

 left with log V02 = -0.54 +^0.08 /logW. Thus, 

 the speed-inclusive effect of W on oxygen-uptake 

 rate actually observed in our experiments was 

 quite small (Figure 3). A multiple regression 

 analysis with speed deleted from the independent- 

 variable list yielded no significant relation be- 

 tween V02 and W (P>0.05). In contrast, the 

 weight-inclusive effect of speed on V02 was readily 

 apparent in a simple plot of log Vbj versus S 

 (means) for all experiments (Figure 4). 



where V02 = oxygen-uptake rate (milligrams O2/ 

 gram per hour), 

 W = mean fish weight (grams), 

 S= swimming speed (lengths/second). 



We hasten to point out an irregularity in the 

 relation just presented. While we offer the equa- 

 tion as a best available predictor of independent 

 weight and speed effects on oxygen uptake in 

 skipjack tuna, we recognize that W and S in our 

 fish were not independent. As indicated in the last 

 section, the fish tended to swim at a characteristic 

 speed inversely proportional to the logarithm of 



600 



800 1000 



2000 



4000 



WEIGHT (g) 



Figure 3. — Lack of significant relation between oxygen-uptake 

 rate and weight of skipjack tuna swimming at voluntary 

 speeds. Weights are means for the fish in each experimental 

 series. 



38 



O 



E 



< 



< 



Q. 



3 



1.0 

 09 

 08 

 07 



06 

 OSl- 



04 

 03 



0.2 



log O2 UPTAKE RATE  -0.4378 +0.1090 -SPEED 

 r •O.ZB 



08 1.0 12 14 16 18 



SWIMMING SPEED (lengths^sec-') 



20 



22 



Figure 4. — Relation between oxygen-uptake rate and relative 

 swimming speed of skipjack tuna (ranging from 60 to 4,000 g). 

 Data are means for each experiment. Length measure is fork 

 length. 



Oxygen Consumption and Activity of 

 Just-Caught Skipjack Tuna 



Skipjack tuna, during their first 2.2 h of captiv- 

 ity, consumed oxygen at rates between 0.9 and 2.5 

 mg Oa/g per h, the median for the 14 determina- 

 tions being 1.3 mg 02/g per h at 75 min (Figure 5). 

 Swimming speeds were correspondingly great — 

 between 2 and 5 L/s. 



Rate of oxygen consumption was not significant- 

 ly correlated with time since capture (Kendall's t 

 = -0.20, P-0.18; Siegel 1956) when all 14 rate 

 determinations were considered as random obser- 

 vations from the same bivariate distribution. 

 However, some features of the experiment sug- 

 gested a decline in the rates of both oxygen con- 



