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



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20 40 60 80 100 120 140 



TIME SINCE CAPTURE (minutes) 



FIGURE 5.— Relation between the rate of oxygen uptake and 

 the time elapsed after capture of skipjack tuna. Dashed lines 

 connect paired determinations of oxygen-uptake rate for the 

 same tank-lot of fish. 



sumption and activity during the 2-h interval 

 following capture. An initial period of frantic 

 swimming, lasting 10-15 min, had already ended 

 before we were able to collect our earliest respira- 

 tion data. Still, the first five determinations of 

 oxygen-consumption rate (within about 1 h of fish 

 capture) were 1.5 mg 02/g per h or more. In two 

 cases, a tank-lot of fish respired at a much reduced 

 rate during the second of two sampling intervals 

 separated by about 1 h. (In a third set of such 

 paired observations, oxygen uptake during the 

 second was . 3 mg O2 /g per h more than during the 

 first, but these determinations were separated by 

 only 20 min.) In conclusion, we believe the oxygen- 

 uptake rate of our fish immediately after their 

 capture was underestimated by the overall me- 

 dian value of 1.3 mg 02/g per h and, in fact, may 

 have exceeded 2.0 mg 02/g per h. 



Low-Oxygen Tolerance Experiments 



General Behavior 



Comparative behavioral responses of the fish in 

 low-oxygen («3.5 mg/1) water were quite consis- 

 tent. However, the sequence of behavior was more 

 accelerated and the fish's reactions were often 

 more violent at the lower oxygen concentrations. 



At the two lowest 02's (1.4 and 2.0 mg/1) the fish 

 showed symptoms of considerable stress within 

 about 30 s of introduction. Stress was manifested 

 as very fast swimming (2.6 L/s), wide mouth-gape. 



and little or no attempt to school. During the last 

 few minutes before the skipjack tuna died, they 

 assumed a steep angle of attack with their snouts 

 out of the water and swam jerkily, with intermit- 

 tent bursts of speed up to 6 L/s. Complete collapse 

 came abruptly; the fish simply ceased swimming 

 and settled to the bottom. At 2.5 and 3.0 mg O2/I, 

 the initial stress reactions were milder and the 

 fish started schooling within a few minutes after 

 introduction. Swimming speeds were still rela- 

 tively high (1.7-2.8 L/s), and the sequence and 

 types of behavior were similar to those at the 

 lowest O2. At 4.0 mg O2/I, both skipjack tuna 

 swam and otherwise behaved as if they were in 

 oxygen-saturated water. 



Resistance Time and Swimming Speed 



There was a marked, direct relation between 

 the logarithms of resistance time and oxygen con- 

 centration at oxygen levels up to 3.5 mg O2/I 

 (Table 2, Figure 6). At 3.5 mg O2/I, four of the 

 experimental fish had resistance times in the 

 same range as the fish exposed to 3.0 mg O2/I, but 

 one fish survived for the 240-min duration of the 

 experiment. The survivor showed few overt signs 

 of stress but did swim faster than the fish in 

 oxygen-saturated water. At 4.0 mg O2/I, both 

 experimental fish survived 240 min. 



The 21 skipjack tuna used in the 12 experiments 

 ranged in weight from 791 to 3,523 g (Table 2). 

 There was no significant correlation between 

 weight and resistance time to low-oxygen levels. 





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log RESISTANCE TIME -0.417 + 2.876 log Oa CONG. 



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20 40 60 80 100 



RESISTANCE TIME (minutes) 



200 



Figure 6. — Relation between resistance time to low-oxygen 

 concentration and dissolved oxygen concentration. Resistance 

 time was the period from the fish's introduction into the low- 

 oxygen tank until it stopped swimming and settled to the 

 bottom. Circled points indicate three fish that were still swim- 

 ming after 240 min. The regression line was fitted to the points 

 for fish that died. 



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