FISHERY BULLETIN: VOL. 79, NO. 1 



second level between 6.2 and 5.6 mg O2/I. For 

 three tank-lots of fish, a second measurement 

 of Vbz was made after resaturation of the tank 

 with oxygen. Because the flow of water through 

 the tanks was maintained throughout the 3- to 

 15-min period of O2 measurement, calculation 

 of Vbz accounted for O2 both supplied to and 

 removed from the tank in the flow of water: 



d[02]c 



dt 



C =([02]/- [02]c)Q-N  W  Vo, 



which gives, upon integration and solution for Vo^ , 



Vo, = 



([O2]/- [02]c)-Q 



N  W 



t)) 



where Vbz 

 [O2]/ 



(l-exp(-Q • C"' 



= oxygen-uptake rate (milligrams 



02/gram per hour), 

 = concentration of O2 in incurrent 



water (milligrams 02/liter), 

 = concentration of O2 in tank and 

 in excurrent water (milligrams 

 02/liter), 

 Q = water exchange rate ( liter s/hoiir), 

 N = number of fish in tank, 

 W = estimated mean weight of fish 



(grams), 

 C = operating volume of tank (liters), 

 t = time (hours) for oxygen concentra- 

 tion in tank to decline from [ O2 ]/ 

 toLOzlc- 



Low-Oxygen Tolerance Experiments 



Apparatus 



The tank used for the low-oxygen tolerance 

 experiments was identical to the smaller respi- 

 rometer tank but was uncovered and was supplied 

 with unaerated water (0.5 mg O2/I) directly from 

 the seawater well. During the time it took to fill 

 the tank, the water took up atmospheric oxygen 

 and O2 increased to 1.4 mg O2/I. 



Experimental Procedure 



Twelve experiments were made, using 21 skip- 

 jack tuna (Table 2). The fish for six of the experi- 

 ments were the same pair that had been used for a 

 preceding series of oxygen-uptake experiments. 

 For the other six experiments, skipjack tuna were 

 taken directly from a holding tank. The fish were 



Table 2. — Swimming speed and resistance time to low oxygen 

 of skipjaci? tuna at various oxygen concentrations. Those fish 

 which continued swimming for 240 min were considered to 

 have survived. Length measure is fork length 



rested in the uncovered respiration tank at air- 

 saturated O2 levels (6.8-7.0 mg O2/I) for about 2 h 

 prior to being transferred to an immediately 

 adjacent low-oxygen test tank. Because of a fish 

 shortage, we were forced to use single rather than 

 paired animals for the last three experiments. 



The experiments were done with O2 ranging 

 from 1.4 to 4.0 mg O2/I and temperatures between 

 23° and 24° C. When O2 in the low-oxygen tank 

 reached the required levels, the fish were netted 

 from the resting tank into the test tank. Each 

 transfer took < 5 s, and two fish were transferred 

 within 30 s. 



The O2 and the behavior and swimming speeds 

 of the fish were observed continuously. During an 

 experiment, the water took up atmospheric oxy- 

 gen at a rate dependent on the air- water pressure 

 gradient. However, at all of the experimental O2 

 levels there would have been a decrease of O2 

 concentration due to the fish's respiration had 

 we not gradually introduced oxygen as it was 

 depleted by the fish; by so doing we continuously 

 maintained O2 concentration within ± 0.2 mg/l of 

 the nominal experimental level. 



A fish's resistance time was the period from 

 introduction into the low-oxygen tank until the 

 animal lost equilibrium and settled to the bottom. 

 A fish was considered to have survived if it was 

 still swimming after 240 min, at which time the 

 experiment was terminated. At the conclusion of 

 each experiment, weight and fork length of each 

 fish were measured. 



36 



