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



ing morning. The skipjack tuna were transferred 

 from the holding tank to the respiration tank 

 through which water was flowing at about 130 1/ 

 min. The fish were observed for 30 min and any 

 that showed unusual behavior was replaced with 

 another fish from the holding tank. Then the 

 cover of the respirometer was installed and the 

 water flow, now supplemented with oxygen, was 

 continued. The animals were allowed to habit- 

 uate until they were schooling and swimming 

 slowly around the chamber at about 1.5 fork 

 lengths/s (L/s) with no overt signs of stress. 

 During this period (1-3 h) O2 concentration in 

 the chamber was maintained at air saturation, 

 6.9-7.0 mg O2/I. The outlet water valve was then 

 closed, the oxygen shut off, and the inflowing 

 water was reduced to about 10 1/min. The cham- 

 ber was slowly filled with water until the acrylic 

 cover domed; then the inflow of water was 

 stopped. When all bubbles were excluded, the 

 sponge rubber ball was positioned to seal the vent 

 pipe and the first experimental run was begun. 

 At the start of a run, O2 concentration in the 

 respirometer was between 6.9 and 7.0 mg O2/I. 

 Oxygen concentration, temperature, swimming 

 speed, and general behavior of the fish were 

 monitored and recorded every 15 min. Swimming 

 speed was estimated by measuring the mean time 

 for three passes by the fish over a straight line 

 distance of 1-2 m. (A comparison of values ob- 

 tained from this technique with mean speeds 

 during complete circuits of fish in the respi- 

 rometer showed that speed was essentially con- 

 stant and that the technique yielded an accurate 

 measure of mean speed during a given circuit.) 

 When the O2 level in the respirometer had been 

 reduced to 5.0±0.1 mg O2/I, final observations 

 were recorded and the run terminated. 



A water flow of about 130 1/min and oxygena- 

 tion were then resumed, which quickly brought 

 the O2 in the respirometer up to about 7.0 mg O2/I 

 where it was maintained for from 1 to 1.5 h until 

 the start of the next run. Following the same 

 procedure, three more runs were made with each 

 group of fish. In all of the experiments, the fish 

 appeared to be in as good condition at the end of a 

 four-run series as they had been at the beginning. 



Oxygen Consumption by 

 Just-Caught Skipjack Tuna 



Rates of oxygen uptake were measured in 11 

 tank-lots of skipjack tuna while they were in 



shipboard transit between the fishing grounds 

 and the shoreside research facility in Honolulu. 

 These observations were made during three sepa- 

 rate fishing trips during December 1972 (Charles 

 H. Gilbert cruise 129). Sea-surface temperature 

 was uniformly 24° C. 



Apparatus 



The five transport tanks served as respirom- 

 eters. Except for the type of cover and differences 

 in plumbing, these tanks were almost identical to 

 the smaller laboratory respirometer described 

 above. The cover of each transport tank consisted 

 of an elliptical fiber glass plate with an open 

 hatchway in its center (see fig. 15 in Nakamura 

 1972). The hatchway was an 80 by 48 cm oval in 

 cross section and extended, chimneylike, 20 cm 

 above the plane of the tank cover. Each cover was 

 tightly bolted to the gasketed rim of its tank. 



Seawater was pumped through each tank at a 

 nearly constant rate between 150 and 250 1/min. 

 Water entered a tank near the bottom at one end 

 and exited at the top through an outlet in the 

 hatchway wall. Oxygenation equipment like that 

 described for the laboratory experiments was used 

 in each tank to supplement O2 , which was mea- 

 sured with the same type of meter used in labora- 

 tory experiments. 



Experimental Procedures 



The transport tanks were made ready before 

 fishing began by establishing a flow of sea- 

 water and 200-300% supersaturation of oxygen. 

 Each fish caught was lowered on the fishing line 

 through a tank's hatch and allowed to escape the 

 barbless hook. On each day, the entire comple- 

 ment of fish was taken from a frenzy-feeding 

 school (Strasburg and Yuen 1960) within a period 

 of about 10 min. A tank-lot ranged from 7 to 

 12 fish, averaging 1.83-2.22 kg; estimated mean 

 weight of captive fish was the actual mean weight 

 of 20 other fish caught from the same school. 

 Such an estimate is quite accurate because the 

 size of individual fish within a skipjack tuna 

 school is remarkably uniform (Brock 1954). 



Within 2 h of capture, oxygen-uptake rate 

 ( Vbz) was measured for each of the 11 tank-lots of 

 fish. In each tank the flow of oxygen was stopped 

 and the time was measured for O2 to decline 

 from the saturation level (6.9 mg O2/I) to a 



35 



