FISHERY BULLETIN: VOL. 79, NO. 1 



Respiration research on tunas is still quite 

 limited. Besides the present work, there are only 

 four published studies — three involving tuna 

 metabolism (Gordon 1968; Stevens 1972; Brill 

 1979) and one involving tunas' low^er tolerance 

 limits for O2 (Anonymous 1965). Gordon (1968), 

 using volumetric microrespirometers, determined 

 rates of oxygen uptake in minced preparations 

 of muscle from skipjack tuna and bigeye tuna, 

 Thunnus obesus. Stevens (1972) measured the 

 oxygen concentration of water entering and 

 leaving the gills of restrained, perfused skipjack 

 tuna; from these data he computed oxygen-uptake 

 rate and utilization. Brill (1979), using a similar 

 technique, estimated the relation between stan- 

 dard metabolism and body weight of skipjack 

 tuna. Stevens (1972) also measured oxygen utili- 

 zation in free-swimming skipjack tuna by sam- 

 pling exhaled water collected via opercular can- 

 nulation. Experiments conducted with skipjack 

 tuna at the Kewalo Research Facility provided 

 the earliest estimate of the lower lethal-oxygen 

 limit for a tuna (Anonymous 1965). 



Our purposes in this work were 1) to determine 

 the magnitude of oxygen-uptake rate in routinely 

 active skipjack tuna, 2) to establish the relation 

 among oxygen-uptake rate, swimming speed, and 

 body weight in skipjack tuna, and 3) to estimate 

 the lowest concentration of O2 that skipjack tuna 

 can withstand for 4 h. The results already have 

 contributed importantly to the development of 

 models of skipjack tuna distribution (Barkley 

 et al. 1978) and bioenergetics (Kitchell et al. 

 1978). 



MATERIALS AND METHODS 



Source and Preexperimental Treatment of 

 Fish for Laboratory Experiments 



Skipjack tuna were caught by angling in 

 Hawaiian waters at sea-surface temperatures be- 

 tween 23° and 24° C. The fish were transported 

 in 2,400 1 shipboard tanks that were supplied 

 with flowing seawater and supplemental oxygen. 

 Upon arrival at the National Marine Fisheries 

 Service's Kewalo Research Facility in Honolulu, 

 the skipjack tuna were transferred into either 

 40,000 or 700,000 1 outdoor holding tanks. Naka- 

 mura (1972) described in greater detail the tech- 

 niques that have been developed at the Kewalo 

 Research Facility for transporting and maintain- 

 ing live tunas. 



The seawater in the holding tanks had 23°-24° M 

 C temperatures, pH 7.4-7.6, 32-33%o salinity, and ~ 

 6.4-6.7 mg/1 O2. The seawater well that supplied 

 water to the holding tanks was also the source of 

 water for the experimental tanks. At night the 

 holding tanks were illuminated at a low level. 



Experiments were conducted with fish that 

 had been in captivity 7-26 d. Once the skipjack 

 tuna started feeding (usually within 3-5 d after 

 capture), they were fed to satiation on thawed 

 northern anchovies, Engraulis mordax, or smelt, 

 Allosmerus sp., once a day. Prior to experiments, 

 the fish were fasted for periods ranging from 24 to 

 27 h, which is more than sufficient time for gut 

 evacuation in skipjack tuna (Magnuson 1969). 

 However, our method of moving fish from the 

 holding to the experimental tanks involved some 

 food ingestion. Two to four hours before data 

 collection, the fish were removed from a holding 

 tank by angling with a baited, barbless hook. 

 Although a small piece of food (1-2 g) was usu- 

 ally swallowed, this transfer technique did select 

 healthy and actively feeding fish. 



Oxygen- Uptake Experiments 

 in Laboratory 



Apparatus 



Two unstirred, sealed respirometers of differ- 

 ent sizes were used. Circulation of water during 

 experiments was provided only by movements 

 of the fish. 



The larger respirometer was used only during 

 the first of the 10 series of experiments (Table 1). 

 The circular chamber was a vinyl-lined plywood 

 tank, 4.57 m in diameter and 1 m deep, with a 

 cover made of transparent vinyl film bonded to 1 

 an inflatable tube, 18 cm in cross section, that ' 

 encircled the tank's inner perimeter just below 

 the rim. After the tube was in place and any 

 trapped air had been removed with an electric 

 pump, the clear plastic cover lay over the entire 

 water surface, forming an effective seal. We ini- 

 tially had intended to run all of the experiments 

 with this respirometer. However, it proved to be 

 difficult to operate and visibility of fish within 

 the chamber was poor. Most importantly, a tank 

 of its volume (16,000 1) required a large biomass 

 of fish in each experiment to effect oxygen reduc- 

 tion in a reasonably short period of time. Skipjack 

 tuna are difficult and expensive to capture and 

 maintain; so, to use fish as economically as pos- 



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