FISHERY BULLETIN: VOL. 79, NO. 1 



Mean swimming speed increased as O2 de- 

 creased (Figure 7), reaching 3.9 L/s at the lowest 

 O2, 1.4 mg O2/I. Mean speed at 4.0 mg O2/I was 

 slightly less than at higher O2 values (oxygen- 

 uptake experiments); therefore, the critical O2 for 

 an increase in swimming speed appeared to lie 

 between 4.0 and 3.5 mg O2/I. 



a 



UJ 

 UJ 



a. 





z 

 < 



1.5 



20 2.5 3.0 3.5 40 45 50 5.5 6.0 6.5 

 DISSOLVED Oz CONCENTRATION (mg-r') 



FIGURE 7. — Relation between mean swimming speed and dis- 

 solved oxygen concentration. The data for O2 of 6.0 and 5.0 

 mg  1' are from the oxygen-uptake experiments. Those at 

 lower concentrations are derived from the low-oxygen tolerance 

 experiments. Length measure is fork length. 



DISCUSSION 



Terminology Relevant to 

 Tuna Metabolism 



In this paper we have strived to quantify the 

 activity and respiration levels of our fish. The 

 question of terminology remains. Doudoroff and 

 Shumway (1970) have emphasized that "different 

 meanings have been attached by different authors 

 to the same term or different terms have been used 



in the same sense " The question of terminology 



is further complicated in tunas because they, un- 

 like typical fishes, must maintain some minimum 

 forward motion for hydrodynamic lift (Magnuson 

 1973) and for gill perfusion (Brown and Muir 1970; 

 Stevens 1972); a stationary tuna both sinks and 

 suffocates. Thus, the notion of "resting" metabolic 

 rate ( Doudoroff and Shumway 1970) is not appli- 

 cable to tunas. 



What we have collected in our laboratory exper- 

 iments were data on "routine" (Fry 1957, 1971) 

 activity and metabolism. Fry (1971) defined rou- 



tine metabolic rate as "the mean rate observed in 

 fish whose metabolic rate is influenced by random 

 activity under experimental conditions in which 

 movements are presumably somewhat restricted 

 and the fish protected from outside stimuli." Our 

 fish were in a postabsorptive state (except for bits 

 of food they may have eaten during the transfer 

 process) and were as quiescent as tunas are ever 

 likely to be when confined in a small tank. Per- 

 haps, our laboratory data reflect minimum me- 

 tabolism for skipjack tuna in that the fish were 

 swimming at speeds actually below the hydrody- 

 namic minima calculated by Magnuson (1973) for 

 skipjack tuna (Figure 2). On first consideration, 

 it would seem unlikely that tunas — which lack 

 ventilatory pumps and are, therefore, obligate 

 ram-ventilators (Brown and Muir 1970) — could 

 achieve minimum swimming speed without also 

 achieving minimum rate of oxygen uptake. How- 

 ever, Stevens (1972) has shown that skipjack tuna 

 have the capability for doubling the amount of 

 oxygen they extract per unit flow of water irri- 

 gating the gills (utilization efficiency, 0.4-0.8) 

 with only a 17% reduction in ventilation rate 

 (from 3.0 to 2.5 1 H20/kg per min). Thus, it is 

 conceivable that a skipjack tuna could decrease 

 swimming speed and simultaneously increase oxy- 

 gen uptake. We must, therefore, recognize the pos- 

 sibility that "excitement" (Fry 1971) associated 

 with the alien and confining environment of our 

 respirometers resulted in heightened rates of oxy- 

 gen uptake compared with those that might obtain 

 in wild, unexcited skipjack tuna swimming in the 

 sea at the same speeds. However, the data we 

 collected do not permit an objective evaluation 

 of this possibility. For purposes of further dis- 

 cussion, we assume that our laboratory measure- 

 ments of oxygen-uptake rate contained no com- 

 ponent of "excitement" metabolism independent 

 of swimming speed. Lack of change in respiration 

 rate among sequential experiments indicates that 

 any activity-independent excitement component 

 of metabolism that may have been present was 

 habituation-time invariate. This has encouraged 

 us to go so far in the following section as to esti- 

 mate the hypothetical "standard" ( = "basal" — see 

 Fry 1971; Brett 1972) metabolism of skipjack tuna 

 from our respiration data; this we did, as Fry 

 (1971) recommends, by simply extrapolating to 

 zero speed the regression equation relating res- 

 piration rate and swimming speed. 



Rates of oxygen uptake measured in the "just- 

 caught" fish can scarcely be considered "routine" 



40 



