FISHERY BULLETIN: VOL. 69, NO. 2 



mg percent in a clupeid, Harengtda Mimeralis. 

 A much higher level of 1866 mg percent was 

 given by Bone (1966) for dogfish. In most 

 cases, the concentration of glycogen in red 

 muscle was considerably higher than in white 

 muscle. 



Liver glycogen controls in jack mackerel were 

 much higher than those reported previously in 

 teleosts. Connor et al. (1964) for example, ob- 

 tained values of about 1 Sr in chinook and sockeye 

 salmon and steelhead trout, and found that 

 moderate exercise associated with ascending 

 fishways had no effect on liver glycogen levels. 

 Black etal. (1960) reported liver glycogen levels 

 of 0.5-4 '/f in rainbow trout, and Dean and Good- 

 night (1964) obtained 0.8-3 9r in four species of 

 warmwater centrachid fishes. Values similar 

 to ours were reported by Wittenberger and 

 Diaciuc (1965) in carp (13.8%) and by Bellamy 

 (1968) in recently fed red piranha (10.3'^f ). 

 Even if a high degree of gluconeogenesis were 

 operative in jack mackerel, it seems unlikely 

 that this could entirely explain the high levels 

 of liver glycogen. 



Control levels of glycogen in jack mackerel 

 white muscle appeared to be similar to those 

 in other fishes. However, in the red muscle and 

 especially in the liver, glycogen levels were usu- 

 ally higher than in fishes studied earlier. 



The most striking finding of this study was 

 the virtually complete depletion of glycogen in 

 the white muscle of fish that failed from ex- 

 haustion. The depletion of glycogen in white 

 muscle occurred in all fish that failed regardless 

 of the speed of swimming or how long they 

 swam. In fish that did not fail at a near thresh- 

 old speed of 21 L" Vsec (Hunter 1971) the gly- 

 cogen in the white muscle did not differ from 

 controls, whereas in the fish that failed, glycogen 

 in the white muscle was at nearly the same 

 low level as it was in fish that failed after a few 

 minutes of exertion at a much higher speed. 

 Red muscle glycogen was also depleted at some 

 swimming speeds but the pattern of glycogen 

 depletion in red muscle closely paralleled that 

 of the liver. Red muscle had one-fifth the lac- 

 tate found in white muscle on a percent basis 

 but only about one-fiftieth on an absolute basis 



because the mass of white muscle exceeds the 

 red by 10 to 1. 



The high lactate levels in the white muscle, 

 the almost complete depletion of glycogen in the 

 white muscle of exhausted fish, and the parallel 

 pattern of glycogen depletion in red muscle and 

 liver all point to the same hypothesis. In jack 

 mackerel at threshold and higher speeds the 

 energy used for swimming was derived primar- 

 ily from glycolysis in the white muscle which 

 was the principal locomotor organ. Red muscle 

 like the liver may serve as a storage organ whose 

 resources could be used to drive the white 

 muscle, given sufficient time for mobilization. 

 Thus at threshold speeds, red muscle function 

 appeared to be tied to that of the white and it 

 could not be considered as acting independently. 

 No change in red muscle glycogen was detected 

 at the highest test speed, possibly because time 

 was insufficient to mobilize the glycogen reserves 

 other than in the white muscle itself. This time 

 dependency for mobilizing red muscle glycogen 

 under conditions of strenuous exercise could ex- 

 plain why Bone (1966), Wittenberger and Di- 

 acuic (1965), Wittenberger (1968), and Fraser 

 et al. (1966) detected no change in red muscle 

 glycogen after strenuous exercise. It must be 

 remembered that in all of these previous studies 

 the strength and the duration of the exercise 

 was unknown, except that it was considered 

 to be extreme. 



The decrease in fat content plus the high 

 lactate levels suggest that the red muscle was 

 used for swimming at subthreshold speeds. 

 Bilinski (1969) showed that the rate of oxidation 

 of fatty acids in red muscle of rainbow trout 

 and sockeye salmon exceeded that in the white 

 muscle by one or more orders of magnitude de- 

 pending on the fatty acid substrate. On the 

 other hand, neither the high oxidative capacity 

 nor the decline in lipid levels in red muscle with 

 moderate exercise are suflicient evidence for an 

 independent locomotor role. In addition, the 

 presence of high lactate levels in white muscle 

 and the drop in the glycogen content of the white 

 muscle indicated that the white muscle was also 

 used at the subthreshold speed of 19 L"" 'sec. 

 The electrophysiological evidence for indepen- 



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