FISHERY BULLETIN; VOL. 69, NO. 2 



that failed to swim the full 6 hr at the same 

 speed the glycogen levels in the white muscle 

 were lower and were different from the controls 

 (P = 0.001 Mann Whitney Latest, Siegel, 1956). 

 Glycogen levels in white muscle of all fish tested 

 at the superthreshold velocity were also much 

 lower and statistically different from the controls 

 (P = 0.05). The lowest glycogen levels of all 

 were in fish that failed from exhaustion at su- 

 perthreshold speeds. The values in these ex- 

 hausted fish were statistically different from 

 those of fish that swam at the same speed but 

 which were removed after 8 min of swimming 

 before they could fall from exhaustion. In sum, 

 strenuous exercise and exhaustion regardless of 

 speed were associated with a marked depletion 

 of glycogen reserves in the white muscle, where- 

 as successful swimming for 6 hr at subthreshold 

 or threshold speed produced no. significant 

 change in white muscle glycogen. 



The glycogen content of the liver and red 

 muscle were lower and different from the con- 

 trols in fish tested at threshold and subthreshold 

 speeds (P = 0.05). At superthreshold speed, 

 on the other hand, the glycogen content of the 

 red muscle was not different from the controls 

 and that of the liver was different only in fish 

 that failed from exhaustion (P = 0.02). 



Thus, the trends in the levels of red muscle 

 and liver glycogen in relation to swimming speed 

 were nearly the reverse of that for white muscle 

 glycogen. Low levels of glycogen in red muscle 

 and liver were associated with slow speeds that 

 could be sustained for extended periods. These 

 results suggest that glycogen from red muscle 

 and liver provide energy to the white muscle at 

 nearly all swimming speeds. We believe that 

 no drop occurred in red muscle glycogen in fish 

 fatigued at high speeds because the time was 

 too short for the white muscle to mobilize sig- 

 nificant amounts of glycogen. This view is sup- 

 ported by the negative correlation between the 

 level of glycogen in the red muscle and swim- 

 ming time to fatigue at threshold speed. This 

 is illustrated in the following table: 



In fish exercised at the superthreshold speed 

 the lactic acid content of the red and white 

 muscle was considerably above that of the con- 

 trols and statistically different from them (P = 

 0.05) (Table 2). At threshold and subthreshold 



Table 2. — Concentration of lactic acid in red and white 

 muscle of jack mackerel following various forced swim- 

 ming conditions. Values given are mg lactic acid per 

 100 g wet weight. 



> Differed from the controls, P ^ 0.05, Mann Whitney i' test (Siegel, 

 1956). 



speeds, the lactic acid concentration in red and 

 white muscle formed no distinct pattern. At 

 threshold speed the lactate levels of red and 

 white muscle were about the same as the con- 

 trols and did not diflfer from them except for 

 one case where the values were actually lower 



382 



