FISHERY BULLETIN: VOL. 69, NO. 2 



20 30 



MINUTES 



100 



Figure 3. — Relation between speed at which 50% Trach- 

 unis fatigued and the duration of the swimming period. 

 Duration of swimming period in minutes plotted on log 

 scale to show exponential trend ; line fit by eye. Speed 

 estimates made at 2-min intervals of cumulated time 

 over the first 100 min of swimming. 



performance in length per second is obtained 

 if we consider smaller fish. For example, fish 

 of mean length 11.2 cm (length range 10.4 to 

 11.9 cm, A'^ = 8) swam for 3 to 5 min (mean = 

 4.5 min) at 139 cm/sec or about 12 L/sec. This 

 difference between large and small fish becomes 

 negligible if 0.6 is used as a coefl^cient of length 

 instead of unity because, as was pointed out 

 previously, the length coeflicient for Trachurus 

 appears to be less than 1. 



DISCUSSION 



The exponential decline in swimming speed 

 with time in fish is well documented; see for 

 example Bainbridge (1960), Brett (1967), and 

 Blaxter (1969). The general form of the re- 

 lation between time and swimming speed in 

 other fish resembles that for Trachurus al- 

 though the speeds and endurance times are dif- 

 ferent in Trachurus. The physiological mech- 

 anisms responsible for the exponential relation- 

 ship between swimming speed and endurance 

 are generally believed to be the limited enery 

 stores in the muscle, the rate these stores can 

 be replaced and the rate catabolites are removed 

 from the muscle (Bainbridge, 1960). A study 



by Pritchard, Hunter, and Lasker (1971) in 

 this issue has provided an explanation for the 

 form of the speed-time relationship in Trach- 

 urus. Pritchard et al. found that at speeds 

 where an exponential relationship exists be- 

 tween time and speed the principal cause of 

 failure of Trachurus was most likely the de- 

 pletion of glycogen in the white muscle. On 

 the othei- hand, fish that failed at speeds near 

 the 6-hr 50% threshold, where a linear relation- 

 ship exists between speed and time, had de- 

 pleted not only the glycogen in the white muscle 

 but that in the red muscle and liver as well. 

 Thus, in Trachurus the form of the time-speed 

 relationship could be explained on the basis of 

 the extent of glycogen reserves available for 

 locomotion and the time required to mobilize 

 them from sites other than the white muscle. 

 An exponential relationship between speed and 

 time could be produced when the speeds are so 

 high that the glycogen supply would be limited 

 almost entirely to the white muscle because the 

 supply in the white muscle would be used up and 

 the fish would fail before significant amounts of 

 glycogen could be mobilized from other sources. 

 A linear relationship could exist where swim- 

 ming speeds are sufficiently low that reserves 

 in the white muscle could not be depleted before 

 other sources in the red muscle and the liver are 

 mobilized. We have, on one hand, a high rate 

 of consumption using a more limited supply of 

 fuel which could lead to an exponential relation- 

 ship between speed and time and, on the other 

 hand, a much lower rate of consumption using 

 a relatively much larger fuel supply which could 

 produce a linear relationship with time. An 

 exponential relationship between energy con- 

 sumption and swimming speed would enhance 

 these effects. 



Let us now consider the significance of the 

 6-hr sustained speed threshold determined for 

 Trachurus. When compared with other deter- 

 minations, this threshold appears to be unique 

 because of different physiological mechanisms 

 and because it is higher than those estimated 

 for other fish. Trachurus at threshold speed 

 appeared to use glycogen as fuel, white muscle 

 for locomotion and maintained a high lactic acid 



270 



