FISHERY BULLETIN: VOL. 69. NO, 2 



to question. Although the roles assigned to the 

 two muscle systems are dependent on swimming 

 speed, no studies have been made on the function 

 of the muscle systems using normally swimming 

 intact animals at known speeds. The objective 

 of this study was to re-examine the metabolic 

 and locomotor roles of red and white muscle by 

 measurement of glycogen, lactate, and fat levels 

 in the muscle and glycogen levels in the liver 

 in fish exposed to various velocity treatments 

 of known strength and duration. Juvenile jack 

 mackerel, Trachums synimetricics, were used in 

 this study because the maximum sustained speed 

 threshold for 6 hr of continuous swimming had 

 already been established for this species (Hun- 

 ter, 1971), and consequently we were able to re- 

 late all of our chemical measurements to known 

 levels of swimming performance. 



METHODS AND PROCEDURES 



SWIMMING TESTS 



Jack mackerel were maintained at a regu- 

 lated seawater temperature of 18.5° C in a plastic 

 swimming pool 4.57 m diameter and fed an 

 abundant ration of brine shrimp, Artemia, and 

 chopped fish and squid each day. The fish were 

 not fed for 20 hr prior to testing. Jack mack- 

 erel were tested in an activity chamber patterned 

 after that of Beamish (1968) and described in 

 detail by Hunter and Zweifel (1971). The swim- 

 ming compartment of the apparatus consisted 

 of a tube 230 cm long and 41 cm in diameter 

 through which seawater could be moved at 

 speeds ranging from 12 to 212 cm/sec. Fish 

 were placed in the tube and forced to swim at 

 a water speed for certain periods varying from 

 8 min to 6 hr. At the end of the swimming 

 period they were removed and dropped imme- 

 diately into liquid nitrogen and the frozen fish 

 were stored at — 30° C until used for chemical 

 analysis. The time required for removal and 

 freezing did not exceed 1 min. 



Speed treatments for the experiments were 

 chosen relative to the 50'^ endurance threshold 

 for jack mackerel at 22 L^^/sec for 6 hr of 

 swimming where L is total length (Hunter, 



1971). Five jack mackerel, nrean length 14.6 

 cm, were tested at the subthreshold speed of 

 19.6 L"'' sec (98 cm sec); 14 jack mackerel, 

 mean length 16.3 cm, were tested at the near 

 threshold speed of 21.1 L''Vsec (113 cm/sec); 

 and 10 jack mackerel, mean length 14.7 cm, were 

 tested at the superthreshold speed of 27.7 L^V 

 sec (139 cm 'sec). Fish tested at the sub- 

 threshold speed swam continuously for 6 hr and 

 were sampled at the end of that period. Fish 

 tested at the threshold speed were divided into 

 two groups: seven fish that were sampled after 

 6 hr of continuous swimming; and seven fish 

 that fell from exhaustion at some time during 

 the 6-hr period. The latter group of seven fish 

 were quickly removed from the apparatus and 

 frozen as soon as they fell against the rear 

 screen. Fish tested at the superthreshold speed 

 were also divided into two groups: those that 

 swam successfully for 8 min; and those that 

 failed after 8 or less minutes of swimming. 

 Ten jack mackerel, mean length 14.5 cm, were 

 used as controls. Five of the control animals 

 were removed from the holding tank, placed in 

 the apparatus, allowed to swim for 30 min at 

 the slow speed of 6.2 L°''/sec (30 cm sec), re- 

 moved, and frozen. The other five control fish 

 were removed from the holding tank and imme- 

 diately frozen. The data from these two con- 

 trol groups were later combined because no dif- 

 ference between them was detected. 



CHEMICAL ANALYSES 



White and red muscle were dissected from 

 the frozen fish while still frozen. One lateral 

 striiJ of red muscle was used for fat analysis 

 and the other divided into two equal portions for 

 lactate and glycogen analysis respectively. 

 About 1 g of white muscle from the dorsal por- 

 tion of the myotome was used for glycogen de- 

 terminations, 0.5 g for lactate, and 0.5 g for fat 

 measurements. Fish were returned to the 

 freezer and liver samples (0.1-0.2 g) were an- 

 alyzed for glycogen about a month after the 

 muscle determinations. 



380 



