white muscle 



FK'iURE 1. — Temperature excess in the 

 heart and in red and white muscles re- 

 corded from Scomber japonicus no. 6(35 

 cm FL, 0.45 kg) swimming at speeds 

 from 1.4 to 3.9 BL/s. Arrows indicate 

 timing and direction of speed changes. 

 Ambient temperature, 19.5°-19.6°C. 



r*! I I ' I I 

 400 600 1000 



Table l. — Temperature excesses as AT (°C) recorded for seven Scomber japonicus swimming at basal and 



moderately fast speeds in body lengths per second (BL/s).' 



'Fish no 5 omitted because it would not swim in the respirometer tube, 



^Thermocouple malfunction 



^Starting temperature is that of the seawater supply from mid-June to mid-July. 



cases within the 3-min swimming bouts. Although 

 the thermal excess was greater in white muscle of 

 fish number 6 ( Figure 1 ) , mean maximum temper- 

 ature excesses recorded in red and white muscles 

 of the seven mackerel were about the same (Table 

 1). 



Variability observed in excess temperature 

 measurements seems attributable to different per- 

 formances of individual fish. Some specimens had 

 more body fat than others and did not swim stead- 

 ily. Others were affected by the trailing ther- 

 mocouple cable as evidenced by their tail-beat pat- 

 terns. The cable also added drag which reduced 

 speed but probably increased total heat production 

 at a specific speed. None of the fish trailing ther- 

 mocouple cables could swim steadily above 5 BL/s, 

 whereas fish trailing the thinner EGG and EMG 



cables could maintain a speed of 6 BL/s. Some of 

 the variability in recorded thermal excesses may 

 have also been due to the slightly differing loca- 

 tions of thermocouples in each fish. In addition, 

 trauma due to thermocouple insertion, which 

 probably interrupts normal blood flow locally may 

 have been a factor influencing thermal convection. 

 In a few cases, thermocouple signals changed 

 abruptly possibly because of insulation failure at 

 the tip due to rapid body flexing of fishes at higher 

 swimming speeds. 



A wide range was found in heart rates of mack- 

 erel cruising at 1-1.5 BL/s (mean, 106; range, 

 80-140 beats/min). With acceleration to 4-5 BL/s, 

 the mean heart rate increased by 549f , (mean, 130; 

 range, 112-150), but rapidly returned to the rest- 

 ing rate within a few minutes of deceleration. 



863 



