30 



Fishery Bulletin 91(1), 1993 



Activity of young red drum closely followed the tra- 

 ditional time-course at all sizes, whether the transfer 

 was upward or downward. The general temporal trends 

 in activity had similar shapes and values within treat- 

 ments. Similar experiments conducted on a single size 

 Cunderyearlings') of Atlantic salmon also showed that 

 spontaneous activity followed the overshoot model for 

 adaptation (Peterson & Anderson 1969). Interestingly, 

 activity of salmon did not decrease dramatically in 

 downward transfers, as predicted. Rather, the time- 

 course for activity followed trends similar to those re- 

 sulting from temperature increases. Peak activity in 

 the overshoot period correlated with the rate of tem- 

 perature change, rather than the magnitude or direc- 

 tion of the change. Also, peak activity for salmon trans- 

 ferred to 12° C from 6°C was considerably lower than 

 that for fish undergoing a similar downward transfer 

 from 18° C. Our data show a similar relationship be- 

 tween the direction of transfer and mean peak (posi- 

 tive or negative) activity during the overshoot period. 

 Upward transfers resulted in lesser overshoots of ac- 

 tivity than downward transfers, and overshoot periods 

 were later and prolonged. The differences were not as 

 great as those found by Peterson & Anderson (1969) 



for salmon, probably because of the smaller differences 

 between acclimation temperatures in our experiments 

 (5° vs. 12°C). 



To our knowledge, no other investigators have ex- 

 amined the influence of acute temperature change on 

 pause characteristics. Pause duration followed trends 

 that were broadly similar to the overshoot model and 

 consistent across sizes. Variability around the trends 

 increased with size of fish. Unlike activity, peak pause 

 duration was essentially independent of the direction 

 of temperature change, but there were differences 

 among sizes. Pause frequency showed fundamentally 

 different time-courses. These were not consistently re- 

 lated to the direction of temperature change or fish 

 size. Pause frequency obviously is not a good indicator 

 of the state of thermal adaptation. Its lack of unifor- 

 mity suggests that it is highly variable and may be 

 influenced by numerous other factors. 



Acclimation differences 



Activity also exhibited the strongest and most consis- 

 tent effects of different constant temperatures in young 

 red drum. In control experiments, the pattern of activ- 



