FISHERY BULLETIN: VOL. 76, NO. 4 



Shifting the isotherm position during an exper- 

 iment required the fish to thermoregulate ac- 

 tively, similar to those studied by Beitinger ( 1976, 

 1977) in his temporal gradient. This technique 

 demonstrated that the fish could follow an 

 isotherm and did not necessarily arbitrarily select 

 a position in the experimental tank. The precision 

 with which a group of fish followed an isotherm 

 varied between species and was related to the size 

 of their preferred temperature range. Juvenile 

 surfperch, Danialichthys vacca, for example, 

 which preferred a narrow range of temperatures. 



closely followed an isotherm ( approximately 1 1°C) 

 (Figure 3). In contrast, juvenile topsmelt, Atheri- 

 nops affinis, after initially selecting approximate- 

 ly 22°C, remained within that compartment, and 

 shifting the gradient did not cause them to move 

 until the temperature reached 26°-27°C. This 

 isotherm was then tracked (Figure 3). Topsmelt 

 are physiologically eurythermal, at least during 

 embryonic stages, and the upper limit for hatching 

 of topsmelt eggs is 26.8°C (Hubbs 1965). Brett 

 (1956) suggested that the preferred temperature 

 may not be a strong enough directing force to move 



I I I 



1 I I I I T 



I • r 



180 240 



TIME (minutes) 



Figure 3. — Changes in fish and isotherm position in the experimental gradient. Juvenile Damalichthys vacca followed the 

 10°- 1 1°C isotherms. Juvenile Atherinops affinis remained in the position they initially selected and moved very little until the 

 temperature reached 26°-27°C. They then followed the temperature range of 25°-27°C. The small numbers indicate isotherms. 

 Large dots indicate the mean temperature selected by nine individuals. 



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