EHRLICH ET AL: THERMAL BEHAVIORAL RESPONSES OF FISHES 



intensities. We employed natural day lengths 

 during preexperimental holding as well as 

 during testing. 



DESIGN OF GRADIENTS 



We built two horizontal gradients: one for juve- 

 niles and adults and another, smaller one, for lar- 

 vae and placed them in a sound-insulated and 

 lightproof room. A differential of at least 10°C 

 between the hot and cold ends of the gradient was 

 established prior to collection of data used for be- 

 havioral analyses. We adjusted the size and po- 

 sition of the temperature gradient to keep the 

 experimental fishes from the extreme end com- 

 partments; differentials of up to 25°C were 

 employed for species with wide preferred tempera- 

 ture ranges. 



Gradient for Juvenile and Adult Fishes 



We divided this gradient (360 cm long x 60 cm 



wide X 60 cm deep, Figure 1) along its longitudi- 

 nal axis to produce two experimental chambers 

 ( 15 and 44 cm wide). The size of the test organisms 

 determined which side of the gradient was used. 

 Heating and cooling were controlled, primarily at 

 the ends of the gradient. A rheostat connected to 

 an immersion heater allowed us to minimize the 

 frequency of the heater turning on and off, reduc- 

 ing fluctuations in heat production. A stainless 

 steel (type 316) coil heat exchanger with cold 

 freshwater coolant (1°C) chilled the seawater in 

 the gradient. Two additional heat exchangers con- 

 structed of polyvinyl chloride and run along the 

 bottom from the ends to the center of the experi- 

 mental chamber served to produce a more even 

 temperature gradient. 



Formation of thermal currents can limit the 

 temperature range and controllability of a hori- 

 zontal gradient. Twelve pairs of surface and bot- 

 tom baffles impeded thermal currents and divided 

 the gradient into 11 experimental compartments. 



28 28 



cml I cm 



360 cm 



Figure l. — Large experimental chamber for temperature selection measurement in fishes: a) daylight-simulating fluorescent light, b) 

 surface baffle, c) screens separating experimental chambers ( j and k) from the end heating and cooling compartments, d) 500- W heaters, 

 e) stainless steel heat exchangers, f) polyvinyl chloride heat exchanger, g) bottom baffles, h) wall separating the large and small 

 exp)erimental chambers, i) drain, j) large experimental chamber, k) small experimental chamber. 



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