386 



riSHEBY BULLETES^ OF THE FISH AND WILDLIFE- SERVICE 



Such average measurements gave no clear indica- 

 tion of the degree of fluctuation. 



Some of the other experimental conditions also 

 could not be controlled completely. The depth 

 of the water in the trough, maintained at 6 to 8 

 inches at the downstream end of the two channels, 

 varied from 4 inches to 10 inches at the ends of 

 the trough when the trough deviated from a hori- 

 zontal position. The downstream end of the 

 trougli had a tendency to settle as a result of the 

 clogging of the screen entrance gate by floating 

 organic debris. Variations of this sort atfected 

 the channels equally, however, and probably are 

 of little significance. 



During the experiments in which water was 

 heated electrically there frequently was a vertical 

 variation in the temperature of the water in the 

 modified channel due to inadequate mixing of the 

 heated water. This was particularly true when 

 only a few heaters were in operation. At such 

 times, the variation was as much as 0.4° C. from 

 the warmer water near the bottom of the channel 

 to the cooler water near the surface. The tem- 

 perature measurements, made 2 inches from the 

 bottom of the channel (the level at which the fish 

 ustially swam), were always of the maximum 

 temperature. 



In interpreting the data collected during these 

 experiments, such variations in experimental con- 

 ditions must be taken into consideration, particu- 

 larly when thresholds are concerned. The thresh- 

 olds for the responses of the fish may actually be 

 lower than those indicated by the data. 



EXPERIMENTS 

 ORIENTATIVE INFLUENCE OF TEMPERATURE 



Experiments were conducted in which the 

 migrating fish were presented with a choice be- 

 tween waters of two different temperatures. The 

 responses of the fish (table 5) indicated a pref- 

 erence for the warmer water. 



The stream tem]ierature during these experi- 

 ments ranged from 11.1° to 22.3° C. The tem- 

 perature differences between channels, created by 

 heating the water of one channel, were varied 

 from 0.4° to 3.0° C. The threshold of the re- 

 sponse api^eared to be at a temperature difference 

 of approximately 0.5° C. 



Periodic eddying which caused a fluctuation in 

 the water temperature of the channel being modi- 



fied (see Aberrations in Experimental Control, 

 }). ;!sr)) was probably responsible for tlie appar- 

 ently intermediate nature of the response to tem-* 

 perature differences of 0.5° to 1.0° C. The tem- 

 perature difference values given in tables 5 and 6 

 are maximum values. The minimum values were 

 approximately 0.5° C. less. For example, during 

 tests at the recorded temperature difference of 

 0.7° C, for at least part of the time, the tempera- 

 ture difference was approximately 0.2° C. 



Therefore, only at recorded temperature dif- 

 ferences above 1.0° C. was the thi-csliold differtMire 

 of 0.5° C. exceeded continually tlirouglioiil tlic 

 entire test. 



TAiii.i': ."). — Rcniioiisc to frmpernttire rliffcrriires <is xhoirii 

 ill c-ri)criiiicii1s of May 7 to June 8, WHO 



[Stream temperatures, n.l° to 22.3° C] 



' Temperature differences are maximum values. Minimum lemiicrature 

 differences were approximately 0.5° C. less. 



2 No response. 



3 Response. 



* See notes on eddying phenomenon, p. 385. 



Taking the eddying into account, the inter- 

 mediate nature of the response to temperature 

 differences from 0.5° to 1.0° C. should probably 

 be discounted and considered as due to imperfect 

 experimental conditions. The data woidd then 

 be interpreted as describing a uniform ungraded 

 response to temperature differences greater than 

 tlie tlireshold. 



