77] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 77 



1. Temperature: The writer agrees with Hubbs that there is relatively- 

 little difference between the surface temperature over deep water and over 

 the shallows if he specifies during the day (which he does not) but with the 

 new turn the problem has taken, a more careful study of temperature 

 relations and conditions must be considered before any conclusions are 

 drawn. 



The following table (16) shows a series of temperature readings made 

 over shallow and deep water at 10 o'clock in the morning and again in the 

 same place at 3 o'clock in the afternoon. These readings were made over 



Table 16 



SURFACE TEMPERATURE OF OCONOMOWOC LAKE OVER SHALLOW 



AND DEEP WATER 



locations in which silversides were found, and represent the relative 

 environmental temperatures selected by the adults (1 meter deep) and the 

 immature (over water 16 meters deep). It must be admitted that the 

 absolute differences are slight, but it must also be remembered that fish 

 are very sensitive to change in temperature: Shelf ord and Powers (1915) 

 report a reaction to a change of 0.2°. The table shows conclusively that 

 the adults are located in water that is uniformly warmer than the immature; 

 hence the young, on establishing themselves permanently in the shallow 

 water are coming into a region of water that is uniformly a little warmer. 

 It does not, however, explain why the young seek the deeper water to 

 begin with. 



Turning now to an examination of the temperature conditions over 

 shallow and deep water during the day and night, I find a situation a little 

 more definite, and results a little more decisive. In compiling the following 

 table, two sets of temperature readings were taken, one just under the 

 surface of water 1 meter deep, the other just under the surface of water 

 16 meters deep. With the aid of an assistant anchored in a row boat over 

 16 meters of water, I was able to obtain thermal readings of the two locali- 

 ties taken at exactly the same time. These records are tabulated in Table 

 17. 



