78 ILLINOIS BIOLOGICAL MONOGILIPHS [78 



Table 17 



TEMPERATURE READINGS OF WATER TAKEN AT FIFTEEN MINUTE 



INTERVALS DURING THE EVENING OVER SHALLOW AND DEEP 



WATER: OCONOMOWOC LAKE, 1923 



Over water 1 meter deep Over water 16 meters deep 



Date 3:00 6:30 6:45 7:00 7:15 7:30 7:45 8:00 3:00 6:30 7:00 7:30 8:00 



A study of this table will show a number of interesting and important 

 facts. To begin with, it will be noted that there is consistently a slight 

 difference in temperature over the shallow and deep water, and that the 

 shallow water is consistently warmer than is the deep. In the second place 

 it should be noted that the shallow water retains its heat better than does 

 the deep water — that is, the surface water in the shallows tends to cool off 

 less rapidly than does the surface water over the deep. Thus a greater 

 drop in temperature is noted, in a short time over the deep water than 

 over the shallows, and this occurs where the water is already cooler. This 

 is due very largely to the fact that much heat is retained by the sand and 

 bottom material, and tends to replace the heat given off by surface radia- 

 tion. Therefore, while the difference in actual temperatures is, as has been 

 said, not great, there is a decidedly greater drop over the deep water, and 

 this is the habitat which the young forsake as the temperature begins to 

 drop. Correlated with the preceding table, the following tabulation gives 

 further information: the time at which the young silversides arrive in the 

 shallow water. Standing at the end of a pier in 70 cm of water the young 

 were plainly seen coming in from the deep water. The hour of their arrival 

 at this point was noted on the same dates as the datain Table 18 were taken. 

 Characteristic of the shoreward journey is the speed with which the min- 

 nows travel: they swim at top speed straight for the shore. Thus, knowing 

 something about the rate of speed with which the fish travel one can figure 

 back and find that the shoreward migration begins at the time the tempera- 

 ture starts to drop over the deep water. Furthermore, be it noted that the 

 minnows arrive together, which fact is significant. In from ten to twenty- 



