79] STUDY OF SOUTHERN WISCONSIN FISHES—CAEN 79 



Table 18. 



TIME OF ARRIVAL OF LABIDESTHES SICCULUS IN THE SHALLOW 



WATER DURING THE EVENING MIGRATIONS 



(To be considered in connection with Table 26) 



Date Time of arrival Weather 



8/8 7:40 



8/10 7:42 



8/11 7:35 



8/13 7:35 



8/14 7:41 



8/17 7:25 cloudy 



8/18 6:55 very cloudy 



8/22 7:20 



8/23 7:35 



8/24 7:15 



8/26 7:08 



9/5 6:55 



9/7 6:50 



five minutes the entire silversides population of the lake is inshore. Since 

 there is a lessening gradient in radiation as one approaches the shallower 

 water, it is evident that the fish over the deeper water start their migra- 

 tion first, and move shoreward, picking up the fish nearer the shore as 

 they advance with the drop in temperature, this bringing all the fish into 

 the shallows within a very short interval of time. 



With the coming of fall and the cooling of the water, the shallows 

 warm up increasingly more than does the deep during the day, and rela- 

 tively more heat is stored up in the bottom for radiation during the hours 

 of darkness. Hence the immature fish, on coming into the shallow water 

 to assume the same habitat as the adults, are seeking an environment of 

 warmer water. Data indicate that, while the diurnal range of variation 

 may be greater in the shallows than over the deep, the average condition 

 is upward of 3° warmer. Since a factor is of increasing importance as its 

 abundance decreases, this excess of warmth in the shallows must be con- 

 sidered as being important. 



One other point needs emphasis in connection with the temperature 

 problem. Observations show that cold water acts as a depressor on the 

 silversides, for the fish are very much less active in winter than in summer. 

 In fact, the fish are so sluggish and slow moving that, when seen under 

 the ice in winter, one hardly recognized the dashing, darting minnow of 

 the summer. The following figures bear out this point: 



August 8, 1923. By means of a stop watch the number of vibrations 

 of the pectoral fin were counted. The average of ten counts of 100 vibra- 

 tions shows the rate of movement per 100 vibrations at a water temperature 

 of 26.2°C as 100 vibrations in 34 seconds. 



