Fish Population Adjustment 161 



Types of Drawdowns. From the standpoint of angling there is some 

 evidence to suggest that prolonged droughts affect fish populations favor- 

 ably. In droughts extending over several years, lakes may gradually de- 

 crease in area. This gradual decrease must cause adjustment in the fish 

 population. However, it is difficult to see how a slow drawdown would 

 have a selective effect in eliminating excessive small fish. 



13-1 



DC DC 



Spring Spring 



1947 1949 



DC 



Spring 



1951 



DC 



Spring 



1953 



DC 



Spring 



1956 



DC 

 Fall 

 1959 



FD FD FD FD FD 



Stable Water Levels 





BASS 



r-' 



'52 '53 

 SUMMERS 



10 

 ■15 



■25 



-7.0 



-6.5 



-6.0 



> 

 < w 



t- 



r a 



2 PI 



-5.0 



Figure 6.10. Changes in average sizes of largemouth bass and bluegills 

 caught by fishermen at Ridge Lake (Illinois) under several types of manage- 

 ment: 1945-1950 biennial draining and culling of small fish; 1951-1955, fall 

 drawdowns with draining censuses in spring of 1953 and spring of 1956; 1956 

 to fall of 1959, stable water levels. DC = draining census; FD = fall draw- 

 down. Fall drawdowns increased the number of bass but reduced their average 

 size. Under stable water levels following the drawdown period, the number 

 of bluegills expanded more than 500 per cent and their average size decreased. 

 After 1957, the average size of the bass increased and their number declined. 



Annual cycles of water levels, such as were described above for the 

 Illinois River and adjacent bottom-land lakes, can be shown to have a 

 pronounced effect on the fish population. Reservoirs with the greatest 

 water-area fluctuations contained the largest per cents (by weight) of 

 predatory species, which included many of our game species. ^^^' ^^^ Large 

 man-made and controlled reservoirs have various types of annual cycles 

 of water fluctuation; these cycles may be only remotely related to cycles 

 of rainfall and runoff. Man-made cycles may vary from one lake to the 



