Fish Population Adjustment 159 



posure of such relevant portions of the bottom may not free them from 

 this vegetation if water levels are normal by the next growing season, 

 although it may be somewhat more sparse and scattered. The drawdown 

 is not an effective method of controlling rooted aquatic plants. In some 

 instances it may be responsible for increasing the extent of beds of rooted 

 aquatics, because plants may gain a root hold in parts of a lake when the 

 water level is down, that ordinarily are too deep for them. For example, 

 in Allerton Lake near Monticello, Illinois, a September drawdown of six 

 feet (maximum depth of lake 14 feet) was maintained throughout a 

 long warm fall (1955). During this period, curly-leaved pond weed, 

 Potamogefon crispiis, gained a start in parts of the lake where the water 

 was seven to eleven feet in depth when the lake was full. Then, as the 

 lake refilled over winter and spring, this pond weed continued to grow 

 so that in the summer of 1956 it reached the surface in all areas seven 

 to eleven feet deep. This created a severe nuisance for boating and 

 swimming and, when the lake was drawn down again in the fall of 1956, 

 the drawdown had a minimum effect upon small green sunfish, red-ear 

 sunfish, and bluegills, because they were protected from bass predation 

 by the dense mats of vegetation in the deeper waters. A fish census made 

 by draining the lake completely a month after the drawdown, exposed 

 excessive populations of small green and red-ear sunfish and demonstrated 

 the importance of pulling the water out of beds of vegetation if a draw- 

 down is to be effective in ridding a lake of small sunfish. 



Effects Upon Invertebrates. When water is released from the basin 

 of an artificial lake through an outlet valve, all motile aquatic animals 

 are either stranded or forced to move down with the water. Animals that 

 escape being stranded are concentrated and exposed to new environmental 

 conditions. Such weak swimmers as many kinds of entomostraca, rotifers, 

 and small insects, particularly those that are littoral, are stranded as the 

 water recedes. Larger aquatic insect larvae, such as dragonfly and mayfly 

 nymphs, may attempt to crawl along with the receding water, but most 

 of them eventually are stranded and die or are eaten by birds or other 

 vertebrates. Some crayfish may be stranded, but most of them burrow 

 into the lake bottom or move down with the receding waters. In draining 

 Ridge Lake,!^ it was not unusual to find 200 to 300 pounds of crayfish in 

 the stilling pit below the gate valve in the outlet tunnel, after all the 

 water had escaped from tlie basin. These crayfish came through the outlet 

 gate with the water during the time the lake was being drained. 



Effects Upon Fishes and Other Vertebrates. The receding water not 

 only strands small invertebrates but many small fishes as well, particularly 

 in the littoral zone where sticks, debris, and mats of rooted vegetation 

 trap these small fishes in temporary water pockets which soon dry up. 

 Certain kinds of small fishes are stranded more often than others. For 



