160 Theories and Techniques of Management 



example, small green sunfish are stranded more often than are small blue- 

 gills and small bluegills more often than small red-ear sunfish. Green 

 sunfish are commonly found in shallow water along the lake edges, as are 

 bluegills. The red-ear prefers deeper water and shows a tendency to 

 move away from the water's edge as the lake level moves down. Neither 

 young largemouth nor young smallmouth bass are ordinarily stranded 

 with receding lake levels, although both may be trapped by dense mats 

 of vegetation. Few large fishes are stranded unless they become trapped 

 in shallow basins on the lake bottom and die later when the water in 

 which they are trapped dries up. 



Small fishes that are not stranded and move down the lake basin with 

 the water are forced from the protection of rooted vegetation and shallow- 

 water debris into the open water of the lake where they are subject to 

 predation from larger fishes, bullfrogs, and fish-eating reptiles, birds, and 

 mammals. These forces and the mechanical stranding of small fish 

 materially reduce the populations of smaller fishes without greatly reduc- 

 ing numbers of the larger ones. The result is a selective culling action 

 which is more specific for sunfish than for bass, and which may not be 

 extensive enough to be beneficial unless the drawdown: (1) reduces the 

 lake surface area by more than 50 per cent (Figure 6.9) and (2) forces 

 the fish from the protection of beds of aquatic plants. The selective culling 

 action resulting in a reduction of sunfish may set the stage for high 

 survival of bass at the next bass-spawning season. Thus, fall drawdowns 

 in several successive years may result in such a numerical buildup of 

 bass that they will be of smaller average size than under more stable water 

 levels (Figure 6.10). 



Heavy predation on the small fish during a fall drawdown may con- 

 tinue as long as their numbers are concentrated and the water remains 

 warm enough for rapid digestion. When the lake cools below 55°F, diges- 

 tion is greatly slowed and the rate of predation diminishes accordingly. 



Although small fish concentrated by a drawdown are vulnerable to 

 predation by many aquatic animals, it seems probable that piscivorous 

 fish account for the death of more small fishes than all other predators 

 together. As yet, no one has been able to evaluate the element of time in 

 relation to the culling of small fish following a drawdown, but it is reason- 

 able to assume that small fish losses, while heaviest at first, may continue 

 with reduced intensity over a period of several weeks or months. 



Flat areas in the bottoms of reservoirs suitable for making seine hauls 

 are sometimes cleared of stumps and debris before the reservoirs are 

 filled. Then, later, when the reservoirs are drawn down, seines may be 

 used to harvest concentrations of carp, buffalo, and other commercial fish, 

 thereby giving an additional assist in the process of population improve- 

 ment. 



