Fish Population Adjustment 155 



the chemical, and plan the mechanical aspects of the operation well in 

 advance. If this has been accomplished, the operation can be performed 

 on short notice— at a time when wind and weather conditions are fa- 

 vorable. 



Artificial Fluctuation of Water Levels 



In Chapter 4, I described certain experiments that demonstrated that 

 the total weight of a population was related to the size of the water 

 area it inhabits. Thus, if a body of water devoid of fish is stocked with a 

 few sexually mature individuals, these fish will reproduce and they and 

 their offspring will add flesh until their total weight approaches the 

 poundage of fish that the water area wall support. This process may 

 require one or more growing seasons, but eventually the poundage of 

 fish will level off at some figure related to the size of the habitat and its 

 food-producing capacity (natural fertility). This poundage adjustment 

 may be downward if more pounds of fish were stocked originally than 

 the lake would support. 



Various levels of population density favor certain species, that is, some 

 are better able to compete for food and space than others. Under extreme 

 competition some become dominant and others, if exposed to this com- 

 petition for several years, may entirely disappear from a population. These 

 species that do poorly when subjected to severe competition may actually 

 become very abundant if stocked with other fishes in a body of water 

 with plenty of space. 



Suppose then, that instead of adjusting the population by netting, 

 seining, or partial poisoning, we subject it to extreme crowding for weeks 

 or months through the release of much of the total volume of water 

 ( Figure 6.9 ) . When the fish are crowded during warm weather, the entire 

 population is under stress. Smaller and weaker fishes of many species 

 starve or are killed through food competition, strife, or predation; the 

 total poundage of the population is adjusted downward to conform with 

 the smaller habitat and reduced food supply. The species, which as 

 adults or fingerlings are best able to withstand crowding, will remain 

 dominant although they may have stopped growing entirely. 



Then the habitat is rapidly expanded by the addition of new water! 

 The exposed lake bottom is reflooded and there is suddenly plenty of 

 space and more food. All fish that have survived the period of crowding 

 begin to grow rapidly. Under conditions of unlimited food and space, 

 certain species that were adversely affected by crowding, produce large 

 broods of young. If these young are piscivorous they may actually check 

 the expansion of some of the species formerly so successful. Thus, there 

 has been a sudden shift in dominance brought about by a drastic change 

 in the habitat.^^ If this change is man-made, we have only taken our cue 



