140 Theories and Techniques of Management 



MANAGEMENT TECHNIQUES 



Once sufficient sampling of a fish population has indicated that man- 

 agement is necessary, one should investigate the known techniques and 

 decide which are applicable. Often several methods seem justifiable and 

 one or more must be selected on the basis of expediency. 



Complete Fish Population Removal 



Complete removal of a population is usually desirable when a lake or 

 pond becomes contaminated with species of no value for angling or fish 

 production. Such fishes as buffalo, suckers (of several kinds), gizzard 

 shad, and sometimes stunted black bullheads may have limited sport 

 fishing value. These species often crowd out more desirable game and 

 pan fishes. Even if these undesirable fishes are present in small numbers, 

 they are always a potential danger to the production of a high sustained 

 yield of more desirable species because of their capacity for producing 

 tremendous numbers of young at a single spawning and their ability to 

 modify their environment ( by stirring silt ) in their search for food. These 

 fishes, and some others unlisted, are completely under control only when 

 they are absent.^- 



Population Removal by Draining. All artificial ponds and lakes should 

 be built with drain outlets of sufficient size to allow their basins to be 

 drained within a period of 3 to 10 days. If a lake with a drain becomes 

 contaminated with undesirable species or must be drained for any other 

 purposes (such as the recovery of stolen goods), a Wolf -type weir 

 ( Figure 6.3 ) can be placed across the outlet, the live fish separated from 

 the water, and the valuable fish saved alive for restocking.^ ^" A Wolf -type 

 weir is more satisfactory than any other type of screen because the water 

 falls through the bottom of the wire-mesh weir instead of flowing through 

 a perpendicular screen. The fish either are left exposed on the wire mesh 

 of the weir bottom or they flop across the bottom screen into a holding 

 box. This is the only type of screen that can handle a large flow of water 

 without frequent shutoffs for cleaning the screen. A Wolf-type weir can 

 be constructed below almost any outlet that will give 6 inches to 2 or 

 more feet of working space below the level of the outflowing water. If 

 it is necessary to catch very small fish or plankton organisms, such a weir 

 may be covered with copper window screening or MS-904 Saran Screen.^^ 

 Usually it is not desirable to use mesh of smaller than one-fourth to three- 

 eighths of an inch. 



Before draining, it is necessary to make some arrangements, either 

 temporary or semi-permanent, for storing desirable fishes. The surface 

 spillways of some artificial lakes may terminate in stilling basins of suf- 



