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UNDESIRABLE SPECIES 351 Figure 14.7 Trap net set as part of a rough fish removal program. to keep fish out of particular reaches of streams but little is known about the fundamental design needs to make them perform efficiently. The purposes for which screens are constructed are important in determining where they should be used, what types should be used, and if the costs are justified (Huber 1974). Designs for screens, bypasses, and fishways must take into account the amount of water diverted, availability of water to attract fish, the swimming ability of the fish, the behavior and sizes of the fish, and the quantity and size of debris that may reach the screen. Electricity and sound are methods sometimes considered in efforts to control or guide fish. Albertson et al. (1965) and McLain (1956) reported limitations in controlling sea lampreys with electricity. A voltage gradient sufficient to control small fish would narcotize, injure, or even kill larger fish in the electrical field. Water resistance, content of dissolved substances, and water temperature also influence the effectiveness of electricity. Portable and boat electroshockers can be used to remove fish from confined areas but do not work well in open, large, or deep waters. Electrical fields have been used to repel fish such as gizzard shad and alewives from water intakes of hydroelectric plants. Although the use of sound has not been thoroughly studied, some preliminary investigations indicate that sound may not be useful in guiding or repelling fish. It may be helpful to use combinations of control methods to capture or eliminate target species under different conditions when they are most vulnerable to particular methods. For example, copper sulfate has been used to increase fish movement that resulted in larger fyke-net catches (Brown 1964). Electrofishing equipment can be used in streams to force fish into gill nets, trammel nets, or trap nets. Successful use of nets and traps has been limited usually because an inadequate portion of the offending population is removed. The sizeable escapement from nets in large lakes and reservoirs requires continuous fishing. Some nets (e.g.,