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The criterion essential for electric barriers to block upstream fish migration requires only that fish receive enough electric current to reduce their swimming ability to a level below in which they are unable to swim against the water flow. Both alternating currents (A.C.) and direct current (D.C.) can be used for upstream fish barriers, however, electric pulses derived from A.C., both single and multiphase, are considered more stressful to fish. The use of half-wave and full-wave undirectional (D.C.) pulses are more commonly utilized to block fish migrations, because pulsed D.C., while tetanizing, does not usually produce as severe or lasting physiological effect in fish and in the case of upstream migrants, the fish are swept clear of the electricfied zones by stream flow, and thereby quickly recover. Upstream barriers should be located in areas of high water velocity, causing fish to spend more time attempting to traverse the barrier, with a second advantage of swiping stunned fish clear of the electrified zone. Downstream barriers, on the other hand, should be located in a area of flow water velocity to allow fish to easily except the unpleasant sensation of the electric zone. Electric fish barriers have proven to be a useful tool for stopping or guiding the passage of fish through natural or artificial waterways. Electric fish barriers have also been used unsuccessfully to increase hatchery returns during seasonal migrations. Other uses of electric barriers include the repelling of fish from water intakes or outlets such as hydroelectric generator intakes, irrigation pump inlets, flumes, spillways and on the dangerous areas where they might otherwise reach or be drawn into by water flow. Use of Underwater Sound to Guide Small Fish Although sound has been used to condition fish to respond as a signal for food, the evidence of attraction to sound along is rare and questionable. In some hatchery experiments, Burner and Moore (1953) summarized their finding by stating "at no time did a sound frequency or intensity influence the action of fish enough to be utilized in guiding young salmon into safe passages around dams and diversions." Biological Control of Fish Populations Frequently it is suggested by biologists that fast-growing populations of warmwater fish might be controlled by introducing a larger game-fish predator capable of reducing the populations. The bass-bluegill combinations that were proposed by Swingle in the 1950s serve as an example of this type of control. Regier (1962) summarizes the evolution of this management practice. The bass become effective predators in some ponds but, in many cases, bluegills grow beyond the size preferred by the bass and become overcrowded and stunted. Other species that are usually suggested as a predator on coarse fish include the northern pike and the walleye. In all biological control programs, the concept of balance among predator and prey must be considered (Anderson and Weithman, 1978). Sometimes rough fish control is clearly unwarranted. The damage from rough fish must be balanced against their benefit as forage, especially in large lakes. For example, tui chubs (Gila bicolor) act as forage for the unique 8