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<br />FISH POPULATION MANIPULATION <br />R.S. Wydoski <br />Division of Federal Aid <br />U.S. Fish and Wildlife Service <br />Denver, Colorado <br />Introduction <br />The objectives of fishery managers are to produce and maintain fish <br />populations that will provide satisfactory fisheries. Few fish populations <br />are managed efficiently and, in many areas, unproductive waters appear to <br />predominate. Most "unproductive" waters contain "undesirable" fish <br />populations. The reasons why undesirable (i.e., rough) fish replace desirable <br />(i.e., sport) fish are not fully understood, but many ecological (physical, <br />chemical, and biological) factors appear to be involved. When a fish <br />population becomes unbalanced or "undesirable," it may be necessary for <br />biologists to manipulate the fish population in favor of desired species. <br />Fish may be determined to be undesirable when: <br />1. They become dominant as a result of angling pressure on the sport fish <br />that changes the species composition or the ratio of predators to <br />prey. <br />2. They become stunted from an overabundance of fish in relation to their <br />food supply (especially true for prolific warm-water species) and few <br />fish of desirable sizes are produced. The species then must be <br />reduced in numbers to balance the population with the available food <br />supply. <br />3. The abundance of predatory fish are limiting the abundance of more <br />desirable species. <br />4. They are obnoxious to anglers or consumers because of heavy parasite <br />infestations. <br />5. They are in direct competition with desirable (sport) species. <br />6. They cause excessive turbidity and prevent the normal growth of <br />desired aquatic vegetation that may affect waterfowl as well as fish. <br />7. They occur in a hatchery water supply where they provide a potential <br />as a reservoir for pathogenic organisms. <br />8. They interfere with management practices for other species of <br />wildlife. <br />Undesirable species have been controlled mechanically with the use of nets <br />(sometimes commercial fishing), weirs, barriers, and electricity; biologically <br />by balancing predatory and prey, regulations, manipulation of water levels, <br />and chemically using fish toxicants.