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r American Fisheries Society Position on Introductions of Aquatic Species Christopher C. Kohler and Walter R. Courtenay, Jr. AFS members are encouraged to comment on the following draft position statement. Comments should be sent, by 1 July 1986, to Paul Brouha, Chairman, Environmental Concerns Committee, 7426 Westwood Park, Falls Church, VA 22046. This position state- ment will be presented to the AFS Executive Committee for approval at the 1986 annual meeting of the society. A. Issue Definition The increased frequenty of inter- and intranational trans- fers of aquatic species carried out over the last 2 decades has prompted concern relative to the potential for debase- ment of integrity of aquatic communities. Past introduc- tions, intentional or otherwise, have run the full gamut from spectacular booms (e.g., Pacific salmon to the Great Lakes) to spectacular busts (e.g., the waterweed hydrilla to portions of the United States). Considering the manifestations of such extremes in terms of ecological and economical impacts, it is not surprising that opposing viewpoints exist with respect to the relative pros and cons of effectuating introductions of aquatic species. Nevertheless, natural resource managers concur that substantially improved measures can and should be taken to increase the odds that benefits of a given intro- duction will exceed risks. Currently, a number of interna- tional commissions have adopted or are considering adopt- ing formal "codes of practice" for regulating the introduction of aquatic species (see Sindermann 1986; Welcomme 1986; Kohler and Courtenay 1986). Implementation of such codes (protocols, guidelines, etc.) can ensure that decisions re- garding future introductions are based on sound ecological evidence, and that introductions effectuated are properly evaluated. B. Negative Impacts on Aquatic Communities The impacts of introduced aquatic organisms on native aquatic communities in North America have been summa- rized by Contreras and Escalante (1984) for Mexico, by Tay- lor et al. (1984) for the continental United States, and by Grossman (1984) for Canada. These impacts can be classified into five broad categories: habitat alteration, trophic alter- ation, spatial alteration, gene pool deterioration, and intro- duction of diseases. Habitat Alteration Introduced plants such as water hyacinth (see Table 1 for scientific names of organisms cited in text), Eurasian wa- March -April 1986 termilfoil, alligator weed, and hydrilla have seriously in- fested anumber ofwater bodies in North America (Shireman 1984). Excessive vegetation interferes with swimming and fishing activities, upsets predator-prey relationships by pro- viding too much cover, causes water quality problems dur- inggrowth and decomposition, and is aesthetically unpleas- ing (Noble 1980). Ironically, exotic fishes, particularly grass carp and the tilapias, are frequently used as biological con- trols. Both the grass carp and the tilapias have reproducing populations in North America, although the habitat require- ment for larval grass carp has so far proved to be limiting and the tilapias are basically limited to the southern extreme of the United States and to Mexico. Although grass carp have proven to be an excellent bio- logical control for aquatic vegetation, a risk exists that aquatic plants (including native forms) might become overly deci- mated as a result of grass carp predation which in turn would limit nursery areas for juvenile fishes, cause bank erosion, and accelerate eutrophication through release of nutrients previously stored in the plants. A risk also exists that grass carp could adversely impact waterfowl habitat and rice fields. However, no major adverse impacts associated with grass carp have yet been documented. Although common carp was not introduced to North America for aquatic weed control, its foraging behavior re- sults in vegetation removal both by direct consumption and by uprooting due to its proclivity to dig through substrate in search of food. The latter activity also results in increased water turbidity. The common carp is the most often cited nuisance introduced fish in North America (Kohler and Stanley 1984) with millions of dollars having been spent for control and eradication, but with little success (Laycock 1966; Courtenay and Robins 1973). Besides grass carp, only the redbelly tilapia has been widely used in weed control programs in North America. No effects on native communities have yet been attributed to vegeta- tionremoval byany ofthe tilapias (Taylor et a1.1984), though increases in turbidity have been attributed to digging activ- ities of the blue tilapia (Noble et al. 1975) and to organic _ enrichment through fecal decomposition by redbelly tilapia (Hickling 1961; Phillippy 1969). Trophic Alteration Taylor et al. (1984) speculated that the introduction of any species into a novel environment should alter community trophic structure, with the nature and extent of such changes being complex and unpredictable. Though this aspect is not well documented, there is little doubt that when an intro- duced fish exhibits explosive population increases, as has occurred with the tilapias (Germany 1977; Knaggs 1977; Shafland 1979), substantial changes in native communities 39