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A_ Issue Definition APPENDIX C INTRODUCTIONS OF AQUATIC SPECIES Christopher C. Kohler and Walter R. Courtcnay, Jr. The increased [requency of inter- and intraatronal transfers of aquatic species carried out over the last two decades has prompted concern relative to the potential for debasement of integrity of aquatic communities. Past introductions, inten- tional or otherwise, have can the full gamut from spectacular booms (e.g., Pacific salmon to the Great Lakes) to spectacular busts (e.g., the waterweed hydrilla to ponans of the United States). Cogsidering the manifestations of such extremes in terms of ecological and economical impacts, it is not surprising that opposing viewpoints exist with respect to the relat"rve pros and cons of effectuating introductions of aquatic species. Nevertheless, natural resource managers concur that substan- tially improved measures can and should be taken to increase the odds:that benefiis of a given introduction will exceed risks. Currently, a number of international commissions have adopted or are considering adopting formal "codes o! practice" for regulating the introduction of aquatic species (see Sinder- mann 1986; WPJcotnme 1986; Kohler and Courtenay 1986). Implementation of such codes (protocols, guidelines, etc.) can ensure that decisions regarding future introductions arc based on sound ecological evidencz, and that introductions effectu- ated are properly evaluated. B. Negative Impacis on Aquatic Communities The impacts of introduced aquatic organistnson native aqua- tic cot-tmunities in North P.merica have beer summarized by Contreras and Escalante (1984) for MexKO, by Taylor et al. i ]984) for the continental United Stales, and b; Crossmar~ (1984) ter Canada. These impacts can be classified into five broad categories: habitat alteration, trophic alteration, spatial alteration, gene pool deterioration, and introduction of diseases. ' Nobitot Alteration r Introduced plants such as water hyacinth (see Table 1 for scientific names of organisms cited in text), Eurasian k~atermil foil, alligator weed, and hydrilla have seriously infested a number of water bodies in North America (Shireman 1984). Excessive vegetation interferes with swimming and fishing activities, upsets predator-prey relationships by.providing too much cover, causes water quality problems during growth and decomposition, and is aesthetically unpleasant (Noble 1980). Ironically, exotic fishes, particularly grass carp and the tilapias, are~requently used as biological controls. Holh the grass carp and the tilapias have reproducing poputatons in Nnrt)i Amer- ica, although the habitat requirement for larval grass carp has so far proved lobe limiting and the tilapias arc basically limited to the southern extreme of the United States and to Mexico. Although grass carp have proven to be an excellent biological control (or aquatic vegetation. a risk exists that aquatic plants (including native forms} might become a~erly decimated as a result of grass carp predation which in turn would limit nursery areas (or. juvenle 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 Amer- ica for aquatic weed control, its foraging behavior results in vegetation removal both by direct consumption and by uproot- ing due to its procfnriry to d'rg through substrate in search of food. The latter activity also results in increased water turbidity. The common carp is the rnQst often ated 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 1956; Courtenayand Robins 1973). 3esides grass carp, only the redbelly tdapia has been widely used in weed cont rot programs in North America. No effects on native communities have yet been attributed to vegetation removal by any of the tlapias (Taylor et al. 1984), though increases in t•;rbidity have been attnbuted to digging activities nt the blue tilapia (Noble et al. 1975) and toorganic enrichment through fecal decompcsilion by redbe~ly tdapia (Hickling 1961; Phillippy 1959) Troph;c Ahcsotion Taylor et al. {19x4) speculated that the introduction of any species into a no,~el environment should alter community tro- Ghic s;r~c:urc, with the nature and extent of s:,ch change. being complex and unpredictable. Though this aspect is not well documented, there is little doubt that when an introduced fish exhibits explosive population increases, as has occurred with the tilapias (Germany 1977; Knaggs 1977; Sha(land 1979), substantial changes in native communities must occur. Like- wise. several dozen studies have documented dietary overlap bet.veen introduced and native fishes (see Taylor et al. 1984). f-io,vever, these studies only demonstrate that the potential for competition exists. Linking dietary overlap to competition has proven to be a difficult task for all but the most controlled ecological st udies regardless of whether non-native species are invol~~ed. (]ocumentat ion of predation by introduced species on native species serves as tl,c most definitive example of impacts on COmmun~l~CS The most frequently cited example in Nonh America concerns declines in populations of-native trouts attributable to brown trout predation (see Moyle 1976a,b; Sharpe 1962; Alexander 1977, 1979). Several other introduced fishes lave been implicated as major causesof mortality among native (isl,es, including pike killifish (Miley 1978; Turner 1981; Anderson 1981, 1982), osear (Hogg 1975), and the bairdiclla (Q~~ast 1961) Tl»u9h frequently Cited as a potential threat of 21