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INTRODUCTIONS OF AQUATIC SPECIES Christopher C. Kohler and Walter R. Courtenay, Jr. 002726 APPENDIX C A. Issue: Definition The increased Irequency 01 inter. and intranational transfers of aquatic species carried out over Ihe Iasl two decades has . prompled concern relalive 10 Ihe polentiallor debasement of integrity of aquatic communities. Past introductions. inten. tional or otherwise. have run the full gamut from spectacular booms (e.g., Pacific salmon to the Great lakes) to spectacular busls (e.g., Ihe walerweed hydnlla to porlions of Ihe United States). Coqsidering ihe manifestalions of such extremes in terms 01 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. NevertheJ~ natural resource managers concur that substan. tially improved measures can and should be taken 10 increase the odds,that benefits of a given introduction will exceed risks. Currently. a number of international commissions have adopted or are considering adopting formal"codes of practice" for regulating the introduction of aquatic species (see Sinder. mann 1986; Welcomme 1986; Kohler and Courtenay 1986). Implementation of such codes (protocols, guidelines, etc.) can er:.sure that decisions regarding future introductions are based on sound ecolog'cal evidence, and that introductions effectu- ated are properly evaluated. B. Negative Impacts on Aquatic Communitie.s The impacts of introduced aquatic organisms on native aqua- tic communities in North America have beer. summarized by Conlreras and Escalanle(l984) for MexX:o, by Taylor et aL 0984) for the contir.ental United States, and by Crossman (1984) for Canada. These impacts can he classified into" five "broad categories: habitat alteration, trophic alteration, spatial alteration, gene pool deterioration, and introduction of diseases_ Habitat Alteration Introduced plants such as water hyacinth (see Table 1 lor scientific names of organisms cited in text). Eurasian watermil. (oil, alligator weed, and hydrilla have seriously infested a number of water bodies in North America (Shireman 1984). Excessive vegetation ;nterferes with swimming and "fishing activities, upsets predator-prey relationshipsby.providing.too much cover, causes water quality problems"during growth and decomposition, and is aesthetically unpleasant (Noble 1980). lronicalIy, exotic fishes, particularly grass carp and the lilapias, are1requcntly used as biological controls. Both the grass carp and the tilapias have reproducing popuiatlOns in North Amer. ica. although rhe habitat requirement (or larval grass carp has so far proved (0 be limiting and the lilapias arc basically limited 10 the SOuthern extreme of the United Stales and to Mexico. Ahhough grass carp have proven to bean excellent biological conlrol for aQualic vegetation. a risk c"xists Ih"l aquatic planls (including native lorms) might become overly decimated as a result of grass carp predation which in turn would fimit nursery areas lor. juvenile fashes, cause bank erosion, and accelerate eutrophication through releaseof nutrients previously stored in the planls. A risk also exists that grass carp could adversely impact waterfowl habitat and rice fields. However, no maior adverse impacls associated with grass carp have yet been documented. Although common carp was not introduced (0 North Amer- ica (or aquatic weed control, its (oraging behavior results in vegetation removal both by direct consumption and by uproot. ing due 10 its procfNity to 019 through substrate in search of food. The Ialler activityalso results in increased water turbidity. The common carp is the 1T\Qst often cited nuisance introduced fISh in North America (Kohle1'and St"';ley 1984) with millions of dollars having been spent for control and eradication, but with little success (u.ycock 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 attnouted to vegetation removal by any of the tilapias (raylor et aL 1984), though increases in f1;rbidity have been attnbuted to digging activities of th~ blue tilapia (Noble et al. 1975) and toorgani..:: enrichment through fecal decomposition by redbelly tnap;" (Hickling 1961; Phi\\;ppy \%9) Trophic Alleranon Taylor et al. (1984) speculated that the introduction of any species into a novel environment should alter ccmmunilY tro- phic strJC:Ufe, with the n.)ture and extent of s~ch change..::; being complex and unpre.di.ctable.. Though this aspect i.s 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; ShaOand 1979), substantial changes in native communities must occur. Like. wise. several dozen studies have documented dietary overlap between introduced and native fishes (see Taylor et a!. 1984). However. these studies only demonstrate that the potential lor compelilion exists_ Linking dietary overlap to competition has proven to be a difficult task for all but the most controlled ecological studies regardless of whether non.native species are involued. Documentalion or predation by introduced species on native species serves as the most definitive example of impacts on communitics" The most frequently cited example in North America concerns declines in populattons of. native trouls attributable to brown trout predation (see Moyle -l976a.b; Sharpe 1962; Alexander 1977. 1979). Several other Introduced lishes have been implicated as major causes of mOi=tality.among native fishes, including pike killifish (Miley 1978; Turner 1981; Andecson 193\, 1982), oscar (Hogg 1976), and the baicdiella (Quc)S! 1961). nlough frequenlly cited as a potential thrNl of 22