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<br />APPENorx c <br /> <br />INTRODUCTIONS OF AQUATIC SPECIES <br /> <br />Christopher C. Kohler and Walter R. Courtenay, Jr. <br /> <br />A. Issue Definition <br /> <br />The increased frequency 01 inter- and intranational transfers <br />01 aquatic species carried out over the last two decades has . <br />prompted concern relativ~ to the potential lor debasement 01 <br />integrity 01 aquatic communities. Past introduetions, inten- <br />tional or otherwise, have run the full gamut from spectacular <br />booms (e.g., Pacific salmon to the Great Lakes) to spectaeular <br />busts (e.g., the waterweed hydn11a to portions of the United <br />States). CoQsidering ihe manilestations 01 such extremes in <br />terms of ecological and economical impacts, it-is not su",rising <br />that opposing viewpoints exist with respect to the relative pros <br />and cons 01 effectuating introductions 01 aquatic species. <br />Nevertheless; natural resource managers concur that substan- <br />tially improved measures can and should be taken to inerease <br />the odds,that benefits of a given introduction will exceed,risks. <br />Currentiy. a number of international commissions have <br />adopted or are considering adopting lonnal"codes 01 praetice" <br />for regulating the introduction 01 aquatic species (see Sinder- <br />mann 1986; Welcomme 1986; Kohler and Courtenay 1986). <br />Implementation 01 sueh codes (protocols, guidelines, ele.) can <br />ensure that decisions regarding future introductions are based <br />on sound ecological evidence, and that introductions eCfectu- <br />aled are properly evaluated. <br /> <br />B. Negative [mpacts On Aquatic CommunitiES <br /> <br />The impactsoCintroduccd aquaticorganisrnson native aqua. <br />tic communities in North America have been summarized by <br />Contreras and Escalante .(1984) for Mexico, by Taylor et al. <br />0984) lor the continental United States, and by Crossman <br />(1984) for Canada. These impacts can be classified into' hve <br />'broad categories: habitat alteration, trophic alteration, spatial <br />alteration, gene pool deterioration. and introduction or <br />diseases. <br /> <br />Habitat Alteration <br /> <br />Inlrodueed plants sueh as water hyaeinth (see T abl. ] lor <br />scientific names or organisms cited in text), Eurasian watermil. <br />loil, alligalor weed, and hydrilla have seriously infested a <br />number of water bodies in North America (Shireman 1984). <br />Excessive vegetation interferes with swimming and 'fishing <br />aClivities, upsets predator.prey relationships by .pr,oviding.loo <br />mueh cover, causes waler quality problems during growth and <br />decomposilion, and is aesthetically unpleasant (Nobl. 1980), <br />Ironically, exotic rishes. particularly grass carp and lhe lilapias. <br />are1requently used as biological controls. Both the-grass carp <br />and the tilapias have reprodueing POpUiatlOns in North Amer. <br />ica, although the habitat requirement lor larval grass ea", has <br />~o rar proved 10 be limiting and the tilapias are basically limited <br />to the southern extreme or the United States and to Mexico. <br />Although grass carp have p;oven 10 bean excellent biological <br />control for aquatic vegelation. a risk c'xiSIS Ihi'll aquatic planls <br /> <br />(including native forms) might become overly decimated as a <br />result 01 grass carp predation which in turn would limit nurseI}' <br />areas for. juvenile fIShes, cause bank erosion, and accelerate <br />eutrophication through releaseol nutrients previously stored in <br />the plants. A risk also exists that grass carp could adversely <br />impact waterlowl habitat and rice fields. However, no major <br />adverse impacls associated with grass carp have yet been <br />documented. <br />Allhough common carp was not introduced to North Amer. <br />ica for aquatic weed contro~ its foraging behavior results in <br />vegetation removal both by direct consumption and by uproot. <br />ing due to its procrlVity to 019 through substrate in search 01 <br />lood. The Ialler activitya\so results in increased water turbidity. <br />The common ca", is the mQSl often cited nuisance introduced <br />fISh in North America (Kohl<!1'andStaOley 1984) with miUionsol <br />dollars having been spent for control and eradication, but with <br />little success (Laycock 1966; Courtenay and Robins 1973)_ <br />Besides grass ca"" only the redbe1ly tilapia has been widely <br />used in weed control programs in North America. No effects on <br />native communities have yet been attnbuted to vegetation <br />removal by any of the tilapias (Taylor et at 1984), though <br />increases in turbidity have been attnouted to digging activities <br />01 the blue tilapia (Noble el aI. 1975) and to organi.: enrichment <br />through lecal deeomposition by redbe1ly tilap:a (Hiekling 1961; <br />Phillippy 1969). <br /> <br />Trophic Alteration <br /> <br />Taylor et al. (1984) speculated Ihat the introduction of any <br />species into a novel environment should alter community tro. <br />phic 5tr'.Jc:ufc, with the n3tur~ and extent or such change5 <br />being complex and unpredictable. Though this aspeet is not <br />well documented, there is lillle doubt that when an introdueed <br />fish exhibits explosive population increases, as- has occurred <br />with the tilapias (Germany 1977; Knaggs 1977; Shafland 1979). <br />substantial changes in native communities must occur. Like. <br />wise, several dozen studies have documented dietary overlap <br />between introduced and native fishes (see Taylor et a!. 1984). <br />However, these studies only demonstrate that the potential for <br />competition exists. Unking dietary overlap to competition has <br />proven to be a, diffieull task for aUbut the most controlled <br />ecological studies regardless or whether-nan.native species are <br />involued. <br />Documentationof predation by introduced species on native" <br />species serves as the most definitive example. or impacts on <br />communilies. The most (requendy cited example in North <br />America concerns declines in populations of. native (routs <br />allributable to brown lrout predation (see Moyle '1976a,b: <br />Sharpe 1%2; Alexander 1977. 1979). Several other introdueed <br />fishes have been implicated as major causes of mOa:talityamong <br />nalive fishes, including pike killifish (Miley 1978; Turner 1981: <br />Anderson 1981.'1982), oscar (Hogg 1976). and the bairdidla <br />(QuaSI 1961). TI10ugh frequently cited as a potential threat b( <br /> <br />22 <br />