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considerable consequence, predation on eggs or young by introduced fishes has not been demonstrated to be a common occurence (Taylor et al. 1984). Spatial Alteration Concommittant overlap in usage of space by non-native and native fishes may Lead to competititve interaction if space is in limited supply or of variable quality. Evidence exists implicating displacement of brook trout by brown trout, but in general, displacements are largely inferential (Taylor et al. 1984). Con- versely, high densities of introduced fishes have been shown to exert negative effects on native fishes. For example, Noble et al. (1975) observed that largemouth bass populations in Trinidad Lake, Texas, declined with no evidence of recruitment as densi- ties ofblue tilapia rose to approximately 2,240 kg/ha' during the period of 1972-:975. Gene Pool Deterr.~ration Through reduction of heterogeneity through inbreeding is clearly a threat to any species Ming produced in a hatchery (Philipp et al. 1983), the risk is most acute with species of intercontinental origin because the initial broodstock invariably represent limited gene pools at the outset. The larger the stock- ingprogram, the more inbreeding among original broodstock is necessary. Thus species introduced to a novel habitat mayor may not have the genetic characteristics necessary for them to adapt and/or perform as predicted. Fortunately, hybriaization events among introduced and native species in open waters are rare (Taylor et al. 1984). Nevertheless, the possibility of native gene pools being altered through such hybridization does exist. For example, brown trout are known to hybridize with native forms in North Amer- ica (Schwartz 1972,1981; Dangel et al. 1973; Chevassus 1979). Introduction of Diseases Diseases caused by bacteria, viruses, and parasites are all too often conveyed along with introduced aquatic species (see Hoffman and Schubert 1984; Shotts and Gratzek 1984 for reviews). This aspect represents one of the most severe threats that an introduced species may pose to a native community. Transfer of diseased fish was no doubt responsible for introduc- tion of whirling disease into North America from Europe. Recently, infectious hypodermal and hematopoietic necrosis virus (IHHNV) has been spread to a number of countries in conjunction with shipments of live penaeid shrimp. [HHNV was first diagnosed in 1981 at shrimp culture facilities in Hawaii among shrimp introduced from Panama (Sindermann 1986). Even "ich," one of the most common fish diseases worldwide, caused by a ciliated protozoan, is thought to have been trans- ferred from Asia throughout the temperate zone with ship- ments of fishes (Hoffman 1970, 1981). C. Courses of Action Introduction of species to aquatic communities are com- monly employed as a fisheries management tool or occur as a result of escapes from aquaculture or ornamental fish holding facilities. It is not feasible, nor desirable, to legislate against all such introductions. What is needed is more education on the role that introduced species can and should play in the context of aquatic resources management. The more informed natural resource managers are about such issues, the less likely that Table 1.Organisms cited in text. Common Name Scientific Name Plants hydrilia Hydrilla verticillata water hyacinth Eichornia crassipes Eurasian watermilfoil Myriophyllum spicatum alligator weed Alternanthera philoxeroides Fsh Pacific salmon Oncorhyncus sp. grass carp Ctenopharyngodon idella common carp Cyprinus corpio tilapias Oreochromis, Sorotherodon and Tilapia sp. blue tilapia Oreochromis oureus (= Tilapia aureau) redbelly tilapia - Tilapia zi7(i brown trout Salmo trutta pike killifish Belonesox belizanus Oscar Astronotus ocellatus bairdiella Bairdiella icistia brook trout Saluelinus fontinalis largemouth bass Micropterus salmoides coho salmon -- -~- Oncorhyncus kisutch striped bass ~ Morone saxatilis walking catfish Clorios batrochus Other whirling disease Myxosoma cerebralis "ich" Ichthyopthirius mulhfiliis mistakes will be made or that legislation will be necessary to enforce an "attitude of caution." The following actions toward that end are recommended. A. The membership reaffirms its endorsement of the 1972 "Position of the American Fisheries Society on Introduction of Exotic Aquatic Species" as modified: Position of American Fisfieries Society on Introduced Aquatic Species. Our purpose is to formulate a broad mechanism for planning, regulating, implementing, and monitoring all introductions of aquatic species. Some introductions of species into ecosystems in which they are not native have been successful and others unfortunate. Species not native to an ecosystem will be termed "intro- duced." Some introductions are in some sense, planned and purposeful for management reasons; others are accidental or are simply ways of disposing of unwanted pets or research organisms. It is recommended that the policy of the American Fisheries Society be: 1. Encourage fish importers, farmers, dealers, and hobbyists to prevent and discourage the accidental or purposeful intro- duction of aquatic species into their local ecosystems. 2. Urge that no city, county, state, province, or federal agency introduce, or allow to be introduced, ary species into any waters within its jurisdiction which might contaminate any waters outside its jurisdiction without official sanction of the exposed jurisdiction. 3. Urge that only ornamental aquarium fish dealers be per- mitted to import such fishes #or sale or distribution to hobbyists. 47