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must occur. Likewise, several dozen studies have docu- mented dietary overlap between introduced and native fishes (see Taylor et al. 1984). However, these studies only dem- onstrate 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 studies re- gardless of whether non-native species are involved. Documentation of predation by introduced species on na- tive species serves as the most definitive example of impacts on communities. The most frequently cited example in North America concerns declines in populations of native trouts attributable to brown trout predation (see Moyle 1976a,b; Sharpe 1962; Alexander 1977, 1979). Several other intro- duced fishes have been implicated as major causes of mor- tality among native fishes, including pike killifish (Miley 1978; Turner 1981; Anderson 1981, 1982), oscar (Hogg 1976), and the bairdiella (Quast 1961). Though frequently cited as a potential threat of considerable consequence, predation on eggs or young by introduced fishes has not been dem- onstrated to be a common occurrence (Taylor et al. 1984). Spatial Alteration Concommittant overlap in usage of space by non-native and native fishes may lead to competitive 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). Conversely, 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 densities of blue tilapia rose to approximately 2,240 kg ha -' during the period 1972-1975. Gene Pool Deterioration Though reduction of heterogeneity through inbreeding is clearly a threat to any species being produced'in a hatchery (Philipp et al. 1983), the risk is most acute with species of intercontinental origin because the initial broodstock invar- iably represent limited gene pools at the outset. The larger the stocking program, the more inbreeding among original broodstock is necessary. Thus species introduced to a novel habitat may or may not have the genetic characteristics nec- essary for them to adapt and/or perform as predicted. Fortunately, hybridization events among introduced and native species in open waters are rare (Taylor et al. 1984). Nevertheless, the possibility of native gene pools being al- tered through such hybridization does exist. For example, brown trout are known to hybridize with native forms in North America (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 mast severe threats that an introduced species may pose to a native community. Transfer of diseased fish was no doubt respon- sible for introduction of whirling disease into North America Table 1. Organisms cited in text. Common Name Scientific Name Plants hydrilla Hydrilla verticillata water hyacinth Eichornia crassipes Eurasian watermilfoil Myriophyl[um spicatum alligator weed Alternanthera philoxeroides Fish Pacific salmon Oncorhyncus sp. grass carp Ctenopharyngodon idella common carp Cyprinus carpio tilapias Oreochromis, Sarotherodon and Tilapia sp. blue tilapia Oreochromis aureus (= Tilapia aurea) redbelly tilapia Tilapia zil[i brown trout Salmo trutta pike killifish Belonesox belizanus oscar Astronotus ocellatus bairdiella Bairdiella icistia brook trout Salvelinus fontinalis largemouth bass Micropterus salmoides coho salmon Oncorhyncus kisutch striped bass Morone saxatilis walking catfish Clarias batrachus Other whirling disease Myxosoma cerebralis "ich" Ichthyopthirius multifiliis from Europe. Recently, infectious hypodermal and hema- topoietic necrosis virus (IHHNV) has been spread to a num- ber of countries in conjunction with shipments of live pen- aeid shrimp. IHHNV 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 pro- tozoan, is thought to have been transferred from Asia throughout the temperate zone with shipments of fishes (Hoffman 1970, 1981). C. Courses of Action Introductions 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 hold- ing facilities. It is not feasible, nor desirable, to legislate against all such introductions. What is needed is more ed- ucation on the role that introduced species can and should play in the context of aquatic resources management. The more informed natural resources managers are about such issues, the less likely that mistakes will be made or that legislation will be necessary to enforce an "attitude of cau- tion." The following actions toward that end are recom- mended. A. The membership reaffirms its endorsement of the 1972 "Position of the American Fisheries Society on Introductions of Exotic Aquatic Species" with modifications as indicated: 40 Fisheries, Vol. 11, No. 2