Our /./v'//;,!,' R('\(nin('.\ — Non-native Species 



429 



Tiibli' I. Kslimaled iiimulalive losses to the United States 

 from selected categones of harmful nonindigenous 

 species. 1906-91 (U.S. Congress 1993). 



Introduction and Dispersal 



Many non-native aquatic species have 

 entered the country in infested stock for aqua- 

 culture or fishery enhancement. For example, 

 the introduction of the Pacific oyster 

 (Crassostrea gigas) to the west coast in the 

 1920"s brought with it a Japanese snail 

 (Ocenebra japouka) that preys on native oys- 

 ters, a flatworm {Pseiidostylochus ostreopha- 

 giis). and possibly also a copepod parasite 

 (Mytilicola orientalis). An Asian tapeworm 

 (Bothriocephahis opsarichthydis) was found in 

 several species of native fish in the 1970"s fol- 

 lowing its introduction via infected grass caip 

 {Ctenopharyngodon idella). A non-native fresh- 

 water snail (Potamopyrgiis antipodarum) that 

 probably escaped from a fish aquaculture facil- 

 ity now threatens indigenous mollusks of the 

 Snake River region. 



The aquarium industry is a significant entry 

 and dispersal pathway for non-native aquatic 

 species. Hydrilla {Hydrilla verticillala). an 

 aquatic weed that causes a major navigation 

 hazard, is believed to have been released by 

 aquarium dealers in an attempt to create a 

 domestic source of the plant (Williams 1980). 

 At least three snail species entered U.S. waters 

 when individual snails were discarded by aquar- 

 ium dealers or their customers over the past few 

 decades. Since 1980, releases from aquaria 

 were the source of at least seven nonindigenous 

 fish species that are now established, and the 

 aquarium fish industry is believed the source of 

 at least 27 nonindigenous fish species now 

 established in the continental United States 

 (Courtenay and Williains 1992: U.S. Congress 

 1993). 



Another major introduction and dispersal 

 pathway for non-nafive aquatic species is via 

 ballast water discharge. Since many ports are 

 infested with non-native aquatic species, bal- 

 lasting operations often bring these species, as 

 well as indigenous species, into the ballast tanks 

 of a vessel. These organisms are then transport- 

 ed around the world within the ballast tanks. 

 When a vessel unloads or picks up cargo, the 

 operator often empties the ballast tanks, thus 



introducing these organisms into new environ- 

 ments. This mode of introduction is probably 

 responsible for the introduction of zebra mus- 

 sels, ruffe (Gymnocephahis cenuais). and the 

 spiny water tlea {Bythotrephes cedcrstrocmi) 

 into the Great Lakes (U.S. Congress 199.^). 



Many non-native aquatic species are inten- 

 tionally imported as pets, for aquaculture. or to 

 supplement recreational fishing. State and fed- 

 eral natural resource agencies have intentional- 

 ly introduced a variety of non-native aquatic 

 species to enhance recreational and commercial 

 interests (e.g., brown trout [Saliiio tnitla], carp, 

 and Pacific oyster). Some animals (e.g., water 

 fieas. freshwater shrimp, crayfish, and others; 

 Wildlife Nurseries, Inc. 1989) can be purchased 

 through the mail and introduced outside their 

 natural range. Many tropical aquariuin species 

 now found in Florida's waters escaped from 

 aquaculture facilities (Courtenay and Williams 

 1992). The Aquatic Nuisance Species (ANS) 

 Task Force suggests that it is inevitable that cul- 

 tured species will eventually escape confine- 

 ment and enter U.S. waterways. 



Assessment and Monitoring 



Efforts to assess or monitor non-native 

 aquatic species are, at best, fragmented. 

 Generally, these species are not monitored until 

 they reach nuisance status, such as purple 

 loosestrife {Lythrum salicaria) or zebra mussels 

 have, and no broad, nationally coordinated pro- 

 gram exists for detecting new species. A nation- 

 ally coordinated effort for providing timely 

 notification to appropriate entities of the detec- 

 tion and dispersal of all non-native aquatic 

 species is needed. There is currently no defini- 

 tive evidence to suggest that rates of introduc- 

 tion for non-native aquatic species are increas- 

 ing or decreasing (Table 2). 



Table 2. Number of new species of foreign origin estab- 

 lished per decade (U.S. Congress 199.3). 



Purple loosestrife (Lythrum salicaria)^ 



Research Strategies 



Three main research strategies are used to 

 limit the damages caused by nonindigenous 

 aquatic species: prevention, control, and detec- 

 tion and monitoiing. Prevention relies on the 

 identification and elimination of pathways 

 through which nonindigenous ANS enter the 

 nation's waters. Although prevention should be 

 the first line of defense, it is unlikely to be 



Zebra mussel {Dreissena polyrtiorptia)* 



Asian clam {Corbicula fluminea)* 



Common carp {Cyprinus carpio)'* 



Figure. Distribution of purple 

 loosestrife. Asian clam, zebra mus- 

 ,sel. and carp in the United States 

 (shading indicates species pres- 

 ence). 



*U.S. Congress 1993. 

 **U.S. Department of the Interior, 

 National Biological Service 1994. 

 Non-native aquatic species data 

 base. 



