| Non-native aquatic species in the United States and coastal waters include species from many plant and animal taxa and span the entire country (Figure). That this problem is extensive is clear by the numbers: 139 nonindigenous species are now established in the Great Lakes (Mills et al. 1993); 32 species of nonindigenous marine organisms were collected from one small Oregon estuary (Carlton 1991); 96 nonindigenous sponges, worms, crustaceans, and other invertebrates are now found in San Francisco Bay (Carlton 1979); and more than half of Hawaii's free-living species are nonindigenous (U.S. Congress 1993). The rate of nonindigenous species' introductions into the Great Lakes has increased in spurts since 1810, largely in response to an expanding human population, development in the basin, and increased transoceanic shipping. |
Figure. Distribution of purple loosestrife, Asian clam, zebra mussel, and carp in the United States (shading indicates species presence). *U.S. Congress 1993. **U.S. Department of the Interior, National Biological Service 1994. Non-native aquatic species data base. |
|
Benefits and Costs |
||
| Nonindigenous aquatic species have been both beneficial and problematic. Beneficial aspects include enhancing recreational opportunities such as sport-fishing; providing reliable, high-quality food via aquaculture and mariculture; and aesthetically improving the human environment via the aquarium industry. Recreational fishing contributed an estimated $24 billion in expenditures and $69.4 billion in economic output in 1991 (SFI 1994). | ||
| Problems associated with nonindigenous aquatic species are primarily related to ecological issues, such as their effects on indigenous species, and financial issues, such as economic losses caused by biofouling of water-intake pipes. For example, nonindigenous species were cited as a contributing cause in the extinction of 27 species and 13 subspecies of North American fishes over the past 100 years (Miller et al. 1989). Federal, state, and local governments, as well as industry, have often borne significant costs related to nonindigenous aquatic species. From 1906 to 1991, estimated losses associated with 79 aquatic and terrestrial nonindigenous species were roughly $97 billion (Table 1), and worst-case estimates for 15 potential high-impact nonindigenous species project future economic losses of another $134 billion (U.S. Congress 1993). |
Table 1. Estimated cumulative losses to the United States from selected categories 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 aquaculture 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 japonica) that preys on native oysters, a flatworm (Pseudostylochus ostreophagus), and possibly also a copepod parasite (Mytilicola orientalis). An Asian tapeworm (Bothriocephalus opsarichthydis) was found in several species of native fish in the 1970's following its introduction via infected grass carp (Ctenopharyngodon idella). A non-native freshwater snail (Potamopyrgus antipodarum) that probably escaped from a fish aquaculture facility 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 verticillata), 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 aquarium 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 Williams 1992; U.S. Congress 1993). | ||
| Another major introduction and dispersal pathway for non-native aquatic species is via ballast water discharge. Since many ports are infested with non-native aquatic species, ballasting operations often bring these species, as well as indigenous species, into the ballast tanks of a vessel. These organisms are then transported 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 environments. This mode of introduction is probably responsible for the introduction of zebra mussels, ruffe (Gymnocephalus cernuus), and the spiny water flea (Bythotrephes cederstroemi) into the Great Lakes (U.S. Congress 1993). | ||
| Many non-native aquatic species are intentionally imported as pets, for aquaculture, or to supplement recreational fishing. State and federal natural resource agencies have intentionally introduced a variety of non-native aquatic species to enhance recreational and commercial interests (e.g., brown trout [Salmo trutta], carp, and Pacific oyster). Some animals (e.g., water fleas, freshwater shrimp, crayfish, and others; Wildlife Nurseries, Inc. 1989) can be purchased through the mail and introduced outside their natural range. Many tropical aquarium 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 cultured species will eventually escape confinement and enter U.S. waterways. | ||
Assessment and Monitoring |
||
Research Strategies |
||
| Three main research strategies are used to limit the damages caused by nonindigenous aquatic species: prevention, control, and detection and monitoring. 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 100% effective and can never eliminate all threats from nonindigenous aquatic species. Therefore, rapid response and control techniques must be identified and in place to control and limit damages caused by nonindigenous ANS. This approach is being used to control ruffe. | ||
| Control is intended to reduce the effects of nonindigenous aquatic species through eradication, reduction in numbers to tolerable levels, and exclusion from sensitive areas. Three general control methods exist to prevent the spread of these species: chemical, biological, and physical. Proper evaluation and use of selective chemicals may provide effective control of non-native aquatic species with an apparent minimum of ecological hazard or other side effects. Increasing concern exists, however, about the long-term environmental safety and impacts of chemicals used to control nonindigenous aquatic species. Efforts to control sea lamprey (Petromyzon marinus) in the Great Lakes are a prime example of chemical control. This control has been highly successful in reducing the population size of an invading species, but carries an enormous price tag: more than $10 million annually (U.S. Congress 1993). | ||
| Carefully planned biological-control programs may provide rapid, cost-effective control and pose negligible ecological problems. The success rate for biological-control programs typically ranges from 16% to 36% (Meyers et al. 1989) and improperly screened biological-control agents have themselves become nuisance species in the past (e.g., blue tilapia [Tilapia aurea]; McClelland 1992). | ||
| Although often very expensive, physical control of aquatic nuisance species can be an appropriate technique in certain circumstances. Physical control has been used to control nuisance aquatic weeds like Eurasian watermilfoil (Myriophyllum spicatum). | ||
| Since no single method is likely to provide the necessary level of control, a comprehensive, integrated control strategy combining techniques is usually necessary for an effective control program. Few, if any, control methods are without some environmental risk. When properly used, and with continual monitoring for effectiveness and ecological side effects, environmentally sound control of at least some aquatic nuisance species can be achieved, as in the Great Lakes sea lamprey control program. | ||
| Detection and monitoring strategies serve as early warning systems that first identify new invasions and then track ranges and populations. This strategy complements or integrates prevention and control to allow for early intervention and assessment of management actions. The capability for early detection of new invasions will allow managers to implement strategies for limiting their spread and reducing negative effects. Timely detection of non-native aquatic species that are or could become nuisances can also help identify gaps in prevention procedures. Monitoring of those organisms will not only allow rapid response if harmful situations arise but will also allow verification or repudiation of assumptions that may have been made during assessments before intentional releases. | ||
| Because of extremely limited resources, cooperative ventures and collaborations between agencies are essential for collecting monitoring information. The Detection and Monitoring Committee of the ANS program is developing a national network to coordinate and provide information regarding occurrences of known nonindigenous aquatic species. This network is intended to provide managers and researchers with an important tool for determining the status of a particular nonindigenous aquatic species, its potential and known effects, and proven or potential control techniques. | ||
| By and large, three interrelated problems associated with nonindigenous ANS remain unsolved: (1) determining levels of acceptable risk; (2) setting thresholds or other variables above which more formal decision making and costly approaches for control are invoked; and (3) identifying trade-offs in terms of costs and economic ramifications in the face of uncertainty as to probable success in controlling ANS. Current federal methods and programs to identify risks of potentially harmful nonindigenous aquatic species have many shortfalls--including long response times. | ||
Summary |
||
| Nonindigenous aquatic species are widespread in the United States. While many of these organisms have been intentionally introduced, many others dispersed via unintended introductions. The potential for ecological and economic harm resulting from introductions of nonindigenous aquatic species can be large. For example, zebra mussels seem to be jeopardizing a number of native North American mussel species (Williams et al. 1993) and could result in economic losses in excess of $3 billion (U.S. Congress 1993). The actual extent of problems associated with non-native aquatic species remains largely unknown. The ability to detect new species and limit their dispersal before they become problematic is critical if we are to limit future nonindigenous species problems. | ||
|
U.S. Fish and Wildlife Service 1849 C Street, NW, Mailstop ARLSQ-820 Washington, DC 20240 |
||
| References | |
|---|---|
|
Carlton, J.T. 1979. Introduced invertebrates of San Francisco Bay. Pages 427-444 in T.J. Conomos, ed. San Francisco Bay: the urban estuary. American Association of the Advancement of Science, Pacific Division, San Francisco, CA. Carlton, J.T. 1991. Man's role in changing the face of the ocean: biological invasions and implications for conservation of near-shore environments. Conservation Biology 3(3):265-273. Courtenay, W.R., and J.D. Williams. 1992. Dispersal of exotic species from aquaculture sources, with emphasis on freshwater fishes. Pages 49-81 in A. Rosenfield and R. Mann, eds. Dispersal of living organisms into aquatic ecosystems. Maryland Sea Grant, College Park. McClelland, M. 1992. Invasion of the bio-snatchers. Florida Environments 6(2):5,13. Meyers, J.H., C. Higgins, and E. Kovacs. 1989. How many insect species are necessary for the biological control of insects? Environmental Entomology 18:541-547. Miller, R.R., J.D. Williams, and J.E. Williams. 1989. Extinctions of North American fishes during the last century. Fisheries 14(6):22-38. |
Mills, E.L., J.H. Leach, J.T. Carlton, and C.L. Secor. 1993. Exotic species in the Great Lakes: a history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19(1):1-54. SFI. 1994. Sport Fishing Institute releases report on economics of sport fishing. Sport Fishing Institute Bull. 452:6, March/April 1994. U.S. Congress, Office of Technology Assessment. 1993. Harmful non-indigenous species in the United States. OTA-F-565. U.S. Government Printing Office, Washington, DC. 391 pp. Wildlife Nurseries, Inc. 1989. What brings them in. Catalogue of animals and plants for wildlife habitat. Oshkosh, WI. Williams, J.D., M.L. Warren, Jr., K.S. Cummings, J.L. Harris, and R.J. Neves. 1993. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18(9):6-22. Williams, M.C. 1980. Purposefully introduced plants that have become noxious or poisonous weeds. Weed Science 28(3):300-305. |
