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Water: Ballast Water

Pathways for Invasive Species Introduction

Globalization has vastly increased long-distance travel and commerce, and highly altered waterways. These and other factors have increased the frequency by orders of magnitude by which non-native plants, animals and pathogens are introduced to new areas, sometimes with costly results. Invasive species can enter important aquatic habitats including riparian zones and wetlands by several common pathways listed below.

  • Ballast Water: Since 95% of all foreign goods by weight enter the U.S. through its ports, the potential for invasive species impacts on coastal communities is immense.
  • Boat Hulls, Fishing Gear and Other Recreational Pathways: Boat hulls, fishing boots (felt-soled wading boots transport whirling disease organisms from stream to stream) and equipment, diving gear, and other recreational items that are transported among several water bodies have been known to spread invasive species problems to new waters. Some zebra mussels and milfoil have been introduced via these pathways.
  • Aquaculture Escapes: Non-native shrimp, oysters and Atlantic salmon in the Pacific Northwest, are just a few examples of non-native mariculture species that have generated concern over disease and other impacts that might arise from their escape.
  • Intentional Introductions: The introduction of non-indigenous species into ecosystems with few controls on reproduction or distribution.
  • Aquaria Releases: Escapes or intentional release of unwanted pets can be a source of new non-native species in all parts of the country. The invasive algae Caulerpa is thought to have been introduced to U.S. waterways after being discarded from aquaria.
  • Live Food Industry: The import of live, exotic foods and the release of those organisms can result in significant control costs, e.g. the snakehead fish in Maryland. Asian swamp eels are spreading through the Southeast after introduction as a food source.
  • Vehicular Transportation: Both private and commercial transportation are major factors in the movement and range expansion of non-native species throughout the U.S.
  • Escaped Ornamental Plants, Nurseries Sales, or Disposals: Many invasive plant problems began as ornamental plantings for sale in nurseries and garden shops. Purple loosestrife, for example, is sold as an ornamental plant but takes over native vegetation in wetlands, and can clog western streams preventing water withdrawal and recreational uses. Only some problem species are currently banned from sale.
  • Cross-basin Connections: From small channels to major intercoastal waterways, new connections between isolated water bodies have allowed the spread of many invasive species. Great Lakes invasions increased markedly after the opening of the St. Lawrence Seaway in 1959.
  • Fishing Bait Releases: Discarding unused bait can introduce species that disrupt their new ecosystems and eliminate competing native species; examples include non-native crayfish, baitfish that overpopulate certain waters, and earthworms that are depleting the organic duff layer in northern forests where no indigenous earthworms existed (Conover, 2000).
  • Illegal Stockings: Although prohibited by law, people release fish into new waters and sometimes cause severe impacts. Yellowstone Lake's world-class cutthroat trout fishery is now jeopardized by an illegal release of lake trout.
  • Domestic Animals Gone Feral: The impact of feral house cats on birds and small mammals in natural areas is well documented; escaped feral pigs from farms have recently begun to do significant damage to soils and plants in the Smokey Mountains.
  • Pathogens Spread by Non-natives to Vulnerable Native Species: Non-native species problems include pathogens carried by resistant non-natives to vulnerable native species. Whirling disease, which has decimated rainbow trout in many western rivers, was originally introduced when European brown trout, tolerant of whirling disease, were imported to U.S. waters and hatcheries.
  • Disposal of Solid Waste or Wastewater: Seeds, viable roots or other propagules of invasive plants may be easily spread to receiving waters through wastewater discharge, then spread by water flow to distant areas downstream.
  • Science/laboratory Escapes, Disposals or Introductions: Accidental or intentional release of laboratory animals has introduced some non-native species into U .S. waters.
  • Seafood Packing and Disposal: Much seafood is packed in seaweed prior to distribution. Because seafood is transported long distances, organisms in packing seaweed may reach new waters as an unintended by-product.
  • Biological Control Introductions: Ideally, introducing a second non-native species to control an invader should result in diminished numbers of both species after control is accomplished, but some introduced controls have backfired because they attack non-target species. Mongoose introduced in Hawaii to control rats have wiped out many native bird species.
  • Past Government Programs: The establishment of a new invader is sometimes an unanticipated outcome of a government program; kudzu, for example, was originally introduced through a government-sponsored erosion control program.
  • Moving and Depositing Fill in Wetlands: Seeds and viable parts of invasive plants contained in fill material may rapidly colonize the new substrate, which then compete with native species within the wetlands.
  • Land/water Alterations That Help Spread Invaders: Many invaders are adept at rapid pioneering where soil has been disturbed or water levels or routes have been changed, leaving a temporary gap in occupation by native flora and fauna.

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