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Home > Special Topics > Invasive Species > Methods of Introduction & Spread

Methods of Introduction & Spread

Although wind and weather can transport and introduce NIS, people are most often responsible for the introduction and spread. These activities are as often unintentional as intentional, conscious as unconscious.


  • Shipping – Non-native species can be transported in cargo, attached to modes of transport (ships, trains, and planes), or attached to transport crew.
  • Ballast water – NIS can be taken aboard ocean-going ships accidentally in ballast water, to be emptied in a foreign port during vessel stabilization procedures. The National Ballast Water Information Clearinghouse collects and shares data about the ships and volume of ballast water exchanged in order to prevent the spread of aquatic invasive species. This method, accepted throughout the world, is the primary method to prevent the accidental spread invasive species from one region of the world to another.
  • Recreational boating – Submersed and floating NIS from a lake or stream can cling to recreational boats, boat trailers, and outboard motors when they are moved from one water body to another.
  • Angling – Some NIS may be transported from one water body to another in bait buckets, live wells, or on fishing tackle.
  • Diving – NIS can cling to diving suits and equipment.
  • Pond stocking – NIS larvae and plants can be transported into private ponds along with the transport water carrying fingerlings from hatcheries. Stocked fishes also may harbor parasites or pathogens.
  • Canals and interbasin waterways – These provide convenient pathways for non-native fishes, mussels, and plants to move from infested into non-infested waters.


  • Agriculture, horticulture, aquaculture, nursery trade – Deliberately planting non-native species can have unintended consequences. While they may produce income and attractive landscapes, they may be capable of out-competing existing species or they may harbor and possibly spread parasites or diseases.
  • Food markets – Deliberately introducing live fishes or produce for specialty food markets can have unintended consequences. While they may meet a market demand, their live release can wreak havoc in weakened habitats. An example.
  • Biological control – Sometimes insects or other non-native species are deliberately introduced to control an existing NIS. Once these biological control agents have consumed or controlled the existing NIS, they may begin to prey on or compete with other native species in order to perpetuate their population.
  • Aquarium, watergarden and pet releases – When hobbyists decide to give up an aquarium or a pet that no longer fits into the family, they sometimes release the animal into the wild. Such animals may establish a wild population or prey on existing native species.

What can people do to minimize the introduction and spread of NIS?

  • Boaters, fishers, divers, and hunters can control the spread of NIS with a few simple steps. A national program, Protect Your Waters, has more information and ideas.
    • Remove visible mud, plants, fish, or other live creatures from boat, motor, clothes, tackle, bait buckets, live wells, decoys, anchors, and other equipment at the boat ramp, dock, or parking lot before leaving the area.
    • Drain all water from equipment before leaving the area.
    • Never put a plant or animal (including live bait) into a body of water unless it came from it. If in doubt, place in trash barrel.
  • Hobbyists, aquarium owners, plant and animal wholesalers and retailers can control the spread of NIS. Nationally, wholesalers and retailers have agreed to follow specific procedures to control the spread of NIS.
    • Never release a plant or animal into a body of water unless it came from it.
    • Destroy aquarium animals humanely or give them to another aquarium owner. If possible, return them to the retailer.
    • Insist upon knowing the scientific name and details about the plant or animal purchased for aquariums and water gardens. Try to use native species.
    • Don’t empty aquariums into a water body or a storm drain.
  • All persons handling or promoting nonindigenous species in any way can follow the codes of conduct provided by the National Invasive Species Council.


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