Pathways of Introduction and Spread

Aquatic invasive species can be introduced both accidentally and intentionally into Canadian waters by human activities. Without effective natural predators or competitors, they can become established in our waters and spread rapidly.

Such species have been entering Canadian waters for centuries; however, recent introductions and spread are much more rapid because of the volume of international shipping.

The most effective approach to prevent the introduction and spread of new invasive species in Canada is to manage the pathways through which invasive species enter and spread in our waters.

Live Animal Trade

Aquariums and ponds: Aquatic invasive species can be introduced when live fishing bait or fish from public or private aquariums are released into natural aquatic ecosystems.

Live fish for consumption: Fish, molluscs and crustaceans for human consumption, and any species unknowingly transported with them, may escape or be released into the environment.


Zebra Mussel, Spiny Waterflea and Green Crab were introduced through the discharge of ballast water and biological fouling.

Regulations and technologies have been implemented to reduce the impact of shipping as a potential source of introductions.

Ballast and bilge water from ships may contain eggs, larvae and juveniles of larger species (fish, molluscs, crustaceans) and the adults of smaller species (tunicates, plankton).

  • Canadian Ballast Water Program
    Transport Canada
  • Ballast Water Management in the Great Lakes Reduces the Introduction of Aquatic Invasive Species
    Fisheries and Oceans Canada Study

Biological fouling occurs when organisms, especially invertebrates, attach to the hulls and other submerged areas of vessels. These small living communities travel with the vessel to new areas where they can fall off or dislodge and become invasive.


  • Vegetation control: Sometimes fish are used in artificial aquaculture ponds and pools to control vegetation. Most aquaculturists use sterile or “triploid” fish for this purpose, but the sterilization process is not always completely effective and “escapees” may establish new populations of invasive fish.
  • Introductions and Transfers: The planned movement of live aquatic organisms, i.e. fish, shellfish and plants, through introductions or transfers supports aquaculture, commercial and recreational fisheries, stock enhancement, research, education, and ecological restoration. These movements are regulated to ensure they take place in a way that limits impacts on other animals and ecosystems.

Recreational Boating and Nautical Activities

Vessels, float planes, sailboats, personal watercraft, kayaks, diving equipment, ropes and fishing gear may transport the attached fragments, larvae and eggs of invasive species to new bodies of water.

Sport Fishery

  • Deliberate introduction: Species that are popular with fishers are sometimes introduced into Canadian waters, without consideration of the threat they could pose to native species.
  • Accidental introduction: Aquatic invasive species are sometimes introduced accidentally when holding tanks are emptied or live bait is thrown into waters. Water from bait buckets may contain juveniles, eggs, larvae or fragments of undesirable aquatic species.

Canals and Water Diversions

Canals, channels and bulk water diversions can be used for a variety of purposes: shipping, hydro-electric power development, recreation, commerce, water supply, flood control, and agriculture. They create artificial connections allowing the free movement of aquatic invasive species across physical barriers, between watersheds.

Cultural Release

Some symbolic cultural practices may involve the deliberate release of a live animal into nature. People may also release a captive fish (e.g. from a food market) with the idea to free it into the wild. Sometimes, those who practise these traditions are unaware of the potentially harmful impacts of these practices.

For further information on important pathways of introduction and spread of aquatic invasive species, consult A Canadian Action Plan to Address the Threat of Aquatic Invasive Species.

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