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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

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June 2005

Pathways of destruction

How invasives spread between waterbodies

Phil Moy

Aquatic invasive species (AIS) are capable of increasing their range without help from people, but human activities greatly increase the ability of these organisms to spread into new areas. Small seeds, microscopic larvae, ability to hide in vegetation and the cryptic nature of non-native organisms are just some of the means by which AIS take advantage of peoples' activities to increase their range.


Aquatic invasive species arrive and move about in our state waters via ballast water, bait releases, escapes from aquaculture, aquarium releases, water garden escape or release, boaters and anglers and others. Once introduced to our rivers and streams they can easily spread by currents. Many aquatic plants spread through fragmentation and are able to establish new colonies from small pieces of a single plant. Zebra mussels (Dreissena polymorpha) produce microscopic larvae that float in the water and are easily entrained in live wells, bait wells and bait buckets. Adult zebra mussels attach to weeds as well as hard surfaces like boat hulls and lower units of motors. In this way they can spread when weeds are tangled onto boat motors and trailers and are transported to another lake or river.

We have recently heard of the Northern snakehead (Channa argus), a predacious Asian species, being found in the Rock River, in a Lake Michigan harbor in Chicago and in rivers in Maryland. These were most likely the result of aquarium releases, where the fish grew too big for the aquarium at home. Rather than returning the fish to the pet shop or selling it to another hobbyist, the owner released the fish into the wild. The Maryland infestation was particularly problematic as males and females were able to reproduce.

Once organisms become established in rivers, eradication may be impossible and their spread is very difficult to control. Although canals provide an inexpensive means to move bulky cargo among broadly separated locations along a waterway, they also offer a means for AIS to move relatively unencumbered between the Midwest and the Great Lakes regions and beyond. The Chicago Sanitary and Ship Canal, for example, connects the Des Plaines River, part of the Mississippi River basin with the Great Lakes drainage basin.

An electric barrier on the Chicago San-Ship Canal will deter the movement of fish but will not be effective on planktonic organisms. Two canals in Ohio connect Lake Erie with the Ohio River drainage basin. The New York State Canal System involves the Erie, Hudson and Champlain canals, and links Lake Erie with the tidal Hudson River. Efforts to control the spread of AIS in these canals are only in the preliminary stages.

Ballast water from ships is a primary pathway for introducing exotic species to the Great Lakes. © DNR Photo
Ballast water from ships is a primary pathway for introducing exotic species to the Great Lakes.

© DNR Photo

Gardeners must be vigilant when shopping for exotic plants online as well as at the local nursery. Plants and animals used in water gardens can spread inadvertently to nearby waters. Snails introduced intentionally or with aquatic plants can crawl to nearby waters or can live out of the water and can quickly establish large destructive populations. Purple loosestrife may have been introduced via ballast water from Europe or as an ornamental garden plant for its pretty bright purple flowers. Often seen in roadside ditches, purple loosestrife is an aggressive wetland invader that produces hundreds of thousands of tiny seeds per plant. These seeds can spread in the wind, in water and can very easily stick to mud on shoes, boots or tires.

Efforts are underway to curb new introductions via ballast water. Regulations established in 1993 require ocean-going vessels to exchange ballast at sea, and new rules will soon require that they meet certain provisions for the number and size of organisms that remain in the ballast water after treatment. The Wisconsin Department of Natural Resources, in partnership with volunteers and Wisconsin Sea Grant, stations watercraft inspectors at boat ramps throughout the state. These individuals help teach boaters and anglers how to prevent the spread of invasive species as they move from one water body to the next. Removing aquatic plants, draining water from bilge, bait and motor, cleaning off mud and other debris and disposing of unused bait in the trash, not in the water, can help prevent the spread of AIS.

Under a new national initiative, Habitattitude, aquarium hobbyists and water gardeners are encouraged to responsibly dispose of unwanted plant and animal species. If you have acquired an undesirable aquatic plant or fish species for your aquarium or water garden, it is important not to release these plants or animals into the environment. While most of these organisms will die, some may be able to survive. A small number of those that do survive have the potential to create negative impacts on our natural environment and our wallets, as well as generate misperceptions about our hobbies. If you are faced with the situation of having an undesirable species, what can you do? By choosing among several alternatives, you can properly dispose of these unwanted aquatic plants or fish.

  • Contact a retailer for proper handling advice or for possible returns
  • Give to or trade with another aquarist, pond owner, or water gardener
  • Donate them to a local aquarium society, school, or aquatic business
  • Seal aquatic plants in plastic bags and dispose in trash
  • Contact a veterinarian or pet retailer for guidance about humane disposal of animals

Aquaculture operations, baitfish dealers and natural resource agencies are also receiving training on how to avoid spreading invasive species through their business and sampling activities. Using a process designed to ensure food safety for NASA, the Great Lakes Sea Grant programs are training resource professionals and business owners how their activities in the field can promote the spread of AIS and how to develop a plan to prevent their spread of AIS.

We all have a stake in keeping our waters free of invasive species. Likewise we each play a role in preventing the spread. Take a few moments to learn how to prevent AIS from spreading through your actions when you're on the water, and teach and encourage others to take steps to keep our waters free of unwanted plants and animals.

Phil Moy is a fisheries and nonindigenous species specialist for the University of Wisconsin Sea Grant.