A Field Guide to Aquatic Invaders

Invader Introductions

Aquatic invaders — non-native plants, animals and fish viruses — introduced into habitats where they don't belong are severe world-wide agents of habitat alteration and degradation. A major cause of biological diversity loss throughout the world, they are considered "biological pollutants."

Introducing species accidentally or intentionally, from one habitat into another, is risky business. Freed from predators, parasites, pathogens, and competitors that have kept their numbers in check, species introduced into new habitats often overrun their new home and crowd out native species. In the presence of enough food and a favorable environment, their numbers will explode. Once established, they rarely can be eliminated.

Most species introductions are the work of humans. Some introductions, such as carp and purple loosestrife, are intentional and do unexpected damage. However, many introductions are accidental. The species are carried in on animals, vehicles, ships, commerical goods, produce, and even clothing. Some introductions are ecologically harmless and some are beneficial. Others are harmful to recreation, ecosystems, and the economy. They have even caused the extinction of native species—especially those of confined habitats such as islands and aquatic ecosystems.

The recent development of fast ocean freighters has greatly increased the risk of new "hitchhikers" in the Great Lakes region. Ships take on ballast water in Europe for stability during the ocean crossing. This water is pumped out when the ships pick up their loads in Great Lakes ports. Because the ships make the crossing so much faster now, and harbors are often less polluted, more potentially invasive species are likely to survive the journey and thrive where they are discharged.

Many of the plants and animals described in this guide arrived in the Great Lakes this way. They can spread to lakes, rivers, and wetlands by "hitching" rides on boats and other recreational equipment.

This guide is designed to help water recreationalists recognize these invaders and help stop their further spread. Most of our waterways are not infested with aquatic invasive species. More and more people recognize the threats these species pose. Protecting and conserving our waters is everyone's responsibility. Visit the Protect Your Waters site to see what you can do.

Midwest Invaders

Round Goby (Apollonia melanostomus)

The round goby is a bottom-dwelling fish, native to eastern Europe, that entered the eastern Great Lakes in ballast water. They can spawn several times per year, grow to about 8 inches, are aggressive, and compete with native bottom-dwellers like sculpins and log perch. They are expected to be harmful to Great Lakes and inland fisheries.

Likely means of spread:
Round gobies could be accidentally transported in livewells, bilge water, bait buckets, and in the ballast water of Great Lakes freighters.

Learn more about the Round Goby...


Sea Lamprey (Petromyzon marinus)

Sea lamprey are predacious, eel-like fish native to the coastal regions of both sides of the Atlantic Ocean. They entered the Great Lakes through the Welland Canal about 1921. They contributed greatly to the decline of whitefish and lake trout in the Great Lakes.

Learn more about Sea Lamprey...


Rusty Crayfish (Orconectes rusticus)

Rusty crayfish are native to streams in the Ohio, Kentucky, and Tennessee region. Spread by anglers who use them as bait, rusty crayfish are prolific and can severely reduce lake and stream vegetation, depriving native fish and their prey of cover and food. They also reduce native crayfish populations.

Likely means of spread:
Rusty crayfish could be accidentally released by anglers, aquarium hobbyists, commercial harvesters, and teachers and students who buy them from biological supply houses and then release them when their study is completed.

Learn more about Rusty Crayfish...


White Perch (Morone americana)

White perch are native to Atlantic coastal regions and invaded the Great Lakes through the Erie and Welland canals. Prolific competitors of native fish species, white perch are believed to have the potential to cause declines of Great Lakes walleye populations.

Likely means of spread:
White perch could be accidentally transported in livewells, bilge water, bait buckets, and in the ballast water of Great Lakes freighters.


Flowering Rush (Botumus umbellatus)

Flowering rush is a perennial plant from Europe and Asia that was introduced to the Midwest as an ornamental plant. It grows in shallow areas of lakes as an emergent plant, and as a submersed form in water up to 10 feet deep. Its dense stands crowd out native species like bulrush. The emergent form has pink, umbellate-shaped flowers, and is 3 feet tall with triangular-shaped stems.

Likely means of spread:
Flowering rush is probably spread over long distances by people who plant it in gardens. Once in a watershed it spreads by rhizomes and root pieces that break off and form new plants. Muskrats may use parts of the plant to build houses and probably contribute to its local spread. Boaters can transport flowering rush on their equipment. Water and ice movements can easily carry flowering rush to new areas of a water body.

Learn more about Flowering Rush...


Curlyleaf Pondweed (Potamogeton crispus)

Curlyleaf pondweed is a non-native plant that forms surface mats that interfere with aquatic recreation. The plant usually drops to the lake bottom by early July. Curlyleaf pondweed was the most harmful aquatic plant in the Midwest until Eurasian watermilfoil appeared. It was accidentally introduced along with common carp.

Likely means of spread:
Curlyleaf pondweed may be carried in livewells, bilge water, boats, trailers, and equipment.


Zebra Mussel (Dreissena polymorpha)

Zebra mussels and a related species, the quagga mussel, are small, fingernail-sized mussels native to the Caspian Sea region of Asia. They were discovered in Lake St. Clair near Detroit in 1988. Tolerant of a wide range of environmental conditions, zebra mussels have now spread to parts of all the Great Lakes, the Mississippi River, and in inland lakes. Zebra mussels clog water-intake systems of power plants and water treatment facilities, as well as irrigation systems, and the cooling systems of boat engines costing millions of dollars in damage each year. They have severely reduced or eliminated native mussel species.

Female zebra mussels can produce as many as 1 million eggs per year. These develop into microscopic, free-swimming larvae (called veligers) that quickly begin to form shells. At about three weeks, the sand grain-sized larvae start to settle and attach to any firm surface using "byssal threads." They will cover rock, metal, rubber, wood, docks, boat hulls, native mussels, and even aquatic plants.

Zebra mussels filter plankton from the surrounding water. Each mussel can filter about one quart of lake water per day. However, not all of what they remove is eaten. What they don't eat is combined with mucus as "pseudofeces" and discharged onto the lake bottom where it accumulates. This material may benefit bottom feeders while reducing the plankton food chain for upper water species.

Diving ducks, the freshwater drum, and other fish eat zebra mussels, but will not significantly control them.

Likely means of spread:
Microscopic larvae may be carried in livewells or bilge water. Adults can attach to aquatic plants, boats, or boating equipment that sit in the water for more than a day.

Learn more about Zebra Mussels...


Ruffe (Gymnocephalus cernuus)

The ruffe is a small European member of the perch family that is native to central and eastern Europe. It was introduced to the Duluth harbor, probably in tanker ballast water, around 1985, and is spreading to other rivers and bays around Lake Superior.

In Europe, the ruffe is a pest species in waters it has invaded. Ruffe have shown explosive population growth and have had harmful impacts on native species and functions of aquatic ecosystems.

In the St. Louis River near Duluth, populations of yellow perch, emerald shiners, and other forage fish caught in survey trawls, have declined dramatically as numbers of ruffe have increased.

The ruffe's ability to displace other species in newly invaded areas is due to:

  1. Its high reproductive rate,
  2. Its feeding efficiency across a wide range of environmental conditions, and
  3. Characteristics that may discourage would-be predators such as walleye and pike.

Ruffe grow rapidly and can reproduce in their first year. In the St. Louis River, females can lay between 45,000 and 90,000 eggs a year. Ruffe are primarily bottom feeders, preferring dark environments where they can hide from predators. Ruffe rarely grow bigger than 5 inches, although the sharp spines on their gill covers, dorsal and anal fins make them difficult for larger fish to eat.

Likely means of spread:
Ruffe could be accidentally transported in livewells, bilge water, bait buckets, and in the ballast water of Great Lakes freighters.

Learn more about Eurasian Ruffe...


This information is adapted from "A Field Guide to Exotic Plants and Animals" brochure (1995) by the Minnesota Department of Natural Resources.


This page last modified on November 13, 2009     © 1996 – 2014 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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