Impacts of Some of the Worst Invasive Aquatic Species

Photo courtesy of Argonne National Laboratory, a U.S. Department of Energy laboratory

Cholera

Cholera is an acute intestinal infection caused by the bacterium Vibrio cholerae. It has a short incubation period, from less than one day to five days, and produces an enterotoxin that causes a copious, painless, watery diarrhea that can quickly lead to severe dehydration and death if treatment is not promptly given. Vomiting also occurs in most patients.

From polluted harbors and bays, ship ballast can carry the Vibrio cholera, concealed in plankton, to estuaries around the world. The virulent El Tor cholera strain, which causes intestinal disease with symptoms of severe diarrhea, was probably carried by ballast water from Asia to Latin America in 1991, and then spread to Mobile Bay, Alabama, where it was found in oysters in closed shellfish beds.(4)

Photo courtesy of U.S. Fish & Wildlife Service

Alewife

Non-indigenous, landlocked alewives impact native ecosystems in a number of ways. They alter the zooplankton community, out-compete other fish species for food, feed on the eggs and larvae of other fish, and cause both reproductive failure in trout and salmon and declines in native species. For example, the disappearance of native Lake Ontario planktivores such as whitefish and lake herring has been attributed to the introduction of alewives which reduced zooplankton populations.

In addition, alewives undergo periodic mass mortalities. When these large-scale die-offs occur, several problems arise. First, any predator fish that utilizes alewife populations as a main source of food will have difficulty finding enough to eat. This results is poor growth rates or declines in game fish such as chinook, coho, brown trout, and lake trout populations in the Great Lakes. Second, the large numbers of alewives that die in these events wash up on beaches, causing foul odors and public health concerns. Stretches of shoreline in the Great Lakes are often closed for weeks at a time after an alewife die-off so that the thousands of fish can be bull-dozed off the beaches, as is often necessary.(5)

Photo courtesy of Minnesota Sea Grant

Eurasian Ruffe

Ruffe pose a threat to native fish because they mature quickly, have a high reproductive capacity, and easily adapt to new environments. Ruffe are more tolerant of poor water conditions and have several anatomical features that give them an advantage over native fishes. Native fish populations – especially yellow perch, emerald and spottail shiners, trout perch, and brown bullhead – have declined in locations where ruffe have become established.

Ruffe were first detected in western Lake Superior in 1986. The ruffe population has increased rapidly in the St. Louis River at Duluth-Superior and has spread to other rivers and bays along the south shore or western Lake Superior. They have also spread past the Ontonagon River in the Upper Peninsula of Michigan. They are now one of the most abundant fish in five tributaries: the Sand, Flag, Iron, Amnicon, and Brule Rivers. Ruffe have also been detected at Thunder Bay, Ontario, and Alpena, Michigan (Lake Huron).(6)

Photo courtesy of University of Wisconsin Sea Grant, Photographer: D. Jude

Round Goby

Gobies are capable of rapid population growth after they reach new areas. They have shown the ability to out-compete native fish for food and habitat because of their aggressiveness, ability to survive in poor water quality conditions, ability to feed in complete darkness, and long spawning period (April through September). Another area of concern involves potential predation on the eggs and fry of lake trout.

After first being discovered in 1990 along the St. Claire River (a Canadian river north of Detroit), gobies have been found in eastern and southern Lake Erie, southern Lake Huron, southern Lake Michigan, and western Lake Superior. They now have access to America’s largest watershed because the Grand Calumet River (which begins at Lake Michigan near Chicago) connects with the Mississippi River.(7)

Photos courtesy of the Virginia Institute of Marine Science

Sea Lamprey

Sea lampreys prey on commercially important fish species; such as lake trout, living off of the blood and body fluids of adult fish. During its life as a parasite, each sea lamprey can kill 40 or more pounds of fish. These organisms were a major cause of the collapse of lake trout, whitefish, and chub populations in the Great Lakes during the 1940s and 1950s.(8)

The sea lamprey was first discovered in Lake Ontario in 1835, Lake Erie in 1921, Lake Huron in 1932, Lake Michigan in 1936, and Lake Superior in 1946. Reproducing populations were found in all of these upper lakes by 1947. The present “hot zone” is the St. Marys River. Sea lampreys produced in the St. Marys River migrate into Lake Huron and northern Lake Michigan. There, the adult sea lamprey population is nearly as large as it was 40 years ago – before sea lamprey control – when lake trout and whitefish stocks were decimated.(9)

Photo courtesy of the Center for Great Lakes and Aquatic Sciences

Zebra Mussels

Zebra mussels, Dreissena polymorpha, are small, fingernail-sized, freshwater mollusks accidentally introduced to North America via ballast water from a transoceanic vessel. Since their introduction in the mid-1980s, they have spread rapidly to all of the Great Lakes and an increasing number of inland waterways in the United States and Canada. Zebra mussels colonize on surfaces, such as docks, boat hulls, commercial fishing nets, water intake pipes and valves, native mollusks, and other zebra mussels. Their only known predators, some diving ducks, freshwater drum, carp, and sturgeon, are not numerous enough to have a significant effect on them. Zebra mussels have greatly impacted the Great Lakes ecosystem and economy.(10)

Photo courtesy of the Minnesota Department of Natural Resources, Photographer: J. Lindgren

Spiny Water Flea

The spiny water flea, Bythotrephes (bith-o-TREH-feez) cederstroemi, a small predacious crustacean, has an average length slightly larger than 1 centimeter (0.4 inches) of which 70% is a long, sharp, barbed tail spine. Their rapid reproduction, general lack of predators, and direct competition with young fish for food gives them the potential to alter the food webs of the Great Lakes.

Spiny water fleas were first introduced into the Great Lakes ecosystem in 1984 via ballast water that was discharged into Lake Huron. By 1987, they had spread to all of the Great Lakes, and currently they infect inland lakes in Michigan and Southern Ontario.(11)

Photo courtesy of the National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory

Opossum Shrimp

The oppssum shrimp, Mysis relicta, is an opportunistic feeder with both predatorial and filter feeding habits. Zooplankton, when abundant, serve as the opposum shrimp’s primary food source; when scarce, Mysis relicta will feed on suspended organic detritus or from the surface of benthic organic deposits. Within its native range, the opossum shrimp has been shown to be an important prey item for freshwater fishes. However, when introduced into what was considered to be an “empty” niche, its impact on the aquatic community was significant. Dramatic changes and species extinctions of native zooplankton communities have been attributed to its opportunistic lifestyle. Declines in the number and size of game fish have been documented since the introduction of opposum shrimp, provoking doubt regarding their utility as a forage base for game fishes.(12)

Photo courtesy of the U.S. Fish & Wildlife Service, Photographer: Dr. Thomas L. Wellborn, Jr.

Whirling Disease

Myxobolus cerebralis is a metazoan parasite that penetrates the head and spinal cartilage of fingerling trout where it multiplies very rapidly, putting pressure on the organ of equilibrium. This causes the fish to swim erratically (whirl), and have difficulty feeding and avoiding predators. In severe infections, the disease can cause high rates of mortality in young-of-the-year fish. Those that survive until the cartilage hardens to bone can live a normal life span, but are marred by skeletal deformities.(13)

Whirling disease originated in Eurasia and is now found in 22 states in the U.S. including: Alabama, California, Colorado, Connecticut, Idaho, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington, West Virginia, and Wyoming. Internationally, South Africa and New Zealand have been invaded by the parasite.(14)

Photos courtesy of the South Carolina Department of Natural Resources

MSX

MSX (Multinucleated Sphere X) disease is caused by a single-celled Protozoan parasite, Haplosporidium nelsoni. MSX is lethal to the eastern oyster, but it is not known to be harmful to humans.(15) Recently, according to the Washington Post, scientists have found genetic evidence that implicates Japanese oysters as the cause of MSX.(16) Its life cycle and means of infecting oysters still remain as mysterious now as they did forty years ago. Its first appearance in mid-Atlantic waters was in Delaware Bay in 1956 where it ravaged oyster beds; the next year it arrived in the Chesapeake Bay.

VHS

VHS (viral haemorrhagic septicaemia) is the most serious viral disease of salmon and trout in Europe. It kills up to 90% of the juveniles in fish farms and hatcheries, and up to 40% of infected adults.

Historically, VHS has been a disease of European rainbow trout and primarily a problem in freshwater. It has been known in rainbow trout in Europe since 1938. The disease is seen in most countries of continental Eastern and Western Europe. Until 1988, it had not been present in the United States. However, the VHS virus has now been found in saltwater, and in the U.S. VHS virus was first isolated here in the U.S. in adult coho salmon returning to a hatchery in the Puget Sound area of Washington state.(18)

Photo courtesy of the U.S. Environmental Protection Agency, Photographer: Karen Holland

Purple Loosestrife

Purple loosestrife (Lythrum salicaria) grows so densely that it crowds out, kills, and replaces native plants. This is particularly devastating because purple loosestrife replaces plants that animals depend on for food and shelter; and, it has no food and little shelter value. Muskrats are dying out in some areas because their diet of cattails has been severely reduced by purple loosestrife. Infestations can become so bad, that they block water flow. Purple loosestrife can reduce biodiversity rates from 900 to 1 species.(19) This invasive plant can produce up to 2.7 million seeds per plant yearly, and spreads across approximately 1 million additional acres of wetlands each year.(20)

Purple loosestrife is a perennial plant native to Europe. It was brought to North America in the early 1800s by immigrants who valued its striking purple flowers. Seeds were also unintentionally transported to the shores of North America in the ballast water of ships. Since then, purple loosestrife has expanded its range; now, it is a serious pest of wetlands and pastures.(21)