Invasive Species Quietly Taking Over American Waterways

Invaders You Rarely See Until It's Too Late

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Most invasive species don't announce themselves. They arrive in ballast water, bait buckets, aquarium dumps or on the hull of a trailered boat, and they spend years at low numbers before the population explodes. By the time anglers or boaters notice, the damage to native fish, plants and shorelines is often permanent. Human activity drives most introductions, through shipping, the aquarium and water-garden trade, deliberate stocking and careless transport between lakes. The costs are large: clogged intakes, collapsed fisheries, fouled boat motors and billions of dollars in cleanup and control. The eight species below were each brought to American waters on purpose or by accident, and each now changes how a river, lake or bay works. Some are famous. Others, like round gobies and giant salvinia, spread quietly while most people look elsewhere. Knowing what they are is the first step in not spreading them any further.

Asian Carp: Silver and Bighead

Bighead, silver and grass carp were brought to the United States in the 1970s mainly for aquaculture and algae control, then escaped into the Mississippi River system. They now dominate stretches of the Mississippi, Illinois and Ohio rivers, where they compete with native fish and mussels for plankton, the base of the food web. Silver carp are famous for leaping out of the water when startled by boat motors, and boaters, kayakers and anglers have been injured by flying fish. Federal and state agencies have built electric barriers in the Chicago Sanitary and Ship Canal to keep the fish out of Lake Michigan, and the Great Lakes fishery is worth billions of dollars a year. Control efforts include commercial harvest, targeted netting and public campaigns encouraging people to eat them. The fish are edible but bony, which has limited their market so far.

Zebra and Quagga Mussels: The Great Lakes Clog

Zebra mussels, native to the Caspian Sea region, were discovered in Lake St. Clair in 1988, almost certainly brought in ballast water from overseas ships. Quagga mussels followed and eventually spread even more widely through the lakes. Both species filter huge volumes of water, removing the plankton that young fish depend on, and they attach to anything solid, including intake pipes, water treatment plants, boat hulls and docks. Utilities have spent billions of dollars over the years clearing pipes and retrofitting systems. Their sharp shells also cut bare feet along the shoreline, and the clearer water they create lets sunlight reach the bottom, fueling more weed growth. The mussels hitch rides west on trailered boats, so many western states now run mandatory inspection stations. Draining, cleaning and drying a boat before moving between lakes is the simplest defense.

Northern Snakehead: The Predator in the Pond

The northern snakehead, native to China, Russia and Korea, made headlines when one was found in a Crofton, Maryland, pond in 2002. It was a fish-market and aquarium-trade escapee, and officials poisoned the pond to eliminate it. Later populations took hold in the Potomac River and other Mid-Atlantic waters, and sightings have continued in several states. Snakeheads are ambush predators with large mouths and sharp teeth, and they can breathe air and survive out of water for a period if kept moist. Early fears that they would wipe out bass fisheries have not fully materialized, but they remain a nuisance and an unknown in new waters. Many states ban possession and require anglers to kill any they catch rather than release them. They're also good eating, and some Mid-Atlantic restaurants serve them. If you catch one, check your state rules and report it.

Hydrilla: The Perfect Aquatic Weed

Hydrilla is a submerged plant that arrived in Florida through the aquarium trade in the 1950s and has since spread across the South and into the Mid-Atlantic and West. Biologists sometimes call it the perfect aquatic weed, because it grows in low light, tolerates a wide range of conditions and reproduces in several ways. It forms dense mats that choke boat propellers, block swimming areas and crowd out native plants. Its tubers can stay dormant in the sediment for years, which means killing the top growth doesn't end the problem. Bass anglers sometimes welcome hydrilla because it holds fish, but too much of it lowers oxygen and ruins the habitat. Management uses herbicides, grass carp, mechanical harvesting and drawdowns, and each has trade-offs. Cleaning plant fragments off trailers and gear is the best way to stop it from reaching the next lake.

Giant Salvinia: The Floating Fern That Smothers

Giant salvinia is a floating fern from South America that can nearly double its coverage in a week under the right conditions. It spreads by fragments, so a tiny piece caught on a boat trailer can start a new infestation in another lake. Dense mats shade out underwater plants, starve the water of oxygen and block boat access, fishing and swimming. It has been found in the Southern coastal states and in California, and Texas and Louisiana waters have been hit particularly hard. Managers use herbicides and a South American weevil that feeds on the plant, though it takes a large number of insects to make a visible difference. Because it floats, it can also travel downstream in floods. The best defense is the same as for other aquatic invaders: inspect boats and trailers, remove plants and let gear dry fully before moving on.

Water Hyacinth: The Pretty Weed That Closed Rivers

Water hyacinth, with its lavender flowers, was shown at the 1884 Cotton Exposition in New Orleans, where visitors took the plants home for ponds. The floating plant escaped, and within a few decades it was clogging bayous, canals and rivers throughout the Gulf states. Its dense mats block navigation, trap sediment, shade out native plants and reduce oxygen in the water below. Florida has spent decades and enormous sums trying to manage it, and it remains a perennial problem. Mats can double in size in a couple of weeks in warm weather. Control uses herbicides, mechanical harvesting and insects such as weevils imported to feed on it. Gardeners can still buy it in some places, though it's banned in many states, which is why checking local laws before adding water plants to a backyard pond matters. Never dump pond plants into a creek or lake.

Round Goby: A Bottom-Dwelling Bully

The round goby is a small, bottom-dwelling fish from the Black and Caspian Sea region that was discovered in the St. Clair River in 1990, probably carried in ballast water. It has since spread across all of the Great Lakes and into connected rivers and inland lakes. Gobies eat the eggs and young of native fish like lake trout and sculpin, crowd native bottom fish out of rocky habitat and feed on mussels, which concentrate contaminants. They also compete aggressively for spawning sites. Anglers often curse them because they steal baits, but smallmouth bass and other predators eat them heavily, so some fisheries have benefited. Researchers have linked gobies and mussels to botulism outbreaks in waterbirds in the Great Lakes. It's illegal in many states to use round gobies as bait or to move them between waters, so never dump a bait bucket.

Sea Lamprey: The Great Lakes Parasite

Sea lampreys are eel-like parasites that attach to fish with a round, toothed sucker and feed on their blood and body fluids. They entered the upper Great Lakes through the Welland Canal around the early twentieth century, and their numbers exploded in the mid-1900s. Lake trout, a top predator in the ecosystem, collapsed in several lakes, and the commercial and sport fisheries suffered badly. A single lamprey can kill dozens of pounds of fish during its parasitic stage. Canada and the United States set up a joint program through the Great Lakes Fishery Commission to control them using barriers on streams and targeted lampricide treatments in spawning tributaries. The program has cut sea lamprey numbers dramatically, though it must be repeated indefinitely. If you catch a fish with a round, scar-like wound, a lamprey may have been responsible.

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