Aquatic Invasive Species Ecology

Small zebra mussels up close
Zebra mussels found on a boat during a Yellowstone AIS inspection.

Aquatic Invasive Species in Yellowstone

During the late 1880s when the Army administered Yellowstone, the US Fish Commission (a predecessor of today’s US Fish and Wildlife Service) stocked nonnative fish in some park waters. These stockings are the first known, deliberate introductions of nonnative fish to Yellowstone.

Other aquatic invasive species (AIS), such as the New Zealand mud snail and the parasite causing whirling disease have arrived since. We may never know exactly how those species were introduced to the park, but it’s likely they were carried here from other parts of the country by unwitting anglers.

2 small, light brown, conical snail shells sit on a dime.

New Zealand Mud Snails

The New Zealand mud snail (Potamopyrgus antipodarum) is an invasive species that was first found in the western United States in the 1980s. Their habitat includes flowing water and some geothermal streams. The snails can form dense colonies on aquatic vegetation and streambed rocks. This crowds out invertebrates that are a primary food for immature trout and other native species.New Zealand mud snails consume large amounts of algae, which is a primary food for native aquatic invertebrates. Its impact on algae is likely to affect entire stream food webs. With its protective shell, the mud snail provides little if any nutrition to its predators. It may pass through a fish alive. Scarcely a quarter-inch long, mud snails may cling to boats, trailers, waders, and other fishing gear. This gear can inadvertently transfer the snails to another waterbody. Because the species can reproduce asexually, a single mud snail is all that is required to establish a new colony.


First detected in the park in 1994, New Zealand mud snails are now in all major watersheds. Although they are abundant in several streams, they are absent or uncommon in others. This suggests that the mud snail's upstream density and distribution is limited by spring runoff. Spring runoff can cause colder temperatures, low productivity, and unstable streambeds.

Impacts of Mud Snails

Once mud snail colonies become established in a stream, removing them without disrupting native invertebrates is not possible. Mud snail research aims to determine the species’ impacts on the aquatic ecosystem. A study of the Gibbon and Madison rivers found that 25–50% of the macroinvertebrates were mud snails. The areas they occupied had fewer native mayflies, stoneflies, and caddisflies. These insects are important in the diet of native fish and several bird species.

An infographic showing the lifecycle of the microscopic organism responsible for whirling disease. The microorganism must first enter an aquatic worm, then in a new form, enter the water column and into a fish, which then gives off the original spores.

Whirling Disease

Whirling disease is caused by a microscopic parasite from Europe (Myxobolus cerebralis) that can infect some trout and salmon. It does not infect humans. Twenty-five states have detected whirling disease. During the parasite’s life cycle, it takes on two different forms. Each form needs its own host. The triactinomyxon (TAM) form, develops in common aquatic worms (Tubifex tubifex) and is released into the water column. Once released they float with the current and can come into contact and attach to trout. Cutthroat trout are susceptible, especially during the first months of life. The parasite feeds on the trout’s cartilage and develops into the myxospore form of the parasite. This can cause skeletal deformities, a blackened tail, and whirling swimming behavior. Because infected trout can't feed normally, and are more vulnerable to predation, whirling disease can be fatal. No practical treatment exists for trout infected with this disease or for the waters containing infected trout or tubificid worms.

Presence and Impact in Yellowstone

Whirling disease was first detected in Yellowstone in 1998 in cutthroat trout from Yellowstone Lake. The disease was described in Europe more than 100 years ago. It was detected in the United States in the mid-1950s before making its way to Yellowstone. It most likely came to the US in frozen fish products.In Yellowstone it has been found in the Firehole, Madison, Gibbon, Gallatin, and Lamar rivers and the Yellowstone Lake watershed. In the lake, the infection has spread to about 20% of cutthroat trout. The parasite is most prevalent in Pelican Creek and the Yellowstone River downstream of the lake. Infection has been most severe in Pelican Creek. This stream once supported nearly 30,000 upstream-migrating cutthroat trout. Whirling disease and nonnative lake trout that eat cutthroat trout in Yellowstone Lake are likely causes of this decline. Some adult cutthroat trout in Yellowstone Lake have survived their initial infection. This suggests some resilience to whirling disease among cutthroat trout.

Studying the Disease

Yellowstone's cutthroat trout spawning streams vary widely in thermal, hydrological, and geological characteristics. This provides an exceptional opportunity to study whirling disease in native trout. Park staff have been working with Montana State University’s Department of Ecology to measure how the infection rate might vary in different stream conditions. Recent laboratory tests suggest cutthroat trout are highly susceptible to whirling disease. Lake trout and grayling appear immune to the disease. Brown trout are resistant to whirling disease but can carry the parasite to other species. Certain fish-eating birds have also been shown to disperse the parasite. Research has found that the parasite can pass alive through the digestive tract of some birds, such as great blue herons. Recent surveys showed that whirling disease risk was still very high in Pelican Creek. But it does not appear that whirling disease has spread widely throughout spawning tributaries to Yellowstone Lake. Also, prevalence of infection in young and adult cutthroat within the lake remains low. Despite this, there are still many unknowns about how the parasite functions in Yellowstone's unique environment.Park staff focus on prevention through education. Visitors who fish, boat, or swim are asked to take steps to help stop the spread of the disease. Thoroughly cleaning mud and aquatic vegetation from all equipment and footwear before moving to another drainage is the best prevention. Anglers should not transport fish or water between drainages. Fish should be cleaned in the body of water where they were caught.

A conical shell with 6 whorls. It is light brown with red flecks.

Red-rimmed Melania

The red-rimmed melania (Melanoides tuberculatus), is a small snail imported by the aquarium trade beginning in the 1930s. It was discovered in the warm waters at the confluence of the Boiling River and the Gardner River in 2009. In 2010, a survey of 18 of the park’s most popular hot springs found red-rimmed melania only in the Boiling River area and downstream for approximately 1 km. The species has a narrow temperature tolerance (64–90°F/18–32°C). It is unlikely to survive downstream of the Boiling River during the winter. It could become established in other thermal water in the park.



Aquatic Invasive Species of Concern

Many AIS species of concern are not yet present in Yellowstone, but they pose a grave and growing threat to the park and the Greater Yellowstone Ecosystem as they continue to spread into the surrounding states.

Further invasion of park waters by AIS could mean devastating effects on the Yellowstone’s thriving and diverse aquatic ecosystem and detrimental impacts to recreational opportunities, waterways, and communities downstream. Yellowstone straddles the Continental Divide and headwaters that flow from the park drain to the Pacific and Atlantic oceans.

Several mussels with dark stripes on a beige shell sit next to a dime. They are much smaller than the dime.
Zebra mussels removed during an inspection from a boat that was attempting to launch in Yellowstone waters.

Dreissenid Mussels

Both zebra mussels (Dreissena polymorpha) and closely related quagga mussels (Dreissena bugensis), are collectively called dreissenids. They are of particular concern because they can be spread easily and cause great harm. The mussels can attach to watercraft and survive many days out of water. Boaters must follow Clean, Drain, Dry to avoid unknowingly spreading them.

Zebra mussels are native to Eastern Europe and western Asia. They were first discovered in North America in 1988 in Lake St. Clair, one of the water bodies connecting the Great Lakes. It is believed they were introduced through ballast water discharges from international shipping.

Following their invasion, zebra mussels spread quickly across most of the eastern United States and Canada. Traveling boaters inadvertently spread the mussels from one body of water to another.

Zebra mussels drastically alter the ecology of infested water bodies. The effect on the ecosystem can be severe. These efficient filter-feeders consume significant amounts of phytoplankton. This depletes the foundation of the aquatic food web. Zebra mussels can attach to most hard surfaces, forming mats that may be up to 18 inches thick. Mussels can impact recreational activities and associated economies. They cover docks, boats, and beaches. Zebra mussels can also completely block water intake pipes. Blockages can severely damage power plants, water treatment plants, irrigation systems, and industrial facilities.Birds and fish that prey upon zebra mussels in Europe are not found in North America.

A mottled brownish green fish with a red eye swims in greenish water above a bottom of cobbles.

Smallmouth Bass

Smallmouth bass are an evolutionarily-advanced, prolific, and highly predatory fish species. They have caused severe losses of native fish populations in other North American locations where they have been introduced. Smallmouth bass will also prey upon many other native species, including macroinvertebrates, amphibians, reptiles, small mammals, and young waterbirds.

Warming river temperatures may have made the upstream movement of smallmouth bass easier in recent years. This could contribute to the species now being present at Yellowstone National Park's northern boundary. Climate-caused changes to river flows and temperatures may continue making it easier for smallmouth bass to move and live upriver. Of particular concern is the ability of smallmouth bass to pass over natural cascades or waterfalls that often stop other fish species. If smallmouth bass move over the Knowles Falls of the Yellowstone River, they will eventually invade the Yellowstone River near Canyon Village. This would also allow them to enter the Lamar River watershed. Both are important habitats for native Yellowstone cutthroat trout. They are also some of the largest remaining strongholds for native Yellowstone cutthroat trout throughout their entire historical range in Montana, Idaho, and Wyoming.

Management may take actions to reduce further upstream invasion of smallmouth bass in Yellowstone National Park. Possible actions include education of park staff, outfitters, and anglers; incorporation of this new invasive species in the park angling regulations booklet; and/or population suppression by NPS biologists using electrofishing or other capture techniques. Any smallmouth bass caught by an angler in Yellowstone must be killed and reported.

4 light to brown, scaly, fish with large tails.

Asian Carp

The bighead carp (Hypophthalmichthys nobilis), black carp (Mylopharyngodon piceus), and silver carp (Hypophthalmichthys molitrix) occur in at least 24 states. All three are types of Asian carp. They out-compete native fish, reduce forage for other fish, and can transmit disease. Silver carp can also jump great distances out of the water when boats travel near them. This can injure boaters.

Silver carp are native to Southeast Asia and east Russia. They were intentionally introduced into the United States in 1973 as an attempt to improve water quality, increase fish production in culture ponds, as biological control, and as a food fish. Silver carp now occur in at least 24 states and reproduce naturally. Both the silver and the bighead carp compete for food (zooplankton) with native fish.

Black carp are native to Asia and eastern Russia. These fish were unintentionally introduced as a stowaway with intentionally introduced grass carp. Black carp now occur in at least nine states. Black carp may reduce populations of native mussels and snails through predation. Fewer native mussels and snails harms the aquatic ecosystem. None of these species are currently found in Wyoming, or Montana.

These invasive species may continue to be spread intentionally or by accident. Accidental introductions happen through improperly using or disposing of live fish or fish eggs used as bait. Asian carp can also move to new areas through connected waterways.

a hand holding many, small, light brown clam shells

Asian Clam

Asian clam (Corbicula fluminea) was introduced to the United States in 1938. It has spread into many of the major waterways of 46 states. The species have not been completely distinguished, but most varieties are small and yellow-green to light brown in color.

The native ranges are in temperate to tropical southern Asia west to the eastern Mediterranean; Africa, except in the Sahara Desert; and southeast Asian islands south into central and eastern Australia. The Asian clam is a filter feeder that removes particles from the water column. It is found at the sediment surface or slightly buried. The Asian clam can reproduce rapidly and has a low tolerance of cold temperatures (36–86°F/2-30°C). This can cause wild swings in population from year to year in northern water bodies.

a hand holds an aquatic plant with  a light green stem and dark green, skinny leaves growing out of the stem.

Eurasian Watermilfoil

Eurasian watermilfoil (Myriophyllum spicatum) has spread to all the United States except Hawaii and Wyoming. In 2007, it was found in Montana.

This nonnative aquatic plant lives in calm waters such as lakes, ponds, and slow-moving parts of streams. It grows well in water with disturbances, sewage spills or abundant motorboat use, like Bridge Bay.

Eurasian watermilfoil is spread by stem fragments moved from one waterbody to another. Boats, gear, and trailers can all carry the stem fragments. Cleaning, rinsing, and inspecting these items before they enter Yellowstone can stop this spread.



Three nonnative plankton species that could harm Yellowstone may be on their way. These species can displace native zooplankton that are important food for cutthroat trout. The three nonnative zooplankton have long spines. The spines make them difficult for young fish to eat.


More Information

A park ranger with a dark beard and wearing a green and grey uniform kneels with a hose next a boat.
Aquatic Invasive Species Management

Learn more about how Yellowstone is working to stop AIS and how you can help.

Range with a net full of fish along a creek
History of Fish Management

Learn about the history of fish management in Yellowstone.

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Science Publications & Reports

View science publications and reports created by Yellowstone's Center for Resources on a variety of park topics.

Canoer paddles on Yellowstone Lake

Take in the view from the water.

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Catch a Fish

Be a responsible angler and understand the regulations before you come.

Yellowstone cutthroat trout
Fish Ecology

Yellowstone is home to 12 native species of fish.

Last updated: April 30, 2024

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