Invasive Species

5 things trying to take over the Hudson – and what can be done

Aquatic invasive species are threatening the ecology of the Hudson River and the tributaries that flow to it by displacing, competing with or feeding on native aquatic species. At least 50 aquatic invasive species are either established, emerging, or potential threats. Most are aquatic plants, but fish, mollusks, crustaceans and other invertebrates are listed as well.

These species are introduced unintentionally by anglers, boats and other watercraft that have been exposed to waters inhabited by invasive species by people releasing unwanted aquarium fish and vegetation to native streams, and via connected waterways. With no natural predators or other ecological processes to control their numbers, aquatic invasive species can multiply quickly.

Invasive Water Chestnut

Photo: Department of Environmental Conservation

Water chestnuts
Water chestnuts are an existing threat to the Hudson River Watershed. Native to Eurasia and introduced to the US as an ornamental in the 1800s, they are now widely established throughout the Hudson Valley. These plants form leafy mats that root in streambeds, choke waterways and block sunlight from native submerged vegetation. As the plants die off and decompose, they consume dissolved oxygen, which suffocates the fish and other aquatic life that requires that oxygen to breathe. The mats grow so thick that they impede the passage of boats and other watercraft, and the spiny seed pods of water chestnuts can injure people stepping on them when wading or swimming.

Once invasive species have become established in a new environment, they can be extremely difficult, if not impossible, to eradicate. Water chestnuts may be managed by manual or mechanical removal, chemical treatment, and biological control. Smaller infestations can be removed by hand pulling, providing that the root systems are removed along with the stems and leaves.


Invasive Hydrilla

Photo: CC, USFWS Pacific

Hydrilla is an Asian species of aquatic plant popular in aquariums but now prohibited in New York State. It was first discovered in Orange County in 2008 and has spread to 11 other counties including Westchester, where it has recently established several “hot spots” in the Croton River, a tributary to the Hudson. Hydrilla grows rapidly, forming large mats that clog infrastructure and impede natural stream flow, damage habitat by displacing natural vegetation, and decrease dissolved oxygen essential to the survival of fish and other aquatic life. While not yet established in the Hudson River, hydrilla is nevertheless an emerging and a potential threat if not contained and eradicated.

The New York State Department of Environmental Conservation (DEC) is in the process of treating hydrilla in the Croton River with the aquatic herbicide fluridone in concentrations well below EPA’s regulated maximum concentration. Chemical treatments are seasonal and may require repeated applications over three to five years before the target species is completely eradicated. However, early detection and spot treatments of hydrilla have avoided the need for widespread or concentrated application of herbicide.


Photo: CC, Landcare Research

Didymo is microscopic algae that form large, slimy mats that cling to rocks in freshwater streams. Also referred to as “rock snot,” didymo is native to streams in northern Canada but has extended its range southward, invading New York waterways in 2007. Didymo is spread by water-based recreation—fishing, boating, tubing, etc.—when the microscopic diatoms cling to boots or equipment and are unwittingly transported from a contaminated stream to a new stream. Didymo mats coat streambeds and suffocate bottom-dwelling organisms that form a critical link in the food chain of fish and other aquatic organisms. Currently didymo is established in the Hudson River tributaries Esopus Creek and Croton River.

Currently, there is no known control method that is effective in eliminating didymo once it has become established.

Zebra mussels

Photo: MNDNR

Zebra mussels
Zebra mussels are native to Eurasia and were introduced to the Great Lakes in ballast water from European cargo ships in the 1980s; they had reached the Hudson River by 1991. These filter-feeding mollusks spread rapidly and clogged boat engines and infrastructure, adhered to the shells of native mollusks and crustaceans, and contributed to the decline of shad and herring populations by consuming 70-80 percent of the plankton upon which those fish feed.

The zebra mussel population, though still significant in the Hudson River, has been in a gradual decline for 20 years—without chemical or biological intervention—as parts of the river continue to recover from the widespread infestation. Suggested reasons for the decline include competitive adaptation by native mussels and shifting prey preferences of blue crabs as they feed more on zebra mussels. So far research has not identified a specific cause or causes of the decline.

Grass Carp

Photo: CC, Peter Halasz

Grass carp
The grass carp is an herbivore native to eastern Asia, widely cultivated in aquaculture for food and used for aquatic weed control in freshwater lakes and streams. Grass carp were introduced to Alabama and Arkansas in 1963 for aquaculture but have now spread to 45 states and the Great Lakes. They have been documented in the Hudson River since the 1980s, where their feeding habits have potential to alter habitats and impact diverse aquatic plant communities and the species that depend on them for food, shelter and breeding habitat.

Grass carp are being managed with biological controls. In 1983, the DEC approved the release of sterile grass carp into New York waters to reduce their numbers through natural attrition as the populations gradually lost the ability to reproduce. Although effective in more confined environments such as lakes or ponds, control by attrition is more difficult and time-consuming in the 315-mile-long Hudson River, where a small number of reproductively viable grass carp have been found in recent years.

What can you do?

1. Stay informed.
Because aquatic invasive species are a regional problem and not confined to individual waterways, New York State has developed an Invasive Species Comprehensive Management Plan (ISCMP) to “help guide invasive species management into the future.”

The ISCMP focuses on eight initiatives:

    • Continue to build partnerships and capacity
    • Commit to a centralized framework for sharing invasive species information
    • Set priorities for invasive species management and advance preparedness
    • Engage and inform the public
    • Advance prevention and early detection
    • Improve the response to invasive species
    • Recover ecosystem resilience
    • Evaluate success

Partners in these initiatives include the Invasive Species Council and Invasive Species Advisory Committee; NYS Invasive Species Research Institute; the invasive species database iMapInvasives; the NYS Department of Environmental Conservation Bureau of Invasive Species and Ecosystem Health; and the creation of the eight Partnerships for Regional Invasive Species Management (PRISM).

Please consider subscribing to listserves of any of the ISCMP partners to keep informed of threats specific to your local waterways. One place to start is New York State PRISMs.

2. Take precautions if you boat, fish or swim in the Hudson watershed.

Although these organisms can be introduced to new waterways when transferred from clothing, pets, fishing gear, boats or other watercraft, there are steps you can take to avoid introducing them to the Hudson River Watershed. For people who boat, fish or swim in the Hudson or its tributaries, the DEC offers guidelines to prevent the spread of aquatic invasive species, such as:

    • Check your boating and fishing equipment for invasive species
    • Clean any visible mud, plants, fish or animals before transporting equipment Discard items in an upland area or in one of the invasive species disposal stations that have been installed at many boat launch sites
    • Zebra mussels can be difficult to remove from a boat hull. They first need to be killed by exposure to water or steam at least 140 degrees F and then removed by brush or pressure washer
    • Drain all water holding compartments including live wells, bait wells and bilge areas
    • Dry or disinfect boats, trailers and equipment before use in another waterbody

Also, be sure to report to the DEC any aquatic invasive species you observe when exploring the Hudson Estuary watershed. Alert users of our waterways are the first line of defense.

Learn more about the threats to Hudson River ecology >

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