DEEP: Examples of Aquatic Invasive Species In Connecticut

Examples of Aquatic Invasive Species In Connecticut
It is critical that boaters take responsibility for stopping the spreading of these plants and animals.
IT IS ILLEGAL TO TRANSPORT on a boat or trailer any vegetation and the following aquatic invasive species (as determined by the Commissioner pursuant to PA 12-167):
  • Zebra mussel (Dreissena polymorpha)
  • Quaqqa mussel (Dreissena bugensis)
  • Chinese mitten crab (Eriocheir sinensis)
  • New Zealand mud snail (Potamopyrgus antipodarum)
  • Asian clam (Corbicula fluminea)
  • Rusty crayfish (Orconectes rusticus)
Invasive or non-native plants and animals crowd out native plants and animals. They can also interfere with recreation by clogging up a motor, tangling around a swimmers foot, crowding out your favorite largemouth bass, trout, perch, etc. Once established, invasive plants and animals are very expensive and virtually impossible to eradicate. To learn more about some of the fresh water invasive plants and animals, select one of the photos below:

{An image of zebra mussels.}
photo credit: S. van Mechelen, University of Amsterdam, The Netherlands  
Zebra mussels have been found in Lake Zoar, Lake Lillinonah and Lake Housatonic. This is the first report of a new infestation of this highly invasive bivalve in Connecticut since 1998 when zebra mussels were first discovered in East and West Twin Lakes in Salisbury. During  2009, zebra mussels were discovered in Massachusetts in Laurel Lake and in the mainstream Housatonic River. At this point, it is uncertain if the mussels found in Lakes Lillinonah, Zoar and Housatonic are the result of downstream migration from these upstream sources of the result of a separate introduction, however, downstream migration is suspected.

Boaters and anglers using any of these waters and western Connecticut in general should use extra care to avoid transporting water, aquatic vegetation, and possibly zebra mussels to new locations.

{An image of zebra mussels.}   

The zebra mussel is a black and white striped, bivalve mollusc which was introduced into North American waters through the discharge of ship ballast water. Since its discovery in Lake St. Clair (Michigan/Ontario) in 1988, the zebra mussel has spread.


Chinese mitten crab was found in Connecticut waters and confirmed by DEEP and Connecticut Sea Grant in June of 2012. The crab was collected from the Mianus Pond fishway on the Mianus
{An image of a chinese mitten crab.}   
River (Greenwich) and is the first confirmed sighting of this invasive crab in this state. Where abundant, Chinese mitten crabs can damage fishing gear, clog pumps and intake pipes, cause riverbank erosion through their burrowing activities and outcompete native species for food and habitat. These crabs are relatively new to the Atlantic coast, however, and at this time it is unclear as to what their effects will actually be here.

Adult Chinese mitten crabs have several distinctive characteristics that aid in identification:

  • Claws are of equal size.
  • "Furry" claws with whitish tips.
  • Brown to green carapace, four spines (the fourth can be small) on each side.
  • Notch between the eyes.
  • Only crab that would be found in freshwater in the Northeast.

Individuals finding a crab that they suspect to be a Chinese mitten crab should keep the crab on ice or freeze it (please do not release the crab), note the exact location it was found, and contact DEEP Marine Fisheries (860-434-6043), DEEP Inland Fisheries (860-424-3474) or CT Sea Grant (Nancy Balcom, 860-407-9107). Any crab found in fresh water should be investigated, as there are no freshwater crabs in New England.

To learn more about this discovery, please visit the Chinese mitten crab press release.  Additionally, an excellent website for more information on Chinese mitten crabs (and other marine invasions) is the Smithsonian Environmental Research Marine Invasions Lab.

{An image of a rusty crayfish.}  

Rusty crayfish is a large crayfish that can be identified by reddish sports on each side of the body just in front of the tail, grayish green color, smooth mandibles (mouth parts without serrated edge) and black bands on tips of claws. Rusty crayfish feed heavily on invertebrates that are important food sources to stream fishes and can destroy aquatic vegetation beds and habits that can impact game fish populations. Rusty crayfish is spread via bait buckets.

Milfoils forms very dense mats of vegetation on the surface of the water, which can make water activities dangerous.

{An image of milfoil in Lake Hortonia}
photo credit: at Lake Hortonia in VT by John D. Madsen, Geosystems Research Institute, Mississippi University

Eurasian water milfoil was first found in Connecticut in 1979 and can now be found in over 40 lakes and ponds in Connecticut as well as the Connecticut River. Eurasian water milfoil is a  rooted, annual plant with a tan or reddish stem, submerged leaves are usually in whorls of four around the stem. It spreads through primarily through fragmentation. When Eurasian water milfoil reaches the surface, it can form a dense mat, which interferes with boating, swimming and other recreational activities, and can alter the ecosystem of the lake. These plants also produce a reddish flower that emerges from the water.

Variable leaf milfoil was first discovered in Connecticut in 1936 and can now be found in approximately 30 lakes and ponds in Connecticut. Variable leaf milfoil is a rooted, annual plant with a thick red stem and has submerged leaves that are typically opposite. It spreads through fragmentation. Like Eurasian water milfoil, variable leaf milfoil can form a dense mat, which interferes with boating, swimming and other recreational activities, and can alter the ecosystem of the lake. These plants also produce flowering spikes that can emerge from the water.

Curly leaf pond weed is one of the most common invasive plants in Connecticut. It was first
{An image of pond weed.}  
discovered in 1932 and can now found in 40 Lakes and pond in Connecticut as well as the Connecticut and Housatonic Rivers. Curly Leaf Pond Weed has a flattened stem with alternate leaves. The leaves are stalkless and have wavy edges (where it gets its name from). These plants spread via turions (soft, hardened leave). Like the invasive milfoil, curly pond weed can produce large, dense mats that interfere with water activities and can alter the ecosystem of the waterbody. These plants differ from most of the invasive plants in Connecticut, in that they often die back by mid-summer. The turions are dormant until the autumn when the water is cool.

  {An image of hydrilla}
photo credit: UCONN IPANE
was first discovered in 1989 and can now be found in two lakes in Connecticut, but is considered extremely aggressive and can outcompete native and invasive species. Hydrilla is a perennial with five whorled leaves which are finely toothed, around the stem. It spreads through fragmentation and tubers that overwinter in the sediment.

Fanwort was first discovered in 1937 and can now be found in more
{An image of fanwort.}  
than 30 lakes in Connecticut and in wetlands along the Connecticut River. Fanwort has submerged leaves that are opposite and have petioles, or short stems. They also have a few small oval, floating leaves at the top of the plant just before the white or pickish flower that can emerge from the water. Fanwort spreads primarily through fragmentation.

Water chestnut was first discovered in Connecticut in 1999 and can be found in the following locations: scattered sites along the Connecticut River from Hartford to Lyme, both in the main stem river and in a number of coves (including White Oaks Cove, Keeney Cove, Hamburg Cove) and connected ponds. Water chestnut has also been found in a number of other waters scattered throughout CT including the Mattabesset, Hockanum and Podunk Rivers, small ponds in Eastford, Thompson and West Hartford, Bantam Lake (eradicated, non found in the last several years), Mudge Pond and at the confluence of the Still River and the Housatonic River.

Anglers fishing in the Connecticut River, its tributaries, and elsewhere should be on the lookout for this highly invasive plant. DEEP and other organizations are involved in eradication efforts.

Water chestnut is a rooted, annual aquatic plant with triangular-shaped floating and feather-like submerged leaves. Its sharp, spiny fruits wash ashore and can inflict painful wounds if stepped on. Dense water chestnut growth can make fishing, boating, swimming and other recreational activities nearly impossible. 

If you find this plant, contact Harry Yamalis at 860-424-3034 or


  {An image of an invasive plant.} {A photo of an invasive species.}  

Didymo is a microscopic diatom (a type of algae) which during “blooms” (a time when the algae goes through explosive and rapid growth) can form large mats of thick cotton-like material that covers large portions of the stream bottom.  Didymo had proven especially problematic in some of New Zealand’s pristine trout fishing waters.

Didymo has been documented in many of blue ribbon trout streams across North America and has been confirmed in several northeastern states. Transport by angler’s waders is considered to be the major source of transfer.  The only confirmation of Didymo in Connecticut to date has been in the West Branch of the Farmington River.  It was first confirmed in the river during March of 2011.  Didymo has not been documented in any other water in Connecticut.

Since its discovery, academic monitoring of Didymo has shown that “blooms” are contained to a small section of the West Branch Farmington River from approximately the Route 20 crossing in Riverton to the confluence with the Still River (approximately one half mile of stream).  This section of stream has ideal chemical and physical characteristics for Didymo.  Small isolated patches have been documented as far downstream as New Hartford (approximately 8 miles downstream).  Anyone coming in contact with the stream bottom in this area is advised to be extra diligent in cleaning their waders/boots or other equipment that has come in contact with the stream bottom before moving to another location on the river or to a different waterbody (or have a second pair of waders/boots available to wear).

Please follow the advice below to help prevent the spread of Didymo or any other potential aquatic species to new waters.

What you can do to prevent the spread of didymo and many other aquatic organisms:

Before leaving the water:

  • CLEAN: your waders, boat, kayak, canoe, tube (especially the soles) including rinsing any sand or mud and removing leaves, plant material, or algae.
  • DRAIN: all water from your boat, kayak, or canoe before transporting.
At home or prior to your next location:
  • DRY: anything that comes into contact with water (boats, trailers, anchors, propellers, fishing equipment, clothing, dogs, etc.) for a minimum of one week during hot/dry weather and four weeks during cool/wet weather.
If drying is not possible:
  • Wash with hot water (preferably high pressure).
  • Dip equipment into 100% vinegar for 20 minutes prior to rinsing.
  • Use a 1% salt solution (1oz. per gallon) or soap and hot water (Lysol, boat soap, etc.) for 10 minutes prior to rinsing.
  • Freeze for at least 24 hours

For more information on didymo:

Vermont DEC

Biosecurity - New Zealand

US EPA Region 8

{Didymo is a threat to trout streams.}

Aquatic Invasive Species