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Aquatic Invasive Species

Help stop the spread!

A number of aquatic invasive species are established in Connecticut and more are threatening to invade. Although zebra mussels continue to expand its distribution along the Housatonic River and remain a great concern a more immediate concern is hydrilla. Unlike zebra mussels, which are somewhat limited due to their water chemistry requirements (hardwater), hydrilla can likely grow in waters all throughout the state and a particularly robust strain has spread throughout the Connecticut River.


Hydrilla can form very dense stands that can limit foraging habitat for predatory species such as Largemouth Bass.

Hydrilla, an emerging new threat to Connecticut’s aquatic resources.

Until recently, hydrilla was found at only a few isolated locations in CT. This invasive aquatic plant can spread aggressively (it can grow up to a foot a day), form dense mats of vegetation, easily outcompetes and displaces native plants, alters aquatic habitats, and interferes with recreational activities. Hydrilla is very difficult to control once established. It typically sprouts later in the growing season, making it difficult to identify until the summer, can sprout in deeper water and/or from fast growing runners, can sometimes fragment easily, and will maintain robust growth through the end of the growing season. Not native to the United States, hydrilla was likely first introduced to this country in the 1950’s. It has since become a major problem in southeastern states and has spread into the northeast, and has now been found in two publicly accessible waterbodies: Coventry Lake. In the fall of 2015, hydrilla was first identified in Coventry Lake. Since then, in collaboration with the Town of Coventry, DEEP has been supporting active management of this population of hydrilla, including plant surveys and herbicide treatments. Results have been good since treatment of the lake with the herbicide fluridone began in 2018. Connecticut River. Hydrilla was first found in the river in 2016, in a back portion of Keeney Cove (East Hartford) and later that year growing in the river at Glastonbury’s Riverfront Park and Boathouse. The observed growth pattern suggested that hydrilla had been in the river for a couple of years. In 2017 hydrilla was found in several new locations including Wethersfield Cove, Crow Point Cove, and scattered along the west shore of the mainstem from the Massachusetts border down to Thompsonville. Since then hydrilla has unfortunately spread widely throughout the river. The presence of hydrilla near the Massachusetts border and a population upstream in Massachusetts suggests the source population was upstream of Connecticut. At this time it is unclear as to what control is possible in a large river system such as the Connecticut River, so it is imperative that boaters and other users practice “Clean, Drain & Dry” methods to help prevent its spread to other waterbodies. See the next page to learn more on “CLEAN, DRAIN & DRY” procedures.

Zebra Mussel

Zebra mussels are now found in a number of locations scattered throughout the Housatonic River and its impoundments. Until late 2010 when adult zebra mussels (Dreissena polymorpha) were found in Lake Zoar and Lake Lillinonah, zebra mussels had only been found (1998) in CT in East Twin Lake and West Twin Lake (Salisbury). Since 2010, zebra mussels have spread throughout the Housatonic River and its impoundments and are now established in the river from the Massachusetts border to the Derby Dam, especially in its impoundments (Lakes Lillinonah, Zoar and Housatonic). In 2016, adults were found at the Rocky River power facility, which provides a connection between the Housatonic River and Candlewood Lake, and in 2020, a small number of mussels were found in Candlewood Lake. The non-native zebra mussel was first found in North America in Lake St. Clair (Michigan/ Ontario) in 1988. Since then they have spread throughout the Great Lakes, the Mississippi River system and most of New York State including Lake Champlain and the Hudson River, and then into a number of western and southwestern states. Zebra mussels have fairly specific water chemistry requirements and are limited to waters with moderate to high calcium concentrations and pH. In Connecticut, suitable habitat for zebra mussels is mostly limited to a number of water bodies in western portions of the state. Under highly favorable conditions, this invasive mussel can disrupt aquatic ecosystems and is notorious for clogging water intakes and fouling boat hulls and engine cooling water systems.

Knobfin Sculpin (Cottus immaculatus)

Knobfin Sculpins are native to the White River drainage in the southern Ozark Region of southern Missouri and northern Arkansas. They were first collected in Connecticut from the lower Weekeepeemee River in 2002, likely arriving accidentally in Connecticut in the early 2000’s with a commercial shipment of live fish from the Ozark Region. Knobfin Sculpins have since colonized the entire lower Pomperaug River drainage, becoming super abundant in the entire Pomperaug River main stem, and in the lower reaches of Sprain Brook, Weekeepeemee River, and Nonewaug River. The upstream spread of this species has been limited by natural bedrock barriers on Sprain Brook and Weekeepeemee River, and by an old mill dam on Nonewaug River. Our historical fish sampling data show that impacts to other resident fish species, including young trout, have been devastating and impacts to aquatic invertebrate species such as crayfish and aquatic insects are probably also significant. It is vital that all Connecticut residents and visitors remain vigilant to prevent this species from spreading. Many of our trout streams appear to have suitable habitat for Knobfin Sculpins, and once established, there is no reasonable method for eliminating them. It is illegal to collect any sculpins for bait or other purposes in Connecticut, and it is illegal to release any fish species into the wild without a DEEP Fish Liberation Permit. DEEP’s Fisheries Division asks that anyone suspecting that Knobfin Sculpins are present in Connecticut in a waterbody outside of the Pomperaug Drainage please contact the Fisheries Division at 860 424-3474. More information can be found on our web page at:

New For 2020 - Aquatic Invasive Species Stamp Fee For Vessels

One of the accomplishments of the 2019 Connecticut legislative session was Public Act 19-190, which established a dedicated funding source for the Connecticut Lakes, Rivers and Ponds Preservation account to combat aquatic invasive species (AIS) in Connecticut. Effective January 1, 2020 this account is funded by an Aquatic Invasive Species fee. For vessels registered in Connecticut: this fee ($5.00) will be automatically included on the boat registration renewal fee (the vessel registration decal will serve as confirmation of the AIS fee payment). For vessels registered in other states: the fee ($20.00) must be paid prior to operating the vessel on inland waters in Connecticut. The non-resident vessel AIS fee can be paid in the same way that Connecticut fishing and hunting licenses are obtained, online at, or in person at one of DEEP’s offices, a town hall that has opted to sell sportsmen’s licenses, or at a number of other vendors that sell fishing equipment. View complete list of vendors on the DEEP website or call DEEP Licensing and Revenue (860-424-3105) for the vendor list. There is no physical decal or stamp; when the AIS fee is paid, a line noting that you have paid the fee will added to your CT sportsmen (fishing, hunting, etc.) license document that can be printed or downloaded to an electronic device (phone, tablet, laptop, etc.). While on inland waters you’ll need to have this document (hard copy or electronically) available on the vessel. Note that this is an annual fee and is good for the calendar year. All of your AIS fees will go into the Connecticut Lakes, Rivers and Ponds Preservation account and will be used for projects involving the restoration and rehabilitation of the state’s lakes, ponds and rivers; programs for the control and eradication of AIS and cyanobacteria; and education and outreach programs to promote AIS awareness and understanding.

Some of the Other Invasives Found in Connecticut

Many boaters and anglers are familiar with freshwater invasive plants, in Connecticut these include:

Eurasian water milfoil was first found in Connecticut in 1979 and has been documented in a number of locations in Connecticut as well as the Connecticut River. Variable Leaf Milfoil was first discovered in Connecticut in 1936 and can now be found in a number of locations throughout Connecticut. Fanwort was first observed in Connecticut in 1937 and is now widely distributed throughout the state. Water chestnut was first discovered in Connecticut in 1999 and can be found in a number of locations throughout the state including scattered sites along the Connecticut River, several tributaries and connected. Water chestnut appears to be expanding its distribution in Connecticut. Annual plant that spreads via sharp, spiny fruits. More information on invasive species in Connecticut can be found online at:

Please prevent new introductions of invasive plants and animals; CLEAN, DRAIN, DRY your boat and gear.

You Can Help! CLEAN, DRAIN, DRY. You can help prevent the spread of problem species by following these simple suggestions.

Before leaving a boat launch

  • CLEAN—Inspect your boat, trailer and equipment. Remove and discard all aquatic plants and animals, mud and debris you may have picked up while on the water.
  • DRAIN—Completely drain all water from the boat, including bilge water, livewells and engine cooling systems.

At home

  • DRY—Dry anything that came in contact with the water (boats, trailers, trailering vehicle) for at least one week.
  • WASH—Rinse boat, trailer and equipment with hot water (high pressure if possible). A bleach solution can be used to clean livewells. Dispose of all rinse materials properly!


  • Do not dump your bait bucket or release live bait! Avoid introducing unwanted plants and animals. Unless your bait was obtained on site, dispose of it in a suitable trash container.
  • Do not transport fish, other animals or plants between water bodies. Release caught fish, other animals and plants only into the waters from which they came.

It’s The Law! (CGS 15-180, CGS 22a-381d)

  • Boaters must inspect their vessel for vegetation and listed invasive invertebrates (zebra mussel, quagga mussel, Chinese mitten crab, New Zealand mud snail, Asian clam, rusty crayfish) and properly remove and dispose of any vegetation or invasives before transporting the vessel. You can be fined $95 per violation.
  • The importation, transportation, sale, purchase, possession, cultivation or distribution of a number of invasive plants including the following aquatic plants is prohibited:
    • Curly leaved pondweed, Fanwort, Eurasian water-milfoil, variable water milfoil, water chestnut, Egeria, hydrilla, common reed, purple loosestrife, pond water-starwort, parrotfeather, brittle water-nymph, American water lotus, yellow floating heart, giant salvinia, yellow iris, watercress, onerow yellowcress. Violators can be fined $50 per plant!

Remove all aquatic organisms, mud & debris!