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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. This year’s Angler’s Guide focuses on the zebra mussel, as it has notably expanded its distribution in Connecticut during the last several years.


thumb_AquaticInvasiveSpeciesZebra 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 been found (1998) in CT only in East Twin Lake and West Twin Lake (Salisbury). Since 2010, adult mussels have also been found in Lake Housatonic and free-floating juveniles (veligers) have been sampled at several sites in the river in Connecticut from the Massachusetts line downstream to Lake Lillinonah.16Surveys completed in late 2012 confirm the continuing presence of zebra mussels adults and free-floating juveniles (veligers) in the Housatonic River and its major impoundments in Connecticut.

One possible source of these mussels is downstream migration from Laurel Lake in Lee/Lenox, Massachusetts. Its short outlet stream drains directly into the Housatonic River. In 2009, an abundant population of zebra mussels was documented in Laurel Lake. Adult mussels were also found that year in the Housatonic River in Massachusetts downstream of the confluence with Laurel Lake’s outlet stream. Since then additional small populations have been found at sites in the river in Massachusetts.

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.

Zebra mussels and other aquatic invasive species can be inadvertently spread by boats, gear and bait buckets. Easy “Clean, Drain & Dry” methods to help prevent their spread can be found below.


Some of the 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.

18Variable Leaf Milfoil was first discovered in Connecticut in 1936 and can now be found in a number of locations throughout Connecticut.

Hydrilla was first discovered in 1989 and can now be found in a few locations scattered throughout the state. Very aggressive and can outcompete native and invasive species.

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.


New arrivals to Connecticut include:

(2011) DIDYMO is a freshwater alga that under suitable conditions can form extensive blooms. Prefers cold, low-nutrient streams. In Connecticut, didymo has only been found in one area, the upper West Branch Farmington River near Riverton.

(2012) Chinese mitten crab A juvenile Chinese mitten crab was collected from the Mianus River in Greenwich. This is the first confirmed report of this invasive in CT. This crab reproduces in saltwater but spends most of its life in freshwater

More information on these and other 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.

Websites of Interest

These websites can provide a good start to learning more about Aquatic nuisance species.


Regulations in red are new this year.

Purple text indicates an important note.

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