A Guide to Aquatic Nuisance Species


Help stop the spread!

An increasing number of damaging aquatic species are invading our waters. Zebra mussels and Eurasian water-milfoil are most familiar to anglers but other exotic species may cause problems as well.

Zebra Mussel

Zebra mussels were recently found (October 2010) in Connecticut’s Lake Zoar and Lake Lillinonah. This is the first report of a new infestation of this highly invasive bivalve in the state 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 mainstem Housatonic River. At this point it is uncertain if the mussels found in Lakes Lillinonah and Zoar are the result of downstream migration from these upstream sources or the result of a separate introduction, however, downstream migration is suspected.

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Anglers fishing in 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.

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 throughout the Great Lakes, the Mississippi River system and most of New York State, including Lake Champlain and the Hudson River. More recently both zebra mussels and quagga mussels (a related species, and also highly invasive) have been expanding their range into a number of western and southwestern states.

This mussel can clog power plant, industrial and public drinking water intakes, foul boat hulls and engine cooling water systems and disrupt aquatic ecosystems.

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Water Chestnut

Dense water chestnut growth can make fishing, boating, swimming and other recreational activities nearly impossible. In Connecticut, water chestnut has been found at 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. DEP 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. If you find this plant, contact Harry Yamalis at 860-424-3034 orharry.yamalis@ct.gov.


Alewife (landlocked)

Landlocked alewives (a member of the herring family) are abundant in many Connecticut lakes and are commonly used by anglers as a bait fish. Accidental introductions, intentional stockings and widespread use of landlocked alewives as bait have all resulted in these fish becoming established in lakes where they can be highly detrimental to other fish species. Once landlocked alewives become established, their population numbers can increase quickly, allowing them to outcompete other fish species for available food. Also, alewives can feed on egg and larval stages of other species such as trout, perch, walleye and bass. Seldom can they be removed from a lake once they establish a population.

East Twin Lake (Salisbury) and Wononskopomuc Lake (Lakeville) provide examples of the effects of landlocked alewives. Each of these lakes had fishable populations of Kokanee salmon for many years. Alewives were accidentally introduced into each of these lakes and the salmon populations declined and then disappeared. Interestingly, over the last 3-4 years the alewife population collapsed in East Twin Lake. Concurrent with this decline, the Kokanee population in East Twin Lake has rebounded (when available, DEP had continued to stock salmon fry into both lakes).

For these reasons we recommend that anglers take great care when discarding their unused bait at the end of a fishing trip. Do not empty bait buckets into the lake (an alternative is to freeze and save bait for the next trip). These precautions will help limit the spread of an unwanted and potentially detrimental species.

Remember: The use of live alewives, blueback herring, gizzard, hickory, or threadfin shad as bait is prohibited in:

Alexander Lake, Colebrook Reservoir, East Twin Lake, Lake Pocotopaug, Saugatuck Reservoir, Shenipsit Lake, West Hill Pond, West Twin Lake, Wononscopomuc Lake, Wangumbaug (Coventry) Lake, West Branch (Hogback) Reservoir

Emerging Diseases

Plants, invertebrates and fish aren’t the only invaders. Just as insidious are a number of new diseases and parasites that affect fish (but are harmless to humans). Largemouth Bass Virus (LMBV) is a relatively new disease that has been found in Connecticut.

  • This virus was first found in 1991 at Lake Weir (Florida). Originally thought to be limited to southern waters, this disease has spread northwards into the Midwest and Northeast.
  • Although LMBV can infect a number of fish species, it is only known to cause mortality of largemouth bass, and has been responsible for a number of notable fish kills throughout its southern distribution.
  • LMBV can be transmitted by consumption of infected prey, through the water, and by fish-to-fish contact. The virus is thought to be most active during the warmer summer months. Because the virus may survive in water for up to a week, it can be transferred between water bodies in improperly cleaned livewells.
  • Stress appears to promote the onset of symptoms that can lead to death. Common stressors include warm water temperatures, low dissolved oxygen concentrations, overcrowding in livewells, and improper handling by anglers.
  • DEP Fisheries biologists first began collecting largemouth bass to test for LMBV in 2005 and a number of Connecticut lakes have been sampled. Bass from Amos Lake (Preston) and Gardner Lake (Salem) have tested positive for LMBV.
  • Properly cleaning and disinfecting boats, livewells and gear will help prevent the spread of this virus. Anglers should never transfer fish between water bodies!

Other emerging diseases in nearby waters include Viral Hemorrhagic Septicemia (VHS) and Spring Viremia of Carp (SVC). Information on these diseases can be found on the DEP website at www.ct.gov/dep/invasivespeciesUH.

Didymo: A threat to trout streams

Your help is needed to prevent the spread of didymo (Didymosphenia geminata). This highly invasive freshwater alga (also called “rock snot”) has now been found in popular trout streams located in a number of northeastern states(New Hampshire, Vermont, New York, Pennsylvania, Maryland, West Virginia and Virginia). Didymo has the potential to alter food webs and degrade habitat in many Connecticut trout streams.

Didymo is typically found in shallow streams with rocky substrate. Thought to be native to northern regions of Europe, Asia and North America, didymo originally was found only in cold, clear, low-nutrient waters.

Didymo’s geographical and ecological ranges have been expanding, and now also include warmer and more nutrient-rich waters. The occurrence and intensity of blooms are also increasing. It is currently unclear why.

The microscopic didymo cell produces a stalk to attach to the substrate. During blooms, didymo can produce large amounts of this stalk material, forming thick mats of cottony material that feels like wet wool on the bottoms of rivers and streams. These mats can potentially smother aquatic plants, mollusks, destroy invertebrate and fish habitat, and impact existing food webs.

Anglers are considered an important vector responsible for the recent spread of didymo. The microscopic cells can cling to fishing gear, waders (felt soles can be especially problematic), boots and boats, and remain viable for months under even slightly moist conditions.

What you can do to prevent the spread of didymo:

  • CHECK: Before leaving a river, stream or lake, remove all obvious clumps of algae and plant material from fishing gear, waders, clothing & footwear, canoes & kayaks, and anything else that has been in the water and look for hidden clumps. Leave them at the site. If you find any later; treat and dispose of all material in the trash.
  • CLEAN: Soak/spray & scrub boats and all other “hard” items for at least one minute in either very hot (140°F) water, a 2% bleach solution, or a 5% dishwashing detergent solution. Absorbant materials such as clothes and felt soles on waders should be soaked for at least 40 minutes in very hot water (140°F), or 30 minutes in hot water (115°F) with 5% dishwashing detergent. Freezing solid will also kill didymo.
  • Dry: Drying will also kill didymo, but items must remain completely dry (inside and out) for at least 48 hours.

There are numerous other freshwater aquatic invaders of concern to Connecticut. Some, including tench, rusty crayfish (Orconectes rusticus), Asian clam (Corbicula fluminea), and hydrilla have been found in Connecticut waters. A number of others including didymo, New Zealand mud snail, landlocked gizzard shad, silver, black and bighead carp, aquarium species such as snakehead fish (family Channidae), and emerging new diseases such as VHS and SVC could eventually find their way to Connecticut.