Aquatic Nuisance Species in New Hampshire:
Didymo or "Rock Snot"

dydimo poster

New Hampshire anglers and boaters: be on the lookout for a new invader! The aquatic nuisance algae known commonly as "Didymo" or "rock snot" has invaded the northern reaches of the Connecticut River, marking the first official report of Didymosphenia geminata in the northeastern U.S.

This diatom species already affects freshwater rivers and streams in other parts of the U.S., Canada (including Quebec province) and New Zealand. It is not known at this time how Didymo will affect water quality, aquatic habitat and fish populations in New Hampshire. N.H. Fish and Game and the N.H. Department of Environmental Services are monitoring the situation closely and working to stop its spread to other waterbodies in the state.

It is critical for anglers and boaters to be aware that Didymo is on the move and is easily spread by even just one cell of the alga breaking off and drifting downstream in infested reaches. It is also very easily spread by waders, fishing gear and other gear that touches the bottoms of streams in infested areas, so it is essential to check and clean your fishing gear to prevent the spread of Didymo and protect New Hampshire's waters.

Anglers and boaters: How to clean your gear

Prevent the spread of Didymo by cleaning all equipment before moving from one location to another, even on the same river. If you see Didymo in New Hampshire, contact Amy Smagula at the N.H. Department of Environmental Services at 603-271-2248 or email

CHECK: Remove everything that sticks to fishing gear, boats and trailers. Remove all visible clumps of algae and plant material from fishing gear, waders, clothing, water shoes and sandals, canoes and kayaks, and anything else that has been in the water. It takes a number of cells present before Didymo is visible to the human eye, so cleaning is also very important.

CLEAN: Use HOT tap water and lots of soap (a good squirt of dishwashing detergent). Scrub boats and other “hard” items thoroughly; scrub all gear at least one minute. Soak clothes, felt-sole waders and other “soft” items for 30 minutes!!

Useful links to find out more about Didymo:

N.H. Department of Environmental Services web page on Didymo, with frequently asked questions, how to identify Didymo, and more details on regional sightings:

U.S. Environmental Protection Agency web page on Didymo:

What Didymo could do to your favorite river (New Zealand Didymo resource page)

Didymo Trout Impact Study (New Zealand):

Interim Report: Invertebrate Drift and Trout Growth Potential in a Didymo-Affected Reach of the Mararoa River (New Zealand):

Be on the lookout for Didymo! If you see it in New Hampshire, contact Amy Smagula at the N.H. Department of Environmental Services at 603-271-2248 or email

To learn more about other species of aquatic wildlife and plants that are unwanted invaders of New Hampshire waters and what you can do to help stop the invasion, click here.

Stop aquatic hitchhikers!

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