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Didymo (Rock Snot)

Didymo covered rock.
Didymo covered rock.
Photo courtesy of Tim Daley,
Pennsylvania Department of
Environmental Protection.

Didymo (Didymosphenia geminata), also known as "rock snot," is now considered likely to be native to New York. Didymo is a microscopic algae (diatom) that can produce large amounts of stalk material to form thick brown mats on stream bottoms. Didymo threatens aquatic habitats, biodiversity and recreational opportunities. Native to northern North America and Europe, didymo has rapidly expanded its range, invading streams in several western states before moving east. Didymo has been found in several major New York water-based recreational rivers.

Historically, didymo has been found in cool, clear, nutrient poor waters but has expanded its worldwide distribution to include nutrient rich waters. Rivers with stable, regulated flows are particularly at risk.


  • Color - tan, brown or white (not green); may form long white "tails"
  • Texture - like wet wool (not slimy)
  • Strength - firmly attached; does not fall apart when rubbed between fingers

Habitat Impacts

Hand holding didymo over a waterbody.
Photo courtesy of
Mark Hoddle,
University of California,

Didymo grows on the bottom of both flowing and still waters. It is characterized by the development of thick mat-like growths (blooms), which can last for months, even in fast flowing streams. During blooms, these mats may completely cover long stretches of stream beds, altering stream conditions and choking out many of the organisms that live on the stream bottom, which can affect trout and other fish by limiting their food. For the recreational user, footing can become very difficult due to the heavy growths.

Confirmed New York Locations

Didymo has been confirmed in the following rivers in New York:

  • Batten Kill and one tributary (Washington County)
  • Kayderosserras Creek (Saratoga County)
  • East Branch Delaware River below Pepacton Reservoir (Delaware County)
  • West Branch Delaware River below Cannonsville Reservoir (Delaware County)
  • West Branch Delaware River below Delhi to Cannonsville Reservoir (Delaware County)
  • Mainstem Delaware River (Delaware and Sullivan Counties)
  • Mouth of Little Delaware River (Delaware County)
  • Esopus Creek downstream of the Shandaken Portal (Ulster County)
  • West Branch Croton River (Westchester County)
  • Croton River (Westchester County)

Please note: these are very popular fishing and water-based recreational rivers. It is very likely that didymo is present in other river systems, but has not been detected yet. Do not assume that rivers not listed above are free of infestation.

Methods of Spread

The primary way for didymo to be spread is unknowingly by anglers, kayakers, canoeists, tubers, boaters and others engaging in water-based recreation. The microscopic algae can cling to waders, boots, boats, clothing, lures, hooks, fishing line and other equipment and remain viable for several weeks, even in seemingly dry conditions.

There are currently no known methods for controlling or eradicating didymo once it infests a water body.

How You Can Help

Water recreationists are urged to inspect and clean their boats and gear to limit the spread of aquatic nuisance or invasive species. Tips for inspecting watercraft and equipment can be found on our Prevent the Spread of Aquatic Invasive Species webpage.

Equipment and gear can also be placed in a freezer until all moisture is frozen solid. Please note that freezing is a didymo specific treatment and will not necessarily prevent the spread of other invasive species of fish diseases and may damage some gear.

Note: If cleaning, drying or freezing is not practical, restrict equipment use to a single water body.

Felt Sole Considerations: DEC encourages anglers to consider alternatives to felt-soled waders such as rubber studded boots. Felt-soles, due to their ability to absorb didymo cells and to stay damp for prolonged periods of time, are a major vector in spreading didymo and require special treatment (prolonged soaking in disinfectant).

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