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Didymo or "Rock Snot" (Didymosphenia geminata)

Didymo (Didymosphenia geminata) or "rock snot" is a brownish alga that can form thick mats on river bottoms and shorelines. Mats can reduce bottom dwelling organisms thereby affecting fish diets and reproduction. In its invasive form, didymo looks like snot, but feels like wet wool or fiberglass insulation. Likely native to Lake Superior and parts of Canada, around 1990 an invasive form spread to some Western rivers and eastward from the Mid-Atlantic to Quebec. It is especially invasive in New Zealand.

Recreational anglers can spread didymo that gets imbedded in felt soles of boots, waders, and field gear. It can survive for days out of water under moist conditions. Eradicating infestations is impossible. Your actions and your help in reporting new infestations are vital for preventing its spread.




Identify Didymo

Identify Didymo

General Characteristics

  • Can form extensive blooms that cover the bottom of lakes and rivers
  • Often found along shorelines subject to wave action
  • Surface of mats often whitewashed; look similar to shredded tissue paper

What You Can Do

  • Learn to recognize didymo.
  • Avoid using felt-soled boots, the most likely way it spreads.
  • Clean all aquatic plants, animals and mud from watercraft, trailers, docks, lifts, anchors and other recreational equipment before leaving access.
  • Drain water-related equipment (boat, ballast tanks, portable bait containers, motor) and drain bilge, livewell and baitwell by removing drain plugs before leaving water access. Keep drain plugs out while transporting watercraft.
  • Spray watercraft and equipment with high-pressure water, or
  • Rinse waders, hip boots and gear with hot water, or soak in 5% salt (2 cup/3 gal) for 30 minutes followed by a tap water rinse, AND/OR
  • Dry gear for five days.

Report New Sightings — note exact location; place a scrape of suspicious material in a sealed plastic bag; and call a Minnesota DNR Invasive Species Specialist (see www.mndnr.gov/invasives/contacts.html), 1-888-MINNDNR or (651) 259-5100; or the Minnesota Sea Grant Program in Duluth, (218) 726-8712.

Know the Rules!

Specimens are needed to confirm sightings, but some jurisdictions prohibit possession and transport of invasive aquatic plants and animals. Contact your local natural resource management agency for instructions. Unauthorized introduction of plants, fish, or invertebrates into the wild is illegal. Protect your property and our waters.

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This page last modified on May 04, 2016     © 1996 – 2019 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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