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Do you know about Didymo?
Help stop Didymo before it stops your fishing, boating, swimming…
Photo - Tim Daley, PA DEP
What’s didymo?

It’s a microscopic alga known as a diatom that’s invading our rivers and streams. Didymosphenia geminata, also known as ‘rock snot’ or ‘didymo’, can smother entire stream beds with mats as thick as eight inches and can ruin just about any river or creek.

What does it look like?

Didymo can be found on rocks in moving water and is often mistaken for fiberglass or toilet tissue. Unlike most other algae, didymo feels like wet cotton and isn’t slimy. It is generally brown, tan or yellow in color.

Where is it?

Didymo has been confirmed in the East and West Branches of the Delaware River (New York and Pennsylvania) as well as in the Delaware River mainstem as far south as Callicoon, New York [view map] --


How does it spread?

Unfortunately, it appears that primary carriers of didymo include fishing equipment and waders, boats, and just about anything else that has come in contact with didymo-infected water.

So what can I do to help? Start by reading the rest of this page. For more information, contact your local PA Fish and Boat Commission Waterways Conservation Officer (or region office) or call 814-359-5129.

Disinfect your boat, trailer, boots & other gear before
entering a new body of water
Remember -- you can’t see individual didymo cells -
so thoroughly disinfect all of your equipment
Photo - Tim Daley, PA DEP
CHECK -- Before you leave a river, stream, or lake, check items and leave debris at site. If you find any later, treat and put in trash. Do not wash down drains.

CLEAN -- There are several ways to kill didymo. Choose the most practical treatment for your situation which will not adversely affect your gear.

Stop aquatic hitchhikersNon-absorbent items

  • Detergent -- soak or spray all surfaces for at least one minute in 5% dishwashing detergent or (2 cups (16 oz.) or 500mls with water added to make 2.5 gal. or 10 litres); OR
  • Hot water -- soak for at least one minute in very hot water kept above 140° F (60° C) (hotter than most tap water) or for at least 20 minutes in hot water kept above 113° F (45° C) (uncomfortable to touch).

Absorbent items -- require longer soaking times to allow thorough saturation. For example, felt-soled waders require:

  • Hot water -- soak for at least 40 minutes in hot water kept above 113° F (45° C) ; OR
  • Hot water plus detergent -- soak for 30 minutes in hot water kept above 113° F (45° C) containing 5% dishwashing detergent; OR
  • Freezing any item until solid will also kill didymo.

DRY -- Drying will kill didymo, but slightly moist didymo can survive for months. To ensure didymo cells are dead by drying, the item must be completely dry to the touch, inside and out, then left dry for at least another 48 hours before use. If cleaning or drying is not practical, restrict equipment to a single waterway.

NOTE: The thicker and denser the material, the better it will be at holding moisture (and live cells), the slower it will be to dry out and the more difficult it will be to soak completely with cleaning solutions.

When cleaning equipment, we recommend that you

  • soak porous materials for longer than the specified times to ensure saturation with cleaning solution
  • choose a decontamination solution that will not adversely affect your equipment
  • follow manufacturer’s safety instructions when using products
  • dispose of cleaning waste well away from waterways

For Boats

  • Inspect every inch of your boat, trailer, and equipment – the hull, drive unit, trim plates, props, anchor, centerboards, paddles, wheels, hitch, chassis, etc. – and remove aquatic plants, animals, and mud from the boat, trailer and equipment before leaving any body of water;
  • Drain any and all water from your boat and equipment on land before leaving the area;
  • Do the following when away from direct drainage areas to lakes or rivers:
    -- Dump any leftover bait on land, especially if the live aquatic bait has been in contact with potentially infested waters;
    -- Disinfect live wells and bait wells, bilges, cooling systems, hulls, and decks with a 1:9 solution of household bleach and water allowing at least 10 minutes contact time. Rinse well to remove all residual chlorine. An easy recipe is a half gallon of bleach into a 5 gallon bucket then fill with water (or a quart of bleach to a half bucket).
  • Rinse your boat after use, preferably with hot water. If hot water is not available use tap water and then allow at least five days to become completely dry before entering a new water body.
  • Do not transport any LIVE FISH, BAIT, OTHER CRITTERS, PLANTS, OR WATER from one body of water to another.
Please help stop Didymo before it stops you!
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