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Invasive Alert: Didymo


Eric Wentz, DRBC

Didymo, Didymosphenia geminate, is the latest invasive aquatic species to reach New Jersey waters. Didymo is a single cell algae that—unlike other algae—prefers cold, clean (low nutrient) waters and poses a serious threat to the Delaware River and New Jersey’s trout streams. Didymo forms massive blooms that smother plants, insects and mollusks and covers stream substrate. Through competition, it reduces critical fish habitat and food. It poses no human health threat or threat to non-aquatic animals. 


Didymo can create large amounts of stalk material that form thick mats of grey, white, or brown, but never green, cottony material on the bottom of rivers and streams. Unlike filamentous algae (which is green in color) didymo does not feel slimy, but rather has a coarse wet wool or cotton-like feel.

The Spread

Didymo has been present in the upper branches of the Delaware River since 2007. In 2012, it was confirmed in the Delaware River from its upper branches all the way to Trenton, spanning a distance of 200 miles, and in the Flat Brook, at it’s confluence with the Delaware River. As with other invasive plants it is spread through direct contact, including fishing equipment and waders, boats, kyacks, life vests, float tubes and just about anything else that has come in contact with didymo-infected water.

Stopping The Spread

Preventing the spread of didymo, is similar to preventing the spread of other aquatic invasives. Only one cell is needed for it to spread!


Before you leave a river, stream or lake, check, remove and leave mud, algae, plants and other debris at the site. If you find any later, treat and put in trash.Do not wash down drains.


Soak or spray and scrub gear with one of the following:

  • 2% bleach
  • 5% detergent
  • 5% salt solution
  • disinfectant cleaner


Dry equipment until dry to the touch, then let dry for at least an additional 48 hours. Dry everything that came in touch with the water including clothes, life jackets, fishing gear and boots. Didymo can survive for months in damp conditions!

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. — then 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!

The most effective way to succeed in containing aquatic invasive species is to report each encounter. Anglers are reminded that possession or release of flathead catfish, snakehead, Asian swamp eel, brook stickleback, oriental weatherfish, green sunfish and warmouth, bighead carp, silver carp and grass carp (diploid) is prohibited. Anglers must destroy these species if encountered and submit specimen(s) to the Bureau of Freshwater Fisheries at (908) 236-2118 for north Jersey and at (856) 629-4950 for south Jersey.

For more information on species threatening New Jersey’s aquatic resources visit NJ Fish and Wildlife’s website at

Regulations in red are new this year.

Purple text indicates an important note.

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