Didymo in the Delaware: Trout Unlimited Issues Caution to Anglers

Erin Mooney, (215) 557-2845


Didymo in the Delaware: Trout Unlimited Issues Caution to Anglers

Anglers should practice clean angling practices to help prevent spread of rock snot.

BUSHKILL, Pa. With today’s National Park Service announcement that didymo, or “rock snot,” has recently been discovered in the Upper Delaware River, a popular area among trout anglers, Trout Unlimited is issuing an urgent call for a multi-agency, multi-state response to the threat posed by this highly aggressive invasive algae.

TU is also calling on anglers throughout the region to practice “clean angling” habits when fishing and boating in the Delaware River system.

“It’s disheartening to learn that, despite years of active stewardship and conservation efforts, didymo has managed to rear its head in a 100-mile stretch of the Delaware River,” said Roger Olsen, chair of Trout Unlimited’s National Leadership Council Delaware River work group. “With this discovery, it is imperative that federal and state agencies promptly address this situation with a multi-state education campaign designed to raise public awareness about the detrimental impact that didymo can have on fish and wildlife, including potential long-term harm to the entire Delaware River ecosystem.”

Extensive mats of didymo were found in a 100-mile stretch of the Delaware River, from an area near the confluence of the East and West branches of the Delaware River, near Hancock, NY, downstream to the vicinity of the Dingmans Ferry Bridge, in Pennsylvania. Scientists from the Delaware River Basin Commission have also revealed studies showing that didymo extends much farther downstream.

Didymo can grow into large mats of algae that attach to rocks and stream beds where it can alter the life cycle of a river by destroying habitat for aquatic insects (a significant source of food for trout), destroying spawning beds and impacting on oxygen levels in the water. By extension, didymo has the potential to significantly impact angling opportunity and success.

Anglers, boaters and other river recreationalists can transmit the invasive algae on boats, waders and other gear. To help prevent the further spread of didymo, anglers need to ensure that this invasive species does not spread further in the Delaware watershed or to other rivers and drainages.

TU urges its members and all recreational users of the Delaware River to follow the “inspect, clean, dry” approach: First, before entering any rivers, anglers should inspect all equipment for any rocks, mud, plants, moss or other materials. Then, they should remove any visible material. Next, thoroughly clean equipment with water and a brush to remove any attached material. After cleaning your equipment, thoroughly dry it in the sun.


Trout Unlimited is a non-profit organization with more than 147,000 members dedicated to conserving, protecting and restoring North America’s trout and salmon fisheries and their watersheds. Follow TU on the TU blog, on Facebook, and on Twitter via @TroutUnlimited.