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"Rock snot" blooms in Upper Delaware

A scientist with the Delaware River Basin Commission recently found large mats of the invasive algae along a 40-mile stretch of the river.

Look out fishermen. "Rock snot" appears to be distressingly alive and well in the Upper Delaware.

Officials knew the stuff, an algae more properly known as Didymo, was in the river. But nothing like this: Last week, an aquatic biologist with the Delaware River Basin Commission found extensive mats of it along a 40-mile stretch of river from the Lackawanna to near the Dingman's Ferry bridge. The section includes portions of the Upper Delaware Scenic and Recreational River and the Delaware Water Gap National Recreation Area.

Samples were collected and sent to the Academy of Natural Sciences of Drexel University in Philadelphia, where academy scientists confirmed the identification of Didymo.

The biologist, Erik Silldorff, said that "the spatial extent and intensity of this bloom is alarming given its potentially detrimental effect on ecosystems and the ease in which it can be spread to nearby tributaries."

Rock snot, an invasive species that got its name from the gooey way it looks, smothers out native algaes that, among other things, are food for aquatic organisms. It covers rock surfaces in water that is cold and flows fast or moderately fast.

After Sildorff's discovery on April 18, scientists with the National Park Service and the Pennsylvania Department of Environmental Protection documented Didymo blooms in a broader area, north to Callicoon, N.Y., and into the East and West branches of the river. While the coverage was dense in some areas and spotty in others, "these findings indicate that the blooms of Didymo now extend across more than 100 miles of river," the DRBC said in a press release.

While Didymo can alter physical and biological conditions within a stream, it is not expected to affect water quality downstream. However, it can ""easily attach to any fishing equipment, especially felt-soled boots, and the chance of it hitchhiking its way into nearby streams or rivers that currently lack this unwanted invader is cause for alarm," according to the DRBC.

Biologists are concerned about neighboring cold-water trout streams that don't have Didymo getting it via anglers fishing the Delaware where it's found and then – without cleaning their equipment – fishing a trout stream where it's not found, a spokeswoman said.

For additional information on Didymo and how to prevent its spread by properly cleaning equipment, visit