The Pennsylvania Fish and Boat Commission and the Department of Conservation and Natural Resources is encouraging anglers to clean all fishing equipment every time they go out to prevent the spread of didymo or "rock snot" as it is more commonly known.
Didymo blooms have been monitored in the Youghiogheny River, the Delaware River as far south as Bucks County and in Dyberry Creek in Wayne County for the past two months.
The United States Department of Agriculture's National Invasive Species Information Center says didymo is native to northern Europe, Russia and northern North America.
It was present in Canada near Vancouver Island in the late 1800s, but did not begin to cause problems until the early 1990s. It was present in the rivers of the Western U.S. by 2004, and it was first discovered east of the Mississippi River in 2005 in Tennessee. It is believed to have been transported to the southern hemisphere on fishing gear. New Zealand has numerous cases of streams being covered.
The invasive aquatic alga can smother entire stream beds and ruin rivers and creeks but is not a risk to human health. It can be up to eight inches thick and appears like wet cotton when the water is squeezed out of it. The commission said it is never slimy to the touch because it has silica in the cell walls and is normally brown, tan or yellow in color.
"It is a diatom," Pennsylvania Fish and Boat invasive species biologist Bob Morgan said. "It's microscopic. You wouldn't see a single cell. It has to be clumped together with others."
Fish and Boat Commission fisheries biologist Mike Depew added, "In a lot of cases it blankets the rocks on a moving stream. The blooms are disgusting to look at. It can choke out the native species of algae and plants that live on the bottom of a stream. Not as many insects feed on it so when it appears it reduces habitat for fish and the insects. There have been a couple of occurrences where it has reduced the insects. That potentially affects game fish like trout. That is why we are urging people to clean their gear every time."
Depew said didymo shows up in rivers below large dams that have cold water discharge. It doesn't do well in warm water like the Ohio River.
"There is a hypothesis that there is a mutant strain," Morgan said. "The USDA and National Park Service is going to be checking to see if it will survive in warm water. Usually didymo doesn't like high nutrient water, but it is changing its characteristics in the Delaware River. The didymo in the Yough has the new characteristics as well."
Depew said that when officers and biologists move from each body of water they thoroughly clean their equipment every time.