Invasive algae now on state’s worry list

Last week, the Pennsylvania Fish and Boat Commission announced the spread of yet another undesirable invasive species here in our state with the news that didymo had been discovered on Pine Creek in Lycoming County.

Didymo is a special type of algae known as a diatom. The common name “didymo” is a shortened form of its Latin name, Didymosphenia geminate. Didymo is also known as “rock snot,” a crude but apt nickname descriptive of its characteristic manner of carpeting a steam bottom with a thick, gooey-looking mat.

Didymo is a microscopic, single-celled organism, but once it gains a foothold in an ecosystem, these tiny algae form huge colonies that can produce dense mats several inches thick and covering vast areas of a stream or river. These mats literally smother the stream bottom and eliminate most other forms of aquatic life, especially insects and other small invertebrates that trout and other species of fish depend on for food. While didymo poses no direct health risks to humans, its presence in a waterway can be devastating to the fishery there as well as having an adverse impact on swimming, boating and other forms of water-based recreation.

I first saw didymo during a fishing trip on the Little Red River in Arkansas four years ago. The Little Red below the Greer’s Ferry Dam is a remarkable tailwater trout fishery and justifiably famous for a world-record brown trout caught there in 1992. That monster weighed 40 pounds 4 ounces and measured 40 1/2 inches long with a girth just under 29 inches, and for almost ten years, I’d hoped to have the chance to fish there.

A thick fog hung over the Little Red as we motored downstream at dawn to a popular and picturesque spot known as Cow Shoals. After beaching the boat on a small mid-river island, we were able to wade the riffles and runs below it. Fishing an assortment of small nymphs and wet flies, I hooked a bunch of trout, both browns and rainbows, over the next several hours until the fog finally burned off the river. While none of the trout I caught were record contenders, the morning was quite enjoyable.

As I waded back to the boat through a shallow riffle, I noticed the rocks were covered with what looked like globs of wet toilet paper undulating in the current. When I asked my guide what the stuff was, he confirmed my suspicions that it was didymo. After seeing that disgusting mess firsthand, I am certain no one will want it in their favorite trout stream.

Didymo is native to waters northern Europe and Asia and was known to be present in western Canada in the late 1800s, but problem outbreaks of the species didn’t occur until the 1990s when it spread to some rivers in Canada and several western states. Because didymo requires cold water, it can be particularly devastating to trout waters. The first incidence of didymo east of the Mississippi River occurred in Tennessee in 2005. Since then, it has been discovered in New York, Maryland, West Virginia and Virginia. Didymo was first discovered in Pennsylvania on the upper Delaware River, followed by an outbreak on the Youghiogheny River in 2012.

The discovery of didymo on Pine Creek occurred last month when routine water samples taken upstream of Waterville by Department of Environmental Protection biologists revealed the presence of the alga. According to acting DEP secretary Chris Abruzzo, biologists have seen no evidence of a full bloom of didymo on Pine Creek or nearby waters so far. What makes didymo so troublesome is it can be easily spread to new waters by anglers and boaters. Just a few cells of didymo are all that is necessary for the organism to invade a waterway, and traces of it readily cling to fishing gear, boats and other recreational equipment.

“We may not be able to eliminate didymo from an infected waterway, but there are easy steps we can take to slow its spread and to prevent it from spreading to other waters,”PFBC Executive Director John Arway said. “Didymo cells can easily be carried downstream and can be picked up by any items which come in contact with the infected water, including fishing tackle, waders, and boats and trailers. We urge anglers and boaters to ‘Clean Your Gear!’ before leaving a water body and entering another one.”

Anglers and boaters using Pine Creek or any other waterway known to contain didymo should be vigilant about cleaning and disinfecting all gear and equipment used there. For more details on how to stop the spread of this, go to