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EGLE aquatic biologist: rock snot algae spotted for first time in Lower Peninsula could spread

Didymo from the Upper Manistee River caught on fishing gear. Credit: Samuel Day, LTBB.Aquatic biologists at the Michigan Department of Environment, Great Lakes, and Energy (EGLE), in response to the didymo algae recently discovered in the Upper Manistee River in Kalkaska County, are stressing the importance of the State's "Clean, Drain, Dry" message to anglers and recreational users of Michigan's cold water streams. The sighting marks the first time the algae - commonly known as "rock snot" - has been spotted blooming in Michigan's Lower Peninsula.

A single-celled alga, didymo, when in bloom covers streambeds and reduces habitat for macroinvertebrates, which are important food for fish.

Despite its name, didymo is not slimy. It looks and feels like white or brown wet wool, and ranges from small, cotton ball-sized patches to thick blankets and long, rope-like strings that flow in currents.

Currently, there are no effective methods to eradicate didymo once it is established in a river or stream, notes Bill Keiper, EGLE aquatic biologist. "To prevent spreading didymo and other aquatic invasive species to new locations, it is critical for recreational users to thoroughly clean, drain and dry waders, equipment and boats upon leaving a waterway," he said.

Keiper says it's unknown what causes the algae to turn from an invisible one into one that develops long stalks when it blooms, making it visible on hard surfaces in the streambed.

Lake Superior State University's researchers, with support from the Michigan Invasive Species Grant Program, have been conducting a extensive studies of didymo in the St. Marys River and Upper Peninsula waters since didymo was first found in 2015. LSSU's ongoing efforts will help guide didymo research and management needs statewide.

The State's "Clean, Drain, Dry" message is the most effective way to prevent spread of rock snot, Keiper emphasized.

  • Clean by removing mud and debris from all surfaces.
  • Drain water from all bilges, wells and tanks.
  • Dry equipment for at least five days or disinfect with hot water or a dilute bleach solution.

"Over the next few months, we'll work with partners to assure aquatic invasive species signs are posted at access sites and to spread the Clean, Drain, Dry message to the fishing community," said Keiper. "We want to encourage local fly shops, fishing guides and conservation groups to help out by stressing the importance of decontaminating gear and equipment to protect these waters from didymo and other aquatic invasive species."

If you observe didymo in the water, either as small, cotton ball-sized patches or thick blankets with rope-like strings that flow in currents, take photos, note the location and report it by using the Midwest Invasive Species Information Network, available online at or as a downloadable smartphone app. The MISIN smartphone app will take a GPS location point if a report is made at the site; it also will allow you to upload photos with a report.

Find more information on didymo and how to identify it at

Michigan's Invasive Species Program is cooperatively implemented by the Department of Environment, Great Lakes, and Energy, the Department of Natural Resources, and the Department of Agriculture and Rural Development.

Caption: Didymo from the Upper Manistee River caught on fishing gear. Credit: Samuel Day, LTBB.

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