DNR – Dreaded didymo – or “rock snot”


December 6, 2021

The Michigan Departments of Environment, Great Lakes, and Energy and Natural Resources have confirmed a report of didymo, a harmful freshwater algae, in a stretch of the Upper Manistee River in Kalkaska County. Also known as rock snot despite its coarse, woolly texture, didymo can grow into thick mats that cover the bottom of the river.

The discovery of the Manistee River marks the first detection of didymo blooms in the lower peninsula. In 2015, extensive didymo mats were found on the Michigan side of the St. Marys River near Sault Ste. Marie in the Upper Peninsula.

“The didymo can attach itself to fishing gear, wading gear and other hard surfaces and be moved to new waterways,” said Bill Keiper, aquatic biologist in the water resources division of ‘EGLE. “With each new detection, it becomes more and more important for people who fish, wade or board to clean boats and equipment, including waders, after each use.”

Anglers who have encountered didymo infested streams in the western or eastern United States know that rock snot is more than just a nuisance.

“Didymo has the potential to be a pest species in Michigan’s cold water fisheries,” said Samuel Day, water quality biologist with the Odawa Indian Bands of Little Traverse Bay. “Unlike the harmful algal blooms that plague Great Lakes regions due to hot temperatures and excess nutrients, didymo blooms form in cold, nutrient-poor streams that most people experience. would generally consider pristine and ideal habitat for trout. Didymo can become a problem when it flowers, covering the beds of streams and reducing the habitat of macroinvertebrates, which are important food for fish. “

didymo carpet in the Manistee river

Day, who studied didymo in rivers in the southeastern United States as a graduate student at Tennessee University of Technology, discovered algae blooms between the Three Mile Bend landings and from Sharon Road Bridge over the Upper Manistee River while fishing with a friend on November 14th. samples, its findings were forwarded to EGLE’s Water Resources division, then verified by Julianne Heinlein, aquatic ecologist and algae taxonomist at the Great Lakes Environmental Center, Inc.

Since 2015, the Michigan Invasive Species Grant Program has supported researchers at Lake Superior State University’s Center for Freshwater Research and Education in a comprehensive study of occurrences of didymo in the waters of the St. Marys River and the Upper Peninsula, the risk of spread and why harmful blooms are on the increase – a phenomenon observed around the world.

Detection of the Manistee River suggests that the distribution of didymo in Michigan waters may be larger than expected. LSSU’s continued efforts will help guide the research and management needs of didymo statewide.

Didymo, a microscopic diatom (unicellular algae), may be present but not detected in some streams until changes in water quality cause it to “bloom” or develop long stems, making it visible on hard surfaces of the stream bed. A better understanding of the changes that trigger blooms can also help address the species’ negative environmental impacts.

Currently, there is no effective method to eradicate didymo once it is established in a river or stream. To prevent the spread of didymo and other aquatic invasive species to new locations, it is essential that recreational users thoroughly clean, drain and dry waders, equipment and boats when leaving a stream. .

  • Clean by removing mud and debris from all surfaces.
  • Drain water from all bilges, wells and tanks.
  • Dry the equipment for at least five days or disinfect it with hot water or a diluted bleach solution.
Enlarged didymo cells and stems

“Over the next few months, we will be working with partners to ensure that 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 fly shops, fishing guides and local conservation groups to help by emphasizing the importance of decontaminating gear and gear to protect these waters from didymo and other aquatic invasive species.”

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

Find more information about didymo and how to identify it at Michigan.gov/Invasives.

Michigan’s Invasive Species Program is a collaborative effort of the Department of the Environment, Great Lakes and Energy, the Department of Natural Resources, and the Department of Agriculture and Rural Development.

/ Note to editors: The accompanying photos are available below for download. Suggested captions and photo credit information are as follows:

Hook: Didymo from the Upper Manistee river caught in fishing gear. Photo courtesy of Samuel Day, LTBB.

Slide: View of Didymo cells and stems under a microscope. Photo courtesy of Samuel Day, LTBB.

Manistee: The growth of Didymo on the gravel of the Manistee river appears dark brown. The areas where the thick shoots break off have a woolly appearance and a light tan, expose a clean substrate underneath. Photo courtesy of EGLE.

Strands: Didymo strands on substrate in the St. Marys River. Photo courtesy of EGLE.

Cell: A single enlarged didymo cell. Photo courtesy of Julianne Heinlein, Great Lakes Environmental Center, Inc./


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