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Trout-snuffing 'rock snot' confirmed in North Shore stream

A nasty cold-water algae nicknamed "rock snot" has been confirmed in the Poplar River near Lutsen along the North Shore of Lake Superior, the first such finding in a Minnesota trout stream. The freshwater algae, officially called didymo, lives in...

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Didymo, also called rock snot, has been confirmed in the Poplar River on Minnesota's North Shore of Lake Superior. The cold water algae has devastated trout populations in other regions (Minnesota DNR photo)
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A nasty cold-water algae nicknamed "rock snot" has been confirmed in the Poplar River near Lutsen along the North Shore of Lake Superior, the first such finding in a Minnesota trout stream.

The freshwater algae, officially called didymo, lives in low nutrient, low temperature environments that are common in North Shore streams and Lake Superior, the Minnesota Department of Natural Resources reported Thursday.

Under the right conditions, didymo can form dense mats of brown slime that smother stream beds and may affect stream invertebrates that are food for fish, birds and other animals.

In other regions of the U.S., and as far away as New Zealand, didymo has been devastating to local trout populations, and anglers are being warned to avoid spreading the algae to other Northland waters. In South Dakota's Rapid Creek, for example, didymo was discovered in 2005 and has since been blamed for a major reduction in the stream's brown trout population.

Didymo was confirmed in Lake Superior at very low levels in 2015 but had never before been seen upstream in rivers. At first thought to be an invasive species, didymo is now believed to be a native species always present in lower levels.

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Algae experts at the Science Museum of Minnesota and University of Wisconsin-Oshkosh have confirmed the Poplar River outbreak. DNR fisheries and invasive species biologists will be searching other nearby streams and rivers in coming weeks to see how far it has spread.

According to Lake Superior State University, didymo resembles toilet tissue or wadded up paper. Fisheries experts are warning anglers and others using the river to thoroughly clean off their gear before going to any other stream or lake.

"It's important for people using the Poplar River to know it's there and use good cleaning practices, so they don't spread it," said Chris Kavanaugh, Minnesota Department of Natural Resources regional fisheries manager. "This species of algae is not well studied in Minnesota and additional surveys are needed to determine if this new find is an expansion to new waters or if didymo was already present and the conditions were right for mats to form."

Didymo generally doesn't directly kill trout but smothers out trout food, Kavanaugh told the News Tribune.

Didymosphenia geminata is a diatom algae native to the northern hemisphere but, for reasons not fully understood, occasionally explodes and takes on characteristics of an invasive species in some areas. It is not considered a significant human health risk, but can affect stream habitats and sources of food for fish, and make recreational activities unpleasant. This microscopic algae can be spread in a single drop of water.

This species has the potential to be harmful, and many North Shore streams are low-phosphorus, cold water systems that provide conditions suitable for didymo growth.

Recreational users should follow a few steps whenever moving between streams and rivers along the North Shore:

• Remove any mud, plants or other material from recreational gear, and drain water.

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• Wash boots and other gear in hot water and dry for 48 hours or freeze for 48 hours before reusing.

• Avoid using materials that absorb water or may be difficult to dry, such as felt-soled waders.

Didymo, also called rock snot, has been confirmed in the Poplar River on Minnesota's North Shore of Lake Superior. The cold water algae has devastated trout populations in other regions. Missouri Conservation Department photo.
Didymo, also called rock snot, has been confirmed in the Poplar River on Minnesota's North Shore of Lake Superior. The cold water algae has devastated trout populations in other regions. Missouri Conservation Department photo.

Related Topics: LAKE SUPERIOR
John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at jmyers@duluthnews.com.
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