OSCODA COUNTY, Mich. – Cells from an invasive algae known as didymo or “rock snot” have been found in the Au Sable River in Oscoda County.
The cells were found in algae samples taken from submerged rocks and debris at two boat launches on the Au Sable River north of Luzerne in Oscoda County.
The samples were taken by Michigan Trout Unlimited in late April. Several didymo cells were found at the Parmalee Canoe Launch and one cell was found downstream at the Whirlpool access off West Cherry Creek Road.
No evidence of didymo was found in samples taken upstream at the McMasters Bridge Road access, downstream, or in the tributaries that were sampled. Didymo is a single-cell alga that thrives in cold, low-nutrient streams.
The cells can form extensive stalks, or blooms, that create dense matts that cover the stream bed. The matts take up habitat that macroinvertebrates like mayfly, caddisfly, stonefly nymphs use. Those macroinvertebrates are a food source for fish.
Anyone who uses the stream is encouraged to properly clean waders, boats, and equipment between each visit to the water.
“With trout season now open across the state and paddle sports gearing up for the summer, it’s important to remind everyone enjoying Michigan’s streams and rivers to take steps to decontaminate to prevent further spread of didymo and aquatic invasive species,” said Bryan Burroughs, executive director of Michigan Trout Unlimited.
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How to identify rock snot
- Didymo is a microscopic alga (diatom) that produces stalks that form thick mats on hard surfaces like rocks in stream beds.
- Looks and feels like white or brown wet wool.
- Ranges from small, cotton ball-sized patches to thick blankets and long, rope-like strings that flow in currents.
- Although often referred to as “rock snot,” didymo is not slimy.
Where is it found?
It thrives in low-nutrient cold water rivers and streams. Didymo cells have been documented in the Great Lakes Basin and Michigan waters in low abundance. Nuisance blooms have been documented in Michigan in the Upper Manistee and St. Marys rivers.
Didymo mats can cover streambeds and reduce habitat for macroinvertebrates including mayfly and caddisfly nymphs, which are important food for fish.
Why is it a concern?
Didymo can be transported on boats, anchors and fishing gear such as waders, felt-soled boots and nets. It can create thick mats that can cover river and stream bottoms. It alter habitat and food resources for fish and make recreation unpleasant.
How you can help
There are no effective ways to eradicate didymo once it’s established in a river or stream.
To prevent the spread of didymo, people should take the following steps:
- Clean, drain, and dry your waders, boots, boats, and other gear between trips or before moving to a new body of water.
- The DNR suggests using different gear for each stream, which can help reduce the chance for didymo spreading.
- People should be extra cautious in places with known or suspected didymo or invasive species activity.
If you use a product to decontaminate gear, the chemicals should be applied on land and at away from the water.
The following products are known to help decontaminate gear:
- Products such as Formula 409 Antibacterial All-Purpose Cleaner: Apply to waders and gear and allow to soak for at least five minutes.
- Bleach: Apply a solution equivalent to 1 ounce of bleach per 1 gallon of water (at least a 2% bleach solution) and let stand for 20 minutes.
- Virkon Aquatic: Apply a solution of 20 grams per liter of water and let stand for 20 minutes (see manufacturer’s label for additional guidance).
If you find didymo, you should report it online through the Eyes in the Field online reporting system.