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“Rock snot” confirmed in Kootenay Lake watershed

‘Rock snot’ confirmed in Kootenay Lake watershed

Anglers asked to take precautions to slow its spread

The scientific name is Didymosphenia geminata – or just Didymo for short – but it’s more commonly used, and perhaps fitting, term is simply “rock snot.”

Its presence has been confirmed in the Kootenay Lake watershed, and its unwelcome arrival has the Ministry of Forest, Lands and Natural Resource Operations (FLNRO) and the Fish & Wildlife Compensation (FWCP) asking anglers and other water users to take extra care in cleaning gear in an effort to slow its spread.

In early May, while monitoring the Gerrard rainbow trout spawning run at the outlet of Trout Lake, FLNRO habitat technician Terry Anderson floated over a mat of Didymo covering at least 400 square metres.

“We know that it is already in the Salmo and Columbia rivers,” said Anderson, “but this is the first time we have confirmed it in the Kootenay Lake watershed on such a significant scale.”

How anglers can help

Some basic actions will help reduce the spread of Didymo:

  • Avoiding the use of felt bottoms on waders;
  • Cleaning all gear (waders, boats nets, etc.) between water bodies, preferably with a light bleach solution;
  • Allow the gear to dry thoroughly.

“It only takes one cell on your gear to move Didymo from one system to another,” added Anderson, “so it’s imperative that we all make an effort to slow its spread.”

What is Didymo?

Didymo is a species of algae that can either be free floating or attached to rocks, gathering together to make gelatinous blobs or thick mats, thus earning its “rock snot” moniker.

Under water, these mats can resemble thick shag-pile carpeting, ranging in colour from pale yellow-brown to white; and if they get washed-up on shore or the water level drops these dried mats often get confused with dried toilet paper.

“In some circumstances, thick mats of Didymo can restrict water flow, and therefore affect oxygen levels in fish spawning and rearing areas,” says senior fisheries biologist James Baxter with the FWCP – a joint partnership between BC Hydro, the Province of B.C., and Fisheries and Oceans Canada. “It can also cause gill irritations and, if the Didymo reaches high enough concentrations, it can even displace some fish species.”

Didymo is not thought to adversely affect drinking water, except for colouration and odour. Swimmers have reported feeling itchy when downstream of heavy concentrations.

Although Didymo is native to some parts of central North America and is often found in clean and unpolluted streams and rivers, it is considered an invasive species here, and its spread over the last 10 years has biologists concerned. It was first discovered in B.C. on Vancouver Island in 1989.

“One theory is that the spread of Didymo is connected with increased ultra violet light levels” says Baxter. “There is no connection with the Kootenay Lake Nutrient Restoration Program, given the fact that this latest discovery was made well upstream.”

If anyone sees large mats of Didymo, they are asked to report them to the Aquatic Invasive Species Coordinator at the Ministry of Environment by emailing

For more information on this story, contact:

Angus Glass
FWCP Communications Coordinator Columbia Region
Phone: 250 352 6874