Rock snot is a type of slimy, yellow-brown, freshwater algae that, like a cold, has spread quickly from a small area on Vancouver Island two decades ago.
It's now a "global invasive species," scientists say, spreading rapidly from B.C. to locations such as Iceland, New Zealand and across North America — including Alberta, Quebec and New Brunswick.
Matthias Herborg, the aquatic invasive species co-ordinator for the B.C. Ministry of Environment, said experts don't know why didymosphenia geminata, or didymo, spread so quickly.
Data as far back as the 1880's shows didymo has been native in rivers in a small area near Nanaimo, B.C., but around 1990 it started to spread.
"We don't know why, but that's when suddenly we had a series of years of really big blooms which kind of spread across Vancouver Island," Herborg said.
"And then in subsequent years it started to spread around the world. . . It's kind of a global invasive species."
While obnoxious looking, didymo isn't a threat to human health, but it can potentially alter food webs in rivers and could impact the fish, said Max Bothwell, a scientist with Environment Canada.
"However, the evidence for this is mixed and the effects of didymo on trout seem to depend on the species and the particular circumstances. There is no evidence that didymo blooms have harmed salmon populations anywhere in the world," Bothwell said in an email to The Canadian Press.
It usually invades pristine rivers and watersheds and is hard to miss under the water, Herborg said.
"It looks like a shag carpet basically — you know, one of those '70s numbers — rolled out on the bottom."
It's only when it's removed from the water when it looks "snotty, so to speak," he said with a chuckle.
There's also an economic and recreational impact, Herborg said, when fish guides take their clients deep into what's supposed to be pristine and untouched areas.
Didymo can slough off the bottom of the river during high water flow. When it dries on the river banks it often looks like dried-up toilet paper, causing people to think there's a sewage problem in the river.
"It greatly degrades the recreational experience," Herborg noted.
Experts agree the most common cause of the spread of didymo is on the bottom of the felt-soled hip waders of recreational fishermen.
At least one manufacture is phasing out the production of the felt-soles and those types of waders have been banned in Alaska.
But Bothwell said anything that moves water or microbes can spread didymo.
Didymo needs a stable rocky bottom and stable water flows to do well and wouldn't survive in many of Canada's waterways, he said.
Herborg said they're aren't sure why it does so well in what are pristine, nutrient-poor rivers.
"We don't know if suddenly someone introduced a different strain, or if something happened to the native population and it changed in a way," he said. "It's still a big unknown."