Didymo Gunks Up US Fisheries: That'snot Funny

January 9, 2008

Disgusting oozes are a perennial hit on the environment beat. This one's especially gross, with a nickname to match: "rock snot," formally known as Didymosphenia geminata ("Didymo," for short).

This algae attaches itself to streambeds with tenacious stalks that ooze long strands of grayish-brown slime that turn whitish at the tip. It's really earned its nickname.

What does it look like? Here is a clear description and some high-resolution copyright-free images from the NH DES.

Also, this YouTube video clip shows what rock snot looks like when you find it in a stream.

So bring your camera, waders, towels, trash bags, cleaning supplies, and eat light beforehand to cover this story. On the bright side, this story can be a good way to wrangle a legitimate fishing trip during working hours.

Rock snot isn't just hideous. It takes over habitat from bottom-dwelling species. It also smothers aquatic insects, depriving freshwater fish of a key food source. And it's spreading - often when fishermen neglect to clean their boots and gear.

Because of the obvious visual hook, this story is a natural for television, magazine, and online coverage. It also offers easy crossover to outdoor recreation news - maybe even in the "weekend" section.

Plus, there's a strong public service hook: Simple how-to tips for preventing the spread of rock snot, and for recognizing and reporting it, make an excellent sidebar. Here's an example of "how to stop rock snot" instructions from the NH DES.

"CHECK: Remove all visible clumps of algae and plant material from fishing gear, waders, clothing, water shoes and sandals, canoes and kayaks, and anything else that has been in the water."

"CLEAN: Using HOT tap water and lots of soap. Scrub boats and other 'hard' items thoroughly; soak clothes, felt-sole waders and other 'soft' items for 30 minutes!!!!"


Rock snot isn't really new, and it's not exactly "exotic." It was reported around Vancouver Island, Canada, more than a century ago, and it may be a native species that has mutated to become more aggressive. It gradually crept into the northwestern states, then into the Rocky Mountain region (especially Colorado) - but in recent years it's expanded quickly into the Mid-Atlantic and New England states.

This January 2007 US EPA white paper describes the nature and spread of rock snot in detail. Of special interest is the map on page 8 showing the confirmed presence of Didymo throughout the US.

This report was co-authored by two leading US experts on Didymo:

Since Didymo is historically from cold, nutrient-poor northern waters, it tends to winter well. Therefore, now might be an especially useful time to remind outdoor enthusiasts to be careful not to spread the ooze.


  • EPA site on Didymo.
  • Check with your state department of environmental protection for local resources and contacts on Didymo. Some states are highlighting this issue on their web sites and through public service brochures and announcements.
  • The American Fisheries Association has chapters in most states and regions.
  • In most communities where rock snot appears, fishermen and outfitters tend to raise the first alarm and keep the closest watch on this issue. Many outfitters actively publicize the problem and provide illustrated literature with clear instructions. Check with local fishing clubs, outfitters, marinas, boating clubs, raft/kayak/canoe rental companies, and sporting-goods stores to find passionate perspectives on this issue.
  • If rock snot is in or near your area, it's especially helpful to get out into the field and check whether local fisherman are aware of rock snot. Ask them (or watch to see) if they take the steps necessary to stop its spread, and wade out to pick up some rocks - it's not hard to spot.
  • Finally, keep an eye on local water resources issues - especially dam releases and large-scale water transfers. Ask local wildlife biologists about whether these activities might spread rock snot infestations.
SEJ Publication Types: 
Topics on the Beat: