Practice Advisory

Practice Advisory – Didymo

Posted by APB
August 6, 2022

Image of DidymoDidymo (Didymosphenia geminata) is a very large, easy to identify diatom. Colonies are commonly referred to as “Rock Snot”. Some historical records have misrepresented its occurrence by other names such as Didymosphenia geminate, Gomphonema geminatum, Didymosphenia geminatum, Gomphonema geminata,and Echinella geminate.

This once rare algae is an invader of freshwater systems across North America, including BC. Didymo has been reported at nuisance levels in a number of river systems around the province, especially throughout central Vancouver Island, where it was first reported in 1989. It has also been found in significant quantities in the Bulkley, South Thompson, Kettle, Columbia and Kootenay Rivers1. The dense algal mats resulting from Didymo colonization stand to negatively impact fish and fish production in a number of ways, including by reducing habitat and food sources for young salmon and trout and by stopping oxygen movement from surface waters to the gravel egg incubation areas2.

Because the algae travel easily by attaching to equipment such as hip waders, fishing gear and boats, government agencies are advising anglers and guides to clean their gear before fishing and are referencing this practice advisory in their Environmental Protection Plans3. Gear can be disinfected by washing in hot water and detergent (5%v/v) for non-porous gear, or soaking in a 2% bleach solution4.

In order to halt the spread of Didymo, the APB recommends that feltless waders be used wherever possible. Where WorkSafeBC or other considerations prevent the use of feltless waders, a cleaning regime should be implemented. For cleaning waders with felts, two approaches have been commonly identified as effective: immersion in very hot (45oC) water with detergent for 30-40 minutes5; or freezing until solid. Bleach and detergent penetration into felts is unreliable and so not recommended. Drying felts is also problematic because inside they can remain moist for extended periods of time.

A cleaning regime should also be in place for any and all field equipment that could contribute to the spread of this pest, including sampling equipment, fishing gear and the inside and outside of watercraft (boats, motors, trailers, kayaks, dinghies). Where suspected Didymo debris (right) is found on equipment after leaving a site, it should be disposed of in the garbage and not washed down a drain.

For more information on specific cleaning procedures for particular materials, refer to the document at the link below:

What Is Didymo and How Can We Prevent It From Spreading In Our Rivers?


  1. This source is no longer available.
  2. Fred Whoriskey, CBC News : Monday, January 8, 2007 | 11:15 AM ET
  3. Peter Cronin, Manager Fisheries Program, New Brunswick Department of Natural Resources
  4. Max Bothwell, Research Scientist, Aquatic Ecosystem Impacts Research Branch, Environment Canada
  5. Field practice in New Zealand involves putting waders into cooler chests, pouring in hot (>45C) water and closing the lid for 40 minutes or more.

Images are courtesy of Wikipedia and can be accessed here:
Scanning electron micrograph of the silica cell wall of D. geminata. Scale bar is 50 μm. Image by Sarah Spaulding, USGS.
Photograph of dead dry didymo beside the Mararoa River in the Southland Region of New Zealand.

This article was previously published as apb Advisory Practice Bulletin #5 and has been edited and updated to remove links that are no longer valid. You can download the original document here.