Didymo (Didymosphenia geminate) or “Rock Snot” is a fresh water diatom most commonly found in high elevation streams and rivers throughout northern Europe and North America. It has long been considered a ‘cold-water’ algae, but has since spread to warmer lower elevation streams and rivers. It can form massive blooms causing significant negative impact to freshwater fish, plants and invertebrates through habitat and food web alteration.
Its most distinguishing feature is a slimy brown layer or ‘ blooms’ that smothers rocks, aquatic plants or any other structures in the water. The blooms resemble thick slimy looking mats and are distinguished from other forms of algae by its brown, beige or white color. Didymo is never green in colour.
Although Didymo looks slimy in appearance, it is spongy and scratchy to the touch and has a texture similar to wet wool, and it doesn’t fall apart when rubbed between your fingers. It forms in large mats that greatly decrease the aesthetic appeal of streams and rivers. The mats also have entrails or long ‘rat tails’ that turn white at the ends and look similar to tissue paper. These “rat-tails” typically break free of the larger mat washing up on shorelines giving them a very unpleasant look.
Occurrence of Didymo on Vancouver Island was reported as far back as 1894; however, documentation did not include specifics such as location or abundance. 100 years later, Didymo blooms are now found in over 12 watersheds throughout Vancouver Island, and it has also been found in significant quantities in the Bulkley, South Thompson, Kettle, Columbia and Kootenay Rivers.
Reasons for its rapid spread are unclear, as Didymo blooms typically occur in cool water. Stability of stream flow and high exposure to ultraviolet light also appear to create favorable conditions in warmer nutrient-poor water for Didymo to flourish. Climate change may also be contributing to rapid expansion as Didymo out competes other algae species under increased levels of ultra violet light radiation.
As it only takes one cell being introduced to create a new didymo bloom, it is important to properly clean fishing gear and boating equipment where it can often easily attach. Boaters, anglers and other water users need to be particularly vigilant when leaving infested areas to make sure they Clean, Drain and Dry their equipment, before launching into a new body of water to prevent further spread.
Fishing waders, dingys, kayaks, boats, motors and boat trailers are all excellent sources of transportation for Didymo to move from one body of water to another. Anglers are asked to wear felt-less soled fishing waders.
Fishing gear should be disinfected with hot water and detergent for non-porous gear. For waders with felt soles, it is recommended to soak the gear in hot water and bleach for 30 – 40 minutes or to freeze until solid.
If Didymo debris is found on equipment after leaving a site, it is advised to dispose of it in the garbage and not down the drain.