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Europe, Asia and parts of North America
Didymo is a tan, light brown, and whitish diatom that clumps in strands and feels cottony to the touch. When submerged it looks similar to wet bathroom tissue.
This diatom can form large masses and reduce the primary productivity of the stream. It may be a cause for the decline of trout in some areas of the US.
Didymo can attach to waders and other gear and can easily be transported to new areas when fishing multiple streams in the same day.