Threat to Trout Fisheries

“Didymo” (Didymosphenia geminata) is a diatom, a form of algae, which is native to the northern hemisphere. Up until recent years, it occurred at low levels in pristine lakes and streams but has become invasive, forming dense mats in many streams in North America. In close proximity to Oklahoma, Didymo has been found in Colorado and the White River in Arkansas. This invasive species attaches to rocks and plants in streambeds and may impact freshwater fish, aquatic plants, and important aquatic insects. Didymo can completely smother rocks and plants and reduce the area of clean substrate on which fish need to spawn and feed. It also tends to outcompete native algal species, many of which are food for aquatic insects, the main diet source of native stream fishes. Didymo mats has become so thick in some areas it became impossible to fish those stream stretches.

Didymo is particularly fond of cold tailwaters and, as such, poses a particular threat to the trout fisheries in the Lower Illinois and Lower Mountain Fork Rivers. Anglers have been principle means of spread, making trout anglers who fish streams in the western United States and Arkansas and return to fish the Lower Illinois and/or the Lower Mountain Fork in particular need of taking precautions to avoid spreading Didymo to state waters.

Anglers need to take the following precautions to avoid spreading Didymo:

  • Before leaving the river, remove all plants and algae from shoes, waders, life vests, boat hulls, trailers
  • Treat equipment with a 2% bleach solution or a 5% salt solution or dishwashing detergent


  • Dry equipment for 5 days before fishing a different stream
  • Never transfer fish or plants from one body of water to another
  • Avoid using felt soled waders
  • Avoid wading through mats of Didymo