Didymo Redux: Keeping Current on Invasive Species

by Ralph Artigliere, Education Director for the Blue Ridge Mountain Chapter; reprinted from the BRM November 2014 Newsletter


  • “Didymo” is a “snotlike” invasive diatom clogging up the world’s rivers
  • Anglers visiting the South Holston are warned to disinfect gear before returning to local waters
  • The jury is out on whether the diatom is carried by sportsmen or whether it has to do with certain chemical dificiencies in water


didymoIt’s been a while since we discussed the diatom accountable for raging “rock snot.” (Up the Creek, Dec. ’09). Our friend and USFS Biologist Jim Wentworth recently sent me a link to a story by BBC on didymo. It is a bit long, but well worth a read in order to get a better understanding of the current status and scientific thought on the pesky organism that threatens to cover the bottom of beautiful trout water with a thick layer of tan ugliness and ruining aesthetics, destabilizing footing, changing the bug pattern, and otherwise forever altering the stream. All of us who love clean, clear waters should be concerned about and up to speed on didymo. Here is the link to the article.

Anyone who has been to the South Holston in Tennessee knows about didymo. In the past, we warned our TU members traveling to fish the Holston on our annual chapter trip about meticulously cleaning their gear when returning to our watersheds because the algae can hitchhike on boots, waders, and even the cork on your rod. The worldwide of didymo was originally blamed on people carrying the algae from river to river on fishing gear and river craft. This fear, along with other invasive species threatening trout habitat, spawned a shift in the industry on how gear like wading boots were made. For example, the techie vibram and rubber soles are supposedly less porous and less likely to carry microscopic passengers from one water to another. While such gear is better protection for the environment, it was not thought to be foolproof and gear still needed to be cleaned.

We are learning more and more about the “rock snot” proliferation caused by a diatom (microscopic algae) scientifically known as Didymosphenia geminata. Turns out the fears and warnings about fishermen and kayakers carrying didymo may not be valid. Current thought is that didymo is present in virtually all areas, but the overwhelming blooms like the one pictured here are caused by the makeup of the water habitat and not fishermen or kayakers carrying a bit of alga from another stream or river. Didymo only transforms into raging rock snot when the water is extremely low in phosphorus, a nutrient found in pollution by certain detergents and fertilizer. This presents a tremendous irony: the less polluted the water is, the more likely Didymo will turn into ugly brown blooms. This is why folks in New Zealand are quite concerned about didymo in their sparklingly clear trout streams.

Regardless of whether didymo is present in all water or not, there is good reason to clean, sanitize, and completely dry gear when going from one watershed to another because there are many organisms that are dangerous when they are carried to a place where they are non-native. Trout Unlimited and others have continued to urge thoroughly cleaning gear to prevent spread of diseases like whirling disease and to prevent invasive species transfer. Also, school is not out on didymo. As we learn more, discretion is urged because of the tremendous downside of a didymo outbreak. I am going to keep carefully cleaning my gear.

BBC did a good job with the article on the worldwide impact of didymo. I commend it to you and continue to urge cleaning and drying equipment before going from one watershed to another. Warm, soapy water works best along with dry-ing in the sun. Frankly, rolling up waders and putting them and wet wading boots in a confined area during transport and then storing without a thorough drying is bad for the gear as well as the environment.

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