A Didymo Debacle Debunking

For those just joining, didymo (or “rock snot”), is an organism that spreads in coldwater environments, choking insect life and in turn, fish populations. A couple of studies, released years ago and [very] aggressively touted, pinned much of the blame for didymo on felt-soled wading boots.

Never mind that the data behind those studies might have been a wee bit light – an entire sport was moved to change. Boot manufacturers introduced a myriad of rubber soles, the environmental lobby spent heavily to promote “clean angling”, and a number of states banned felt soles altogether.

All was [supposedly] right with the world. Rather than ruffle feathers (because there was no chance in hell migrating waterfowl or any other wild creature or environmental phenomena could possibly have any causal relationship to rock snot infestations) the skeptics just went carp fishing instead.

The news of late should provide ample amusement, as responsible parties attempt to explain …

There’s new evidence published today that’ll have the fishing community in a tizzy, given their common belief that unclean anglers and felt soles are the root cause of the intercontinental spread of Didymo.

The article, “The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms,” cites research done in both Canada and New Zealand (by their respective governments) that suggests anglers have little to do with Didymo blooms.

Read the rest, as only Master Barton could muster with gentlemanly political incorrectness. Those who shared the “common belief” might also benefit from reviewing Charles MacKay’s Extraordinary Popular Delusions and the Madness of Crowds.

MG signing off (because shooting first and asking questions later only works in perpetuity if nobody has the nuts to actually ask the questions)


Doug K says:

the evidence that felt soles transport invasive species such as didymo and many others, is overwhelming. In CO it is about whirling disease rather than didymo, so refraining from felt soles is still a good idea.

The blooms can only happen once the didymo is in the stream. How did it get there ? The new paper has nothing to say about that. It was probably felt soles..

An overview here,

You can’t call a bullet back once it’s fired. Similarly you can’t eradicate didymo (or any other invasive species) once it’s been transported on felt soles into a new river. The prudent approach is better than shooting blindly..


Thanks for the input. I reviewed the original studies long ago, and yet am still curious about the evidence. Is it “overwhelming” due to …

1) The study that didn’t specify where on the boots the particulate was extracted from (a point that at minimum was overlooked, or at maximum obfuscated);


2) The study that reflected a single-digit sample size (which would likely get most any student of statistics a failing grade and would definitely get anyone who used it in an accountable scenario fired)?

The point of the post is not to debate invasive species other than didymo nor the relative benefits of cleaning gear, but to highlight that a decision was made that has had significant regulatory and out-of-pocket (consumer) cost, may have been based on questionable analyses, and (in light of this new data) was probably made in haste. The fact that it was trumpeted by organizations that should be exercising more professional discretion only furthers the issue as I presented it.

As you put it, “It was probably felt soles..”, sure seems like speculation to me.

Now we get a rigorous analysis by constituents (whom I should also point out could otherwise be regarded as biased towards proving the original studies) and find that alternative and quite plausible explanations exist instead.

And that, I believe, is the real shame.

Nate says:

Making the assumption that felt soles must have been the issue with no data to support it is no different than if I claimed migratory geese were the cause. There is no data either way, there is data however, to support a convenient economic surge by promoting a point of view that benefits companies that supply the products.

On a lighter note I was unaware there were any natural fish populations left in Colorado that would be susceptible to whirling disease. I thought they were all just hatchery fish sitting below dams. I would like to see the data on the reproductive capabilities of the fish that do exist in most of the rivers, to see if there would be a sustainable population without constant intervention on behalf of the state.

Anyone for a shoelace ban?

And speaking of migratory waterfowl, Nate, they probably ARE the cause of much of the problem of aquatic invasive species. While felt makes a half decent at best vector for things like Didymo, the digestive tract of ducks and geese is probably a much better ride to be hitched.

Think about it for a sec – you wear your boots for a day or two at a time in this or that watershed, then they sit in the garage for a week or two until the next trip to another watershed. If you’re really responsible you 409’d or froze em in between, but not the filthy ducks and geese, they eat that rock snot up like it’s candy, then decide to stretch their wings a bit and fly to the next watershed, when they get there, they’ve got to drop a deuce like a dog after a walk, and you get the picture.

So the real answer to this problem is to declare war on waterfowl (I did back in 1989, and have been engaged in combat with these dirty birds each year between October and February, since).

I would encourage you to join me, take up arms and do your part to win the war on Didymo, or whatever else we can pin on the ducks.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.