Stopping Didymo

Learn what you need to know to stop the spread of didymo.

Stop Didymo!

What we know about didymo

Since we last reported on didymo, the invasive alga has been found in Vermont's Batten Kill and Connecticut rivers. Given the itinerant nature of New England anglers and didymo's propensity to spread, it seems only a matter of time before didymo is found in other nearby river systems. This is not good news by any measure, but what it means for our fisheries, and just how bad things could get, is far from certain-because, unfortunately, we don't know all that much about didymo.

Here's what we do know: Didymo is considered native to North America (which means its presence in New England is not that anomalous). However, its distribution and the number of river-choking nuisance blooms across the country have increased dramatically over the past 20 years. The big question that scientists are trying to answer is why the nuisance blooms are occurring so frequently.

Unfortunately, finding the answer will likely take some time due to the fact that there are currently no projects dedicated to studying didymo in the US. Funding for research projects-in the form of federal and state grants-has not materialized because there has been little outcry from interest groups such as anglers and boaters. That is, until didymo is spotted in their favorite river. In fact, the leading didymo expert in the US, Sarah Spaulding, a Denver-based ecologist with the US Geological Survey, says she works on didymo in between her funded projects. In contrast, she says, New Zealand's response to the discovery of didymo on the Lower Waiau River in 2004 was immediate and effective. "There was a crisis, then they figured out what they needed to do, and two years later there is a plan on how to address the problem," she says. The New Zealand plan includes the inspection of angling gear at customs, signs at river access points with information about didymo, and certain regions, such as the Fiordland National Park, have banned felt-sole wading boots.

It is not entirely clear what effect didymo has on trout populations, but some studies indicate that when the didymo blooms occur the numbers of large aquatic insects (e.g. stoneflies and mayflies) decrease, while the number of midge larvae in the stream increases. Ultimately, a change in a stream's insect community has to have some effect on trout but, at this point, how much and in what way is unknown.

Most mysterious, however, is the steady increase of nuisance blooms over the past 20 years. From her research, Spaulding offers three possibilities: 1) We're changing the environment in some way that makes conditions perfect for didymo to bloom, such as building dams and diverting water from streams (nuisance blooms usually occur during droughts); 2) anglers are spreading didymo on their felt-bottomed wading boots to rivers it would normally not inhabit; or, 3) didymo has mutated into a much more aggressive form. Of course, to find the answer to these questions, Spaulding says, "We need to have more formal studies."

Whatever the cause may be, there doesn't seem to be a magic bullet to control didymo. A number of experiments have been conducted using various chemicals in New Zealand, but Spaulding doesn't expect it is possible to remove a microorganism from streams and rivers. "You can't eradicate it," she says. "Until we know more about didymo in the US, it's not responsible to say 'let's go treat the water' because we don't know why the blooms are happening."

While the scientists continue to learn more about didymo, anglers can take some measures on their own to prevent further spreads. The Federation of Fly Fishers has launched the Clean Angler program to teach anglers how to stop the spread of didymo and other invasive species. The message is simple: "Inspect, Clean, and Dry" your gear after fishing, something we all should be doing.


The following guidelines were created jointly by the EPA and the Federation of Fly Fishers to stop the spread of didymo and other invasive species and incorporate the latest science. It should be noted that Biosecurity NZ has a stricter decontamination standard that requires anglers to immerse wading boots in no less than 113 F degree water for 40 minutes, or place wading boots in the freezer until frozen solid. Authorities from both countries discourage the use of felt-sole boots.


Before leaving a river, look for clumps of algae and sediment, and remove them.


Soak all gear for at least one minute in a 2 percent (by volume) solution of household bleach, or a 5 percent (by volume) solution of dishwashing detergent or salt. All surfaces must be in contact with the cleaning solution for a full minute. Water-absorbent equipment (lifejackets, waders) should be soaked thoroughly to ensure complete contact.


If cleaning is not practical, after the item is dry to the touch, leave it to dry for at least another 48 hours before using in another freshwater system.

-Federation of Fly Fishers