Didymo In Western Rivers

Didymo Below Ruedi Dam, CO
My two recent trips out West to Colorado and Montana provided some good fishing opportunities on new water. The trips also enlightened me on the current range of Didymo in the West. On my second day nymphing on the Frying Pan, I was frequently catching the green stringy algae, typical in most rivers. Yet, on a few occasions I pulled in some rather large, snotty fibrous growths. I immediately recognized that white, gelatinous, dripping mass that enveloped my fly. No one in the parking lot or fly shops could confirm if its presence was documented, and many had never heard of Didymo. I did see signs posted warning of the spread of Whirling disease. Later that day I phoned a biologist at the Colorado Division of Wildlife who verified Didymo’s presence in the river. I was informed the growths were generally not a nuisance, except in certain sections of river. In a week of fishing I noticed the bottom in the upper mile was totally covered like the rock below.
Didymo On Frying Pan River Rock
Fast forward to Montana a month later, and I noticed how white the bottom of the Blackfoot River appeared. On closer inspection the swaying of long white “rat tails” could be seen on the dark river bottom. The growths in some sections literally “blanketed” the bottom under inches of tan-gray algae, far worse than anything I’ve seen during the worst blooms back East. Later in the week I saw thick growths of Didymo on the Bitterroot River. I contacted Montana Fish, Wildlife & Parks, and also Bob Wiltshire, Executive Director For Center Of Aquatic Nuisance Species. I was curious of the extent of Didymo’s known range in Montana, and what effects it had on those rivers? My email from MTFWP was quickly returned with an email detailing how Didymo was considered native to Montana. They also verified the presence of Didymo in all the rivers I mentioned. I also received an email back from Bob, and his response offered a bit more insight into the current situation regarding Didymo in Montana. I included his response below.


You are right about the Didymo in MT. Many of the rivers you fished have just been identified with the algae this year. I have worked with a number of guides and others in Western MT to get proper identification as we have a couple of other stalked diatoms that it can be confused with. However, the bottom line is that MT has it all over the place. In addition to the waters you mentioned, it is in tributaries to the Yellowstone River, in the main Kootenai River and in many streams around Glacier Park. The new discoveries in MT this year have mostly been in the Bitterroot and Flathead basins, both of which are part of the presumed native range.

Didymo presents a special problem in Montana. It is actually a native species here so Fish, Wildlife & Parks does not consider it to be an invasive species. Therefore, they are effectively ignoring its presence and spread. In terms of the native range of Didymo, there is little accurate info about the exact range. The literature states the the native range is the “northern latitudes” of North America. As far as I know, no one has done much work to try to establish the exact boundaries of the native range. However, I think it is probably safe to assume that it was native to all of Montana and much of the Northern Rockies. I don’t know the specifics of Colorado native distribution. The problem is that no one has any idea of why it is sometimes a serious nuisance and other times nothing more than a patch of fuzz on a rock. We have historic records of nuisance blooms in the 1980’s, so we know it has happened in the past. Yet, the past problems vanished and we had no nuisance problems until about 3 years ago. Now the nuisance blooms are spreading rapidly. There are a lot of theories about why but the bottom line is that no one knows and that we are unlikely to find out any time soon.
Bob Wiltshire

Didymo Growth From Gunpowder River
For East Coast anglers, it is still unclear what the effects this invasive diatom will have on our fisheries. If anything, the West can offer some insight into the blooms within its native range. One popular myth I’ve heard is that only tail waters are at risk, but the majority of Montana rivers with the worst blooms I witnessed were freestones. Another unknown factor with this algae is the “dormancy period” where it’s unnoticeable one year, and the next year, unbearable. On the Gunpowder we have seen a similar seasonal fluctuation in blooms, but know little other than it seems to prefer cold water. We have yet to see whether the Savage will experience the same fluctuations and cycles, but I did find small traces this Summer. For something so widespread, it is surprising how little is known about this invasive algae.