By Captain Ed McCoy

It felt daunting when approached to write about my personal experiences with didymo (rock snot) on the Upper Manistee River. As I reflect upon all the emotions and the sheer disbelief I experienced, writing this piece began to feel necessary.

I recall the first day I fished the Upper Manistee River spring of 2022 (Editor’s Note: Michigan is fortunate to boast several year-round trout fisheries). I remember thinking to myself how drastically different it looked: The river had a very odd brownish-grey hue, with white chunks of didymo suspended.

This was unlike any typical spring high water event I have ever experienced on the Upper Manistee River. A considerable amount of didymo sloughing off the bottom of the stream and being transported downstream throughout the entire float. There was so much debris that it would stick to our flies, knots and lines with every cast. It was almost apocalyptic in appearance.

I remember feeling hopeful that the spring runoff may be scouring the didymo mat from the bottom of the stream. However, as the water began to drop and clear for the first time, that was the moment the reality of the situation began to set in. I didn’t expect to see how extensive the didymo bloom was.

Now that I could see the didymo mats covering the bottom, I finally understood how substantial the bloom was. At this point, my anger shifted to depression and anxiety. An intense feeling of uncertainty for the future of the Upper Manistee River crept into my mind. How could we let this happen? What does didymo mean for the future of our hatches and the trout fishery on the Upper Manistee River? I had a million thoughts racing through my mind all at once.

didymo covers the Upper Manistee substrate.

After floating several sections with coworkers and clients, it became evident that a line was drawn in the sand just downstream of the CCC Bridge.

We did not visually detect any didymo above the CCC Bridge this spring and the Michigan Department of Environment, Great Lakes, and Energy (EGLE) supported that observation. However, as we moved below the CCC Bridge, we found patchy coverage areas in the section running along the CCC campground. As we approached the 4-mile access site and pushed further downstream, the percent coverage of didymo increased dramatically.

The didymo bloom appeared to begin approximately around the CCC campground and continued downstream for over 40 miles extending past the 131 bridge access site. We visually confirmed didymo coating the substrate and found large amounts of debris within every section we fished downstream of the CCC bridge.

Throughout most of the fishing season, we encountered chunks of didymo suspended in the water column downstream of the CCC Bridge.
The density of the suspended particles was exceptionally high throughout April, May and June.

More debris was visible in the water column during busy weekends and after storm events. It was becoming apparent that human activity was increasing the mechanical scour of didymo particles from the substrate and raised multiple concerns for us regarding the effort to stop its spread.
This stuff flowed downstream for months, and there was an increasing probability of didymo colonizing other river sections and neighboring streams. No one I spoke with could give me an honest answer to the nagging question in my mind: Do the didymo chunks contain viable cells, and are they capable of spreading didymo into new areas? Every time I fished the river this spring, it looked more like a losing battle to prevent the spread of didymo. This stuff was everywhere and collected on everything touching the water.

The busy season

The Michigan trout opener was fast approaching, and I couldn’t stop thinking about one of the busiest fishing weekends in Northern Michigan. Many anglers fished their favorite trout streams, and some likely fished multiple rivers and streams.

Yet, with the trout opener days away, there still was NOT ONE SIGN posted at any of the access points along the Upper Manistee River. Not a single sign addresses the highly-transmissible nature of didymo.

The Michigan departments of EGLE and Michigan Department of Natural Resources (MDNR) announced the discovery of didymo in the Upper Manistee River with a press release detailing their findings in December of 2021. However, since that press release was published, there was not one sign posted nor one biologist interviewed during the lead-up to the trout opener about how to wash, rinse and dry all gear thoroughly and avoid wearing felt-bottom wading boots.

From my point of view, our leading state agencies dropped the ball big time. They failed to create an effective media campaign highlighting the threats didymo poses to our streams.

Like many, I was out opening weekend, and I was utterly blown away by the number of people that did not know about the didymo problem. Somewhere along the way, the messaging was lost, and it became evident that there was a considerable need for a more extensive awareness campaign. As a result, a few local fly shops and guides started taking their own media campaigns to social media to build more awareness. After several weeks of pleading and communications from local guides and fly shops, the state installed the signs a few days before Memorial Day weekend.

ProcessingDidymo displayed on a boat. It lives up to its moniker: rock snot.

I knew in the beginning that I had to let the emotion wear off before I could logically process the overall impacts of didymo on the Upper Manistee River. My coworker Jon Ray compared didymo to cancer or diabetes; basically, he viewed didymo as a disease affecting an unhealthy stream.

At that moment, my mind could finally process what I was seeing more clearly. We both decided not to run off to another river for the season but instead to gut it out and run our standard programs. Every morning we would discuss our findings over coffee. The collaboration we had amongst ourselves and a few other guides made it easier to stay focused and constructively share our observations.

As the season moved on, we were hopeful we could help shed some light on the effects of didymo in the Upper Manistee River.

Inconsistent hatches

Didymo immediately impacted this season, especially in the sections where didymo bloomed downstream of the CCC Bridge. The didymo mat that formed during the fall of 2021 blanketed almost all the rigid substrates in the river. It had extensive coverage of the stream bottom throughout the majority of the river. In some areas, we observed over 95% of the substrates and stream channel coverage. One interesting observation was the large-scale impact in areas where spring seeps and brook trout numbers are typically the greatest.

The didymo mat was thicker and darker in these areas, covering every inch of the hard bottom. The insect and fish activity was noticeably off for these zones, and they seemed void of life. Insect activity was greatly reduced throughout the impacted zones, and we experienced visual inconsistencies with our hatches this season. Several hatches were nonexistent this year, and most major insect hatches showed inconsistent hatch times. Overall, insect abundance throughout May and June was the lowest I have ever experienced on the Upper Manistee River.

Trout and didymo

Didymo immediately impacted the trout population in the Upper Manistee River, and there was a noticeable decline in abundance in all of the sections affected by the didymo bloom. Every section we fished with visible didymo growth suffered from diminishing returns. In other words, you can’t catch what’s not there!

Fishing in the impacted zone yielded a 60-85% decline in our catch rate compared to 2021. This trend became even more evident as we entered our peak hatch season. Even during the most prolific hatches of the year, we were in awe of the lack of surface-feeding activity. Most of the other guides we conversed with reported similar observations.

It appeared as if the trout, especially brown trout, had an adverse reaction to the presence of didymo. There is ample research out of New Zealand documenting declines in brown trout biomass by as much as 70% within steams affected by didymo. This decline aligns with our fishing experiences this year and highlights potential concerns for the Upper Manistee River.

There has been chatter amongst the angling community for years regarding declining insect hatches and lower catch rates on the Upper Manistee River. No one has been able to pinpoint the root cause, but visible changes have accelerated over the past decade.

Regional changes affecting the surrounding landscape will usually take time to manifest themselves. I firmly believe didymo is the result of this change and that habitat decline is the primary catalyst. It is widely accepted that deficient phosphorus levels in streams cause didymo to bloom. But this is only one variable in a long list of inconclusive factors allowing didymo to take over a stream. Didymo flourishes in cold, cool, clear, low-nutrient streams with stable, moderate flows that experience high levels of sun exposure. This describes the Upper Manistee River and many of our other trout streams in a nutshell.

I don’t think it’s a coincidence that the didymo bloom has a line in the sand starting at the CCC Bridge Campground.

King Road’s contribution to didymo?

My initial thought is that King Road is negatively impacting that section of the river. The road parallels the river for approximately six to seven miles from King Trout Ranch to the 3-Mile Access site. The road is mostly a stone’s throw away from the stream bank along the entire course. King Road is a constant source of dust contamination for that area and has been treated with a road brine solution as far back as I can remember.

After the brine application to the road, contaminated dust and sediments enter the river all summer. The local road commission began using a new mineral brine solution in approximately 2015. This timing coincides with the start of our declining insect hatches on the Upper Manistee River.

Brook trout and aquatic insects decline in streams with increasing salt concentrations. Conversely, didymo appears more resilient to increasing salt concentrations and can flourish under these conditions. The mineral brine applications on the roads near the river significantly contribute to the didymo bloom.

2022 didn’t provide optimismCaptain Ed McCoy holds didymo on the Upper Manistee River.

The Upper Manistee River Trout Fishing continued throughout the 2022 season.

The sections above the CCC Bridge fished noticeably better than the sections downstream from the bridge. However, the reaches above the CCC Bridge still had some inconsistencies that occurred for no apparent reason.

One unusual thing we observed back in June of 2021 was the off-colored appearance of the river. The water had a light tea stain when we were experiencing long hot days and drought-like conditions. The water should have been gin clear, but it wasn’t!

The river surface had an unusual black appearance during low light periods and was slightly tannic during the daytime. This phenomenon was detected again above the CCC Bridge in June of 2022.

These observations were confirmed through conversations with other guides who regularly fish the river.

You might ask why this is important. Individual didymo cells are amber or golden brown in coloration. I firmly believe didymo was present in the drift during the off-colored water conditions of 2021 and 2022. Hindsight is 20/20, but I firmly believed didymo already impacted fish movement when the river was off-colored.

The trout knew it was coming and were starting to migrate into cleaner water. There is currently no known treatment for didymo, and it appears only to bloom when conditions are right.

There is still no agreement in the scientific literature regarding the primary didymo bloom conditions. Didymo is a new threat and is a significant concern for the surrounding watersheds. Currently, the State of Michigan doesn’t have a plan to combat didymo; without a strategy in place, we will continue to experience the negative impacts of didymo on the Upper Manistee River for years to come.

Trout are poised to be negatively impacted by didymo's presence in the Upper Manistee River.

Didymo and habitat

Habitat decline has reached a tipping point over the past decade. Habitat loss is now at a critical threshold, and the river’s ability to buffer against change is losing ground. Large swings in water temperature are all too familiar now. Changing hydrologic conditions, more significant rain events, longer low-flow periods and disproportionate losses of large woody debris are increasing sedimentation issues and widening the stream width. Subsequently, critical deep-water habitat loss has accelerated over the past five years.

The Upper Manistee River has become dominated by broad, shallow, structureless, sandy areas with long sections of uniform depth. Vast regions of shade providing deciduous trees have also been lost due to disease and invasive species. This combination of declining habitat variables along the stream corridor allows more light penetration to reach the stream bed and causes the water to warm differently. Unfortunately, the river has become too shallow and impacted with sand to maintain its characteristic cold water flows during May, June and July.

The Upper Manistee River experienced exceptionally low flows during the past two summers. As a result, large daily swings in water temperature were more prevalent throughout the stream. Water temperatures fluctuated by as much as 8-12 degrees over a 24-hour period. At night, water temperatures would drop to lows near 58-62 degrees and climb during the daytime to highs reaching 70-73 degrees or more. The biggest temperature swings were commonly observed on warm sunny days during June and July. We also experienced 70-plus-degree water temperatures on sunny days, with air temperatures peaking at 75 degrees. The most alarming change recently occurred was the loss of a two-degree buffer between the M-72 stream gauge and the 4-mile access monitoring station.

This section became uniform in temperature this year during the longest summer days. A healthy stream shouldn’t show this kind of temperature profile.

The big 24-hour swings in water temperature are a giant red flag for the Upper Manistee River. The increasingly uniform habitats have created instability within the stream and impacted the river’s cold water conditions. Increased solar exposure is winning the battle, and the stream’s resilience to extreme change is eroding alarmingly. From what I have observed, these are the conditions didymo requires to take over a stream.

In August, we found new didymo growth above the CCC Bridge, where it was not previously discovered. I would argue that this new growth is a probable result of further habitat decline. So how do we change the conversation surrounding didymo? How do we minimize the ecological and economic impacts on the Upper Manistee River and other affected streams? The river has undergone considerable change, and didymo is simply capitalizing on an unhealthy system.

In August of this year, didymo was discovered on the Boardman River in Traverse City, MI. It was found in a section sampled earlier this year and didymo was not in that area. During the spring, the state sampled several streams, and didymo was not found outside the Upper Manistee River Watershed.

The uncomfortable fact is that didymo is already in the Manistee and Boardman Rivers and will likely spread further. Didymo isn’t going to disappear magically; in my opinion, we don’t have a very effective strategy to stop the spread. Simply put, a hands-off approach will have severe environmental and economic costs moving forward. After witnessing the magnitude of the Didymo bloom firsthand this summer, there is a critical need for early detection and habitat rehabilitation programs statewide. Holistic management strategies and establishing long-term stream monitoring programs should be top priorities.

Our only option is to voice our concerns and demand change. It’s time to rattle the cage, shake the tree, and demand more accountability from our governing agencies; at minimum, our streams deserve a more concerted effort.

If you encounter what you believe to be didymo in an undocumented stretch of stream, please email photos, a location and a description to

Captain Ed McCoy grew up in Northern Michigan fishing for trout, salmon and steelhead. After graduating from Michigan State University with degrees in Zoology and Fisheries & Wildlife, he began guiding anglers professionally across Northern Michigan. Ed guides full-time for Mangled Fly and is passionate about fishing, conservation and protecting our cherished waters for future generations.