So much of our sport comes down to the necessity of trout to eat. And having a strong food base in the form of healthy macroinvertebrate populations is paramount. That’s why we pay attention to available surveys of not only how our fish populations are doing, but how healthy our streams’ bug populations are.
Thankfully for us Upper Madison enthusiasts, NorthWestern Energy (NWE) produces an annual report (29 years running!) on aquatic benthic macroinvetebrate (BMI) surveys as part of its environmental biomonitoring for hydroelectric facilities on the Madison and Missouri Rivers (FERC Project 2188). This document is very informative to the status of macroinvertebrates. What’s a little tricky about macroinvertebrate populations is that they can be volatile from one year to the next as far as population numbers go. They also tend to be cyclical and heavily influenced by either drought or high water years as well as other environmental factors. In our case on the Upper Madison, that means the ice jamming near Ennis in colder winters. This really seems to be a limiting factor for macroinvertebrate populations. But thankfully, rivers that are highly resilient like the Upper Madison, tend to see quick rebounds in bug populations after such events.
NWE just published the summary report for surveys completed in 2023 (always conducted in August, FYI). Although there’s 11 monitoring sites from Yellowstone to Morony Dam on the Missouri, I only focus on the 3 sites on the Upper Madison here. Those are: Hebgen, Kirby, and Ennis. Hebgen and Kirby are obviously on the upstream ends of the Upper Madison, and Ennis on the lower end. I wish we had a monitoring site somewhere in the middle like McAtee, because I feel like based on the differences in assemblages observed from Hebgen to Ennis, it’d be nice to know what’s going on in the middle section.
If you’re wondering how they conduct the survey, they use a modified kick net procedure. Basically a very formalized San Juan shuffle with a big bug net downstream to collect the critters for preservation and analysis in the lab to species level. They do this collection effort 5 times at each site. It’s important to keep in mind when looking at the results that it’s only a small spot at a single moment in time. Are the species results exhaustive and complete for each site? No, they’re probably missing some contributions of some bugs, but the idea is to do it effectively enough 5 times per site, to get a pretty decent idea of the relative abundance of bugs per site. The method has proven to be effective at that. So if you don’t see March Browns, for example, listed with any relative abundance contributions at, say, Kirby, but you know you’ve seen them hatch there before, it’s not as if NWE is saying they aren’t present. The results are simply saying that they didn’t find them in the 5 samples at that site in any given year. So, it may be that March Browns are at Kirby, but we can surmise that the overall abundance of that bug at that site isn’t very high for that year.
Approximately 179,370 macroinvertebrates (±10,100 individuals) from 11 sites (55 samples) were collected on the Missouri and Madison Rivers during August 2023, which is significantly higher than the number of individuals collected in 2022 (144,595). One of the reasons for this appears to be the massive increase in density at Kirby…more on that in a bit.
Before we break down Upper Madison data, here’s the overall graph of all taxonomic groups at all sites in 2023.
So let’s break down the three Upper Madison Sites. In total, 95 unique species of macroinvertebrates were surveyed at the three sites on the Upper Madison in 2023.
For us anglers on the Upper Madison, among all taxonomic groups, Mayflies, Stoneflies, Caddisflies and Midges are easily the most important to our sport. I’ll focus largely on those. Here’s a breakdown of those groups at the three sites on the Upper Madison.
Ennis
Mayflies were fairly abundant in Ennis in 2023. Ennis Mayflies were dominated by BWOs (5 species). Other notable mentions at Ennis were strong contributions from March Browns (not found at Kirby or Hebgen in 2023), one species of Western Green Drakes, one species of Pink Ladies, Dark Hendrickson, and a heavy dose of Tricos. Among the 5 species of BWOs at Ennis, Baetis tricaudatus was easily the most dominant, as with the last 4 years at this site. This species is the most widespread and abundant baetid on the continent. Although it can hatch most of the summer season, it seems to be most prominent in spring and fall.
Some might be surprised to see Stonflies representing a low 1-4% of the bug population at all sites (4% at Ennis). But this is fairly normal for the Upper Madison, despite the outsized notoriety they receive (rightfully so). They may not be as numerous in samples as other groups, but their contributions to available biomass is likely equally if not more important than all other species. They’re big bugs afterall. We see this in other animals too. Think of moose versus deer. Elephant versus impala. The big critters usually don’t outnumber the smaller critters. Stonefly contributions to the economy are also enormous. Throngs of people follow their hatch every year. Who doesn’t like casting dry flies 3” long?
Take this video for example, of a kicknet sample I took at Windy Point on the Upper Madison this spring. The first thing you notice are the three giant Salmonflies, and the sculpin. But surrounding those are dozens and dozens of smaller Mayfly, Caddisflies, and Midge species.
Ennis had more species of Stoneflies (6) than the other sites in 2023. Skwala were the most dominant, followed by Golden Stones (2 species), and one species of Salmonflies, Willowflies, and Sallflies. Interestingly, the number of Stonefly individuals sampled at Ennis in 2023 was the highest in the last 4 years. I certainly saw this borne out with the Skwala hatch this spring in town. A great hatch. Hopefully this portends a solid Salmonfly and Golden Stone hatch this summer.
Caddisflies were the dominant taxa group at 39% of the survey at Ennis in 2023. Among Caddisflies, the Speckled Peter (Helicopsyche) was the most dominant, followed by Short-horned Sedges (Glossosoma), Little Brown Sedges (Lepidostoma), and Mother’s Day Caddis (Brachycentrus) among about 8 other species of Caddisflies. The Speckled Peter without fail is the annual leader of Caddisflies at Ennis. In 2021 this single species made up 61% of all bugs total at Ennis! This species makes its larval home in a sand case shaped like a seashell and hatches for a few weeks from mid-June to early-July.
Kirby
As you can see from the first chart, among all 11 sites, Kirby (located near the West Fork Madison) stands out as quite unique in its BMI assemblage. Approximately 66%(!) of all macroinvertebrates sampled at Kirby are non-insects. Comparatively, Ennis contains a mere 5% of non-insects. And Hebgen only 7%. Of that 66% at Kirby, two species of freshwater snails (Physella sp. - also known as Bladder snails, and Fossaria sp. - both are native) make up 63% of relative abundance of all macroinvertebrates at Kirby. Another 2 species of snails represent the 3% bringing the total percent relative abundance of snails at Kirby to 66%. That’s a helluva lot of snails.
When you look closer at the individual species data from Kirby, compared to the other Upper Madison sites below Hebgen and at Ennis, you notice that the insanely high abundance of snails at Kirby is replacing mostly Midges, Caddisflies, and Mayflies in the assemblage (17% relative abundance in all in 2023 at this site). Not ideal for fly fishermen and women, and not ideal for stream health. As I highlighted my theory of what’s going on in my last Blog post HERE, there’s some significant water quality impairment going on in this reach. Something to keep an eye on for sure.
Among Mayflies at Kirby, again BWOs dominate, although with the heavy presence of snails here, their influence is dampened substantially. Even for a dominant taxa group like BWOs, only 69 individuals of BWOs were found in kicknet samples! Compare that to the mind blowing 1,108 snails picked up in those same kicknets! Western Green Drakes, Pink Ladies, and Mahogany Duns were also present at this site.
As for Stoneflies, only Salmonflies and two species of Golden Stones were sampled at Kirby. Only 15 individuals of Stoneflies were sampled, representing a low water mark for Stonefly populations at Kirby over the last 4 years. A down year coming for Stones in this reach?
For Caddisflies, a slight rearrangement of species importance from Ennis. At this site, Little Brown Sedges were most dominant (68 individuals), followed by Spotted Sedges (Hydropsyche - 26), Long-horned Sedges (Oecetis - 25), and Mother’s Day Caddis (11) among about 9 other species of Caddisflies.
Hebgen
The third and final site on the Upper Madison is located just downstream of Hebgen Dam. Here, Midges were more abundant in 2023 than at Ennis or Kirby representing over a quarter of the bug population at Hebgen.
Among Mayflies at Hebgen in 2023, BWOs made a giant rebound from the last 4 years. Four species of BWOs were represented by 450 individuals sampled, more than doubling last year’s numbers! Bug populations can be volatile from year to year, and this is a great example. At Hebgen, the dominant BWO species is Attenella margarita, and has been for the last four years. Attenella are often called Little Western BWOs for their size, which is often a 20. Tiny little guys. Gonna be a great year for dry fly fishing near Hebgen. Hebgen also showed higher numbers of Western Green Drakes than the other sites in 2023, with 3 species present. Most common was Drunella grandis (94 individuals). Drake numbers the last several years have been very steady at Hebgen. Although likely present river-wide, Hebgen was the only site to register PMDs in samples which was interesting.
Stoneflies at Hebgen have been pretty stable over the last 4 years, with Salmonflies easily the most dominant (24 individuals), followed by 2 species of Goldens, and a single species of Winter Blacks.
Curiously, Caddisflies at Hebgen were at a 4 year low. The Scaly-wing Sedge (Ceraclea) was the most dominant of Caddisflies (only 20 individuals), followed by Long-horned Sedges, Little Western Weedy Water Sedges, and Mother’s Day Caddis.
What really seems odd in the snail department is that even though the proximity of Hebgen to Kirby is relatively close, the stark difference in snail numbers is notable. Upstream of Kirby, snails represent only 7% of all bugs, but by the time you get to Kirby that jumps to 66% in 2023, then back to 5% at Ennis. Something in the water is clearly assisting snails in becoming so dominant near Kirby that isn’t present at the other sites. I think that something is Chlorophyll-a…but again, if you’re interested in those thoughts, visit the last Blog post HERE.
That sums up the 2023 macroinvertebrate report for the Upper Madison. Enjoy!