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State of the Upper Madison: Macroinvertebrates

Updated: Apr 9

I wanted to start an annual series each spring where I summarize all the research and reports available from the previous year on various important topics to anglers and lovers of the Upper Madison. Because any day now we're expecting a report out on the fisheries itself from 2023 surveys, I'll wait to do that one and instead start with macroinvertebrates. Bugs. Lots of cool info out there for us to check out. So here goes. Maybe get a bourbon ready for this one. It's a doozy. Other topics I'll cover include: water quality, recreation data, fisheries data, among others.

When one thinks of bugs on the Upper Madison, they likely immediately have visions of massive salmonflies in their heads. I do. Like little unicorns dancing in my dreams every June. Rightfully so. In fact, I dedicated a separate section on salmonflies at the end of this summary just to review those big beautiful bugs. The Upper Madison is one of the most famous streams in the world for giant salmonflies. They’re big and flashy and elicit big and flashy eats from trout. But they’re not the only bug in the river. In fact, in the grand scheme of all macroinvertebrates, salmonflies make up a very very small portion of the total abundance of bugs. Less than 3% of all bugs on the Upper Madison in fact. Pretty normal for stonefly rivers. If measuring in biomass, I'd bet stoneflies look much more important. Dozens of other taxa are present in far greater numbers, and the abundance and densities of each of those taxa can tell us a lot about the health of our river. However, you could argue that the salmonfly, despite being outnumbered in a big way, is perhaps one of the most important species of macroinvertebrates in the system simply because of the massive resource pulse it provides to fish, birds, and other wildlife. 

I put together this summary to help us all learn more about the river we love so much. In my opinion, once we stop learning about our fishery we stop growing and developing as anglers. So I hope this helps folks.

First off, everyone should know that the Madison River is arguably one of the most studied rivers in the country. There’s very little we don’t already monitor on this river and we shouldn’t take that for granted. Other communities on other rivers would love to have the amount of information we do. Much of that is due in part because we have a Federally licensed dam controlling flows. With that Federal license permit (FERC License 2188 Permit) come responsibilities mandated by law for the owners of said dam to monitor everything from fisheries to water quality to cultural resources. Pages and pages of reports are available to anyone who is curious. Another reason is our local government organization called the Madison Conservation District, very thoroughly monitors our tributaries and spearheads many of our restoration projects which often require research first. We just need to be able to consume the information and use it to make sound management decisions. It’s a major advantage for us, and one of the reasons why the Upper Madison continues to shine even when some of its neighboring streams aren’t. 

To that end, NorthWestern Energy (NWE) who owns the dam has been annually conducting aquatic benthic macroinvertebrate (BMI) surveys since 1995 as part of its environmental biomonitoring for hydroelectric facilities on the Madison and Missouri Rivers. Every August, they conduct their surveys and produce annual reports on BMI (aka bugs). We’re now in the 29th year of the dataset. Very robust. We don’t yet have the 2023 report, hence the last year of data presented is 2022.

It’s important to consider where and on what frequency macroinvertebrates are sampled. Bugs aren’t sampled everywhere all the time. So when talking about the condition of the Madison River’s macroinvertebrate community, we’re basing that knowledge off of a select few sites which are collected on an annual basis. Macroinvertebrates are collected annually from Yellowstone to Morony Dam on the Madison-Missouri system at 11 sites as part of the 2188 requirements. Seven of those sites are on the Madison. And three of those are on the Upper Madison between Hebgen and Ennis Lake, which I’ll focus on in this summary. Those are: below Hebgen, Kirby ranch near the West Fork confluence, and Ennis. See maps below for specific locations.

Below Hebgen Site (Ghost Village area, downstream of Campfire Lodge)

Kirby Site (just upstream of West Fork confluence)

Ennis Site (upstream of Ennis ramp)

This summary distills information from three NWE reports documenting macroinvertebrates on the Madison. There are occasionally other academic studies done on bits and pieces of the macroinvertebrate assemblage in the Madison, but NWE is the only group that consistently studies our bugs exhaustively on a professional level in the context of long term datasets. One of those reports is the Madison River Flushing Flow Program Report from 2022. In that study, two sites were used on the Upper Madison (Kirby and Ennis). The second report is more comprehensive and thorough and is conducted annually. It’s called the Madison and Missouri River Macroinvertebrate Biomonitoring: 2022 Data Summary Report. It’s dedicated to only looking at bugs every year and includes three total sites on the Upper Madison (Hebgen, Kirby, and Ennis) among 8 other sites from YNP to Morony. Although individual years like this one are not available online, you can request it from NWE. Or you can view the decadal summary, which includes data gathered from the annual reports. It’s called: Water Quality and Biological Monitoring Trend Analysis Missouri-Madison Water Quality Monitoring Program. Basically, most if not all of what we understand about macroinvertebrates on the Upper Madison comes from these three regularly produced reports which are focused on three specific study sites.

I’ll structure my summary by breaking down the data for each of the 3 sites on the Upper Madison below with supporting figures at the end. 

But first, because flows are so important to macroinvertebrates, we’ll start there. NWE summarizes annual flow trends out of Hebgen in these reports. Here is that chart going back to 1995:

Right away you notice a trend. However, if you take out 1995-2000 (not sure if there was a reason for elevated flows back then in the requirements?) the trend becomes pretty stable if not slightly increasing over the last 20 years. Not sure why they have 2021 and 2022 in red…no explanation there.

For us fishers, oftentimes the most important bugs to us are mayflies, stoneflies, and caddisflies. Researchers consider them important too, and lump all these bugs into one category for analyses called EPT. Generally speaking, higher amounts of EPT taxa indicate healthier streams. However, some taxa of bugs, like Diptera (midges), in higher amounts indicate poorer stream health. The letters in EPT stand for each group's taxonomic name: E= Ephemeroptera (mayflies), P= Plecoptera (stoneflies), T= Tricoptera (caddisflies). 

The flushing flow report which had a chapter on macroinvertebrates documented a total of 104 macroinvertebrate taxa in 2022 on the Madison. Dipterans (midges) were the most diverse insect order with 31 taxa including 24 chironomid genera. Mayflies were represented by 18 species, and caddisflies included 24 taxa. Five stonefly taxa and 4 riffle beetle genera were found. A single species of dragonfly, and one aquatic moth taxon were also collected. Non-insects were represented by 19 taxa including 4 snail genera, 3 worm taxa, 3 leech taxa, 2 genera of fingernail clams, 2 amphipod taxa, and single taxon of sowbug, crayfish, flatworm, and water mite. 


Ennis Macroinvertebrate Site:

Insects dominated the Madison River macroinvertebrate bug community at Ennis, accounting for more than 90% of all bugs. The remaining 10% were non-insects. Fifty six total taxa were observed here in 2022. The Ennis site appears to be mostly dominated by caddisflies but fluctuates with the cyclical resurgence of mayflies every 2 to 3 years. Caddisfly abundance was 65.6% in 2017, gradually declined to 33% relative abundance by 2019, increased again to 76.4% by 2021, only to be reduced again to 32% in 2022. Caddisfly abundances appear to be replaced with mayflies, rising from 7.7% in 2017 to 35.7% abundance in 2019 (Figure 3-9). This fluctuation was followed with another three-year cycle, with low abundances in 2020 and 2021 (<10%) and peaking again in 2022 at 38.3%. Stoneflies show a similar, but much smaller trend. Water beetles (Coleoptera), midges (Diptera), and non-insect contributions generally contribute around 25% of the total relative abundance in a given year, with 2020 and 2021 being two exceptions. As a layperson, I guess I sort of expected bug communities to be more static than that, so it’s interesting to see how much annual variation occurs. 

The modified Hilsenhoff Biotic Index (HBI) is primarily a measure of organic pollution and trophic status that is often used in conjunction with macroinvertebrate surveys to monitor stream health. Montana foothill and valley streams free of significant nutrient or organic pollution are characterized by values less than 4. Ennis has consistently shown “very good” conditions, averaging 3.51 over the entire 11 years and 3.37 over the last 5 years. This is one of those metrics you have to force yourself to remember lower values are better than higher values.

Excess sedimentation can negatively affect macroinvertebrates. And in healthier streams, higher numbers of sediment intolerant species are found. Ennis consistently registers higher numbers of intolerant taxa, generally in the range of 6 to 10 taxa which is great news. Slight increases in relative abundance of sediment intolerant taxa were observed in 2018, 2019, and 2020, during years with above average flows peaking at 33.3% in 2018, 26.3% in 2019, and 27.3% in 2020 before declining in 2021 and 2022.

On the flip side, finding higher numbers of macroinvertebrate taxa which are considered sediment tolerant can be an indicator of poorer stream health. Relative abundances of sediment tolerant taxa at Ennis have fluctuated less than at other sites, generally ranging from 8% to 12%, although dropping as low as 2.8% in 2021.That also seems like great news. 

The temperature tolerances and thresholds for macroinvertebrate taxa have been derived from the published literature and professional opinion. These thermal tolerances have been categorized as cold-water taxa (thermal tolerance less than or equal to ~18° C) and warm-water taxa (thermal tolerance greater than ~22° C). Ennis, as well as other sites, is seeing continued effects of warming water temperatures in the period from 2008-2021 with an upward trend in the number and relative abundance of warm-water taxa (with associated declines in cold-water taxa) in the bug community. An upward trend in cold-water taxa observed in 2018 to 2020 that was a response to colder, above average streamflows, did not continue into 2021 for Ennis. Percent of cold-water taxa declined substantially in Ennis since 2020 and now comprise <5% of the community. So it’s not until you get down into the nitty gritty that you see there are some concerning signs in the bug community at Ennis. But as we’ll see, cold-water taxa declines are not unique to Ennis alone.

Another index called the multimetric bioassessment index quantifies attributes of community composition, structural, and functional organization into a single number estimate of biological integrity. The multimetric index score ranges 0 to 30 and is reported as a percentage of the possible maximum score, ranging from 0 to 100%. High scores (> 75%) are characteristic of minimally impacted stream reaches. So, in contrast to the biotic index from before, higher bioassessment index scores are better than lower. From 2018 to 2022 the Ennis score averaged 93%. Ennis routinely had the highest bioassessment scores of the Madison River sampling sites, and it had been consistently over 90% since 2008. Pretty darn good. 

Currently, mayflies and caddisflies are nearly equally abundant at Ennis with 38% and 33% relative abundance, respectively. Mayflies were represented prominently by the minnow-tailed mayfly Baetis tricaudatus (BWOs). Baetis tricaudatus accounted for 26% of the total 38% relative abundance of mayflies at Ennis. Other mayfly species included little blue-winged olives, hendricksons, tricos, march browns, mahogany duns, and a lone pink lady. Interestingly, no drakes or PMDs were found in samples at Ennis in 2022. I know they’re present, as I’ve seen them in town, but surveys didn't pick them up. The Ennis site was also the only site to pick up march browns. Though the furthest site downstream was Ennis, from personal experience I notice even greater abundance of march browns the closer you get to Ennis lake. 

Caddisflies at Ennis were mostly comprised of the snail-case caddisfly Helicopysche borealis (22.8%) commonly known as the speckled peter. At Ennis, they represent 66% of all caddisflies (375 in the net samples!). Interestingly, as dominant as this taxa is at Ennis, it was virtually absent from Kirby and totally absent below Hebgen. Quite the change upstream. Next closest in abundance was another species of snail-cased caddisfly. The ones in the small long skinny cases stuck to all the rocks. The third most common caddis at Ennis was the popular Mother’s Day Caddis (Brachycentrus occidentalis) coming in at 13 individuals in samples. 

Stonefly contributions to relative abundances are currently small at all Madison River sites, but somewhat more prevalent at Ennis (2.5%). This may surprise a lot of folks who’ve experienced heavy Stonefly hatches on the Upper Madison. That just goes to show you just how many bugs there really are, if all those swarming stones make up less than 3%! Five stonefly taxa were collected at Ennis during sampling. In the kick net samples they found skwalas, goldens, salmfonflies, and green stoneflies (often called sallflies). I’ve seen the green stonefly adults in town before in summer. Super bright green stonefly, on the smaller side. Almost a neon green.

Most recent data suggest that, overall, mean bug taxa richness per sample at Ennis is right at the long term average. Moreover, in 2022, Ennis was pretty much right at average as well for EPT taxa richness per sample compared to the last decade. In 2022, mean percent EPT in the Madison River was ~55%; scores ranged from 28% at Kirby to a high of 75% at Ennis. It’s worth noting that Ennis reported an all time high of 87% EPT in 2021. That’s remarkable.


Kirby Macroinvertebrate Site:

The Kirby site could not be much different than the Ennis site. The Kirby site is dominated by non-insects…mainly snails, in most years, but these were lower in 2017 and 2018. The lower relative abundance of molluscs in these years was accompanied by increases in caddisflies at the site. Since 2019, non-insects have increased to greater than 50% of the total relative abundance, thus reducing EPT taxa compositions into the 30% to 40% range. There has been small fluctuations in the contribution of mayflies over the past six years, rising in 2018, falling in 2019 and 2020, and slowly rising again through 2022.

For the last 5 years of data, mean taxa richness at Kirby was 31.6, lowest among all sites. Comparatively, mayflies, stoneflies and caddisflies were fairly abundant and diverse at most other Madison River sites. Over the last decade, mean EPT richness has generally been lowest at Kirby, with the exception of 2015, 2016, and most recently 2022. The combined relative percentage of mayflies, stoneflies and caddisflies is significantly lower than 50% at the Kirby site with a site average in the most recent 5-year period (2018-2022) of 28.2%. Big contrast to the 73% EPT at Ennis…

Mayflies were represented prominently by various species of BWOs at Kirby in 2022, which accounted for 9.5% of the total 12% relative abundance of mayflies. Interestingly, little western green drakes were picked up in the samples at Kirby. This compared to none found at the Ennis site. Aside from that, Mayfly composition at Kirby similarly included smaller abundances of mahogany duns, tricos, and another single pink lady. No PMDs or march browns were picked up at Kirby.

Stonefly contributions to relative abundances were small at Kirby (1.9%). Three stonefly taxa were collected at Kirby, compared to five at Ennis, comprising two species of goldens and the salmonfly. Interestingly, no skwalas were present.

For caddisflies at Kirby, where the total relative abundance was only 13%, the long-horned caddisfly Oecetis (4.9%) and the little brown sedge caddisfly Lepidostoma (2.9%) were the most prevalent taxa of the 18 species found (compared to 17 species at Ennis). Other notable mentions here was the great grey sedge (Arctopsyche grandis), and little sister sedge (Cheumatopsyche) not found at Ennis. 

In 2022 non-insect taxa were a combination of worms, crustaceans, and molluscs. At Kirby, the samples were dominated by the physid “bladder snail” Physella sp. (39.1%) and the lymnaeid Fossaria sp. (11.7%), and these were found in all sample habitat locations except the fast and deep habitat. They apparently prefer slow water. At the other three sites, these snail taxa accumulated no more than 0.5% of relative abundance. The next highest relative abundance for non-insect taxa was 11.9% at Norris. That’s quite the difference.

Looking at relative abundances of sediment tolerant taxa, Kirby shows a substantially higher percentage, likely due to the increased numbers of snails. The two snail taxa dominant at Kirby are classified as sediment tolerant, and also have higher tolerance values to organic pollution. Therefore, many of the metrics indicating environmental stress or disturbance at Kirby likely stems from the dominance of snails, and the environmental conditions that are favoring their presence.

On that note, over the previous 10 years of monitoring, biotic index values at Kirby indicate some fairly significant pollution between 2012 and 2014 (average of 5.49), and significant pollution between 2019 and 2022 (average of 6.00). The site average in the most recent 5-year period (2018-2022) was 5.75 at Kirby. Remember, anything above 4 is not good. Six is very not good.

Over the last three years, sediment tolerant taxa numbers at Kirby are more comparable to Lower Madison site numbers than other Upper Madison site numbers. Looking at relative abundances of sediment-tolerant taxa at four sites on the Madison River over the 2012 to 2022 period, Kirby shows a trend of much higher percentages of sediment tolerance taxa starting in 2015, likely due to the increased numbers of snails in recent years. Kirby continued to exhibit the effects of warming water temperatures in the period from 2008-2021 with an upward trend in the number and relative abundance of warm-water taxa (with associated declines in cold-water taxa) in the bug community.

Mean biotic index scores in the Madison River were 4.6 (moderate nutrient impairment). In 2022 scores at Kirby hit 6.2, indicating significant impairment. Overall, 2022 bioassessment scores were at or above average for 10 of the 11 sites in the Madison and Missouri Rivers, only 1 Madison River site was below the long-term average, and that was Kirby (63%). This, after reporting a score of 80% in 2018! Percent cold-water taxa at Kirby has declined significantly since 2020 and make up less than 5% of the bug community. Interestingly, biointegrity was high in the 1990s at the Kirby site but has fluctuated in the impaired range since 2002.

So, why all the snails at Kirby? Seems like a big clue something is off. Interestingly, snails really provide no nutrient value to trout. They can pass through the fish without dying. Combine that with their asexual reproduction, and apparently those little buggers can reproduce freely without much fear of predation. The snails are native, so that part’s not a concern, but it is a red flag for me from a water quality standpoint. In much the same way that a higher percentage of bugs below Hebgen are Midges (as you’ll see in the next section), which indicates some level of impairment. 

Special mention: New Zealand Mud Snails (NZMS - invasive species) were found in relatively low densities at Kirby, with a few individuals (<10 per sq. meter) collected.


Below Hebgen Dam Macroinvertebrate Site:

First a little background on Hebgen Dam. Madison River flows are controlled, to a large extent, by operation of Hebgen Dam. Hebgen has no power generating abilities and primarily serves as a storage reservoir for downstream energy producing dams. Heben holds about 380,000 acre-feet of water. In 1959, an earthquake caused a major landslide across the Madison River about five miles downstream of Hebgen Dam. The landslide blocked a section of the Madison River. This blockage created what we now call Quake Lake. At the time the slide occurred, Quake was approximately 174 feet deep. The lake is shallower now as a result of erosion of the outlet. 

To limit erosion of the outlet from Quake Lake, NWE is required by the FERC License to limit the maximum releases from Hebgen Dam to 3,500 cfs (at the Kirby guage) and a minimum of 150 cfs out of the dam itself. No more than 10% per day is allowed in flow change. Engineers have determined that the 3,500 cfs limitation effectively limits erosion at the outlet from Quake Lake. Why are they worried so much about erosion of the Quake outlet? Excessive high flows would weaken the outlet structure of Quake Lake. The volume of material that blocked the Madison River below Hebgen Dam and formed Quake Lake has been estimated to be something north of 37 cubic yards. This volume of material would become an excessive source of downstream sediment for one thing, leading to excess turbidity levels, potentially disrupting the geomorphic integrity of the Madison River, and increasing the risk of downstream flooding. For these reasons, maintaining the structural integrity of the outlet of Quake Lake is important. We’re kinda stuck with it. Which, in my opinion, is too bad. I’d love to have several miles of canyon river back. But, Quake is ours now. Better take care of it. 

Midges represented 30% of the bug community at Hebgen compared to 10% at Ennis and 5% at Kirby. Non-insects at Hebgen represented roughly 19% of the community. So right away, half of the bug community below Hebgen represent taxa that are indicative of some level of impairment. Weird.

Mayflies below Hebgen represented 22% of the community. Among the mayflies, again the most dominant taxa were various species of BWOs with the little western BWO representing the biggest portion of the BWO species. Among all three sites, this one had the most species of drakes (3), with the western green drakes representing the largest relative abundance of the three (5%). All that was left in the Mayfly community here were lots of Tricos and a small amount of PMDs. Interestingly, both Ennis and Kirby found mahogany duns, but below Hebgen none were present. Same with little BWOs. Abundance of little BWOs was largest at Ennis, smaller at Kirby, and non-existent below Hebgen.

Caddisflies represented 17% of the bug community below Hebgen. While no one or two species ran away with dominant numbers, the long horned caddis and little brown short horned sedge were most common. Ochrotrichia caddisflies (a purse maker caddis) and Amiocentrus caddisflies (case maker) were fairly common below Hebgen and totally absent in Ennis. And again, most interesting among caddisfly trends was the extreme dominance of speckled peter’s (Helicopsyche borealis) at Ennis, and the total absence of them below Hebgen.

As for Stoneflies below Hebgen, they represented 1.7% of the community. Species observed included goldens, skwalas, salmonflies, and interestingly the least salmonfly (Pteronarcella badia) which wasn’t observed at either Ennis or Kirby. 

Riffle beetles were close to 12% below Hebgen, compared to 9% at Kirby and 8% at Ennis. 

Despite being quite literally at the base of a dam, the site below Hebgen, continues to exhibit the effects of warming water temperatures in the period from 2008 -2021 with an upward trend in the number and relative abundance of warm-water taxa (with associated declines in cold-water taxa) in the BMI community.

Indicators of declining stream health at this site are numerous. Dipterans, especially midges (Chironomidae), were the most dominant benthic macroinvertebrates in samples collected from below Hebgen (29% of the community). New Zealand Mud Snails were relatively common at this site, with around 15 per sq. meter. And over a decade ago, this site contained upwards of 23 species of cold-water taxa, contributing as much as 30% to the BMI community. In 2022, Hebgen site contained 13. Hebgen has bounced above and below levels of biotic impairment since 2018, but currently stands as technically non-impaired. 

Interestingly, Hebgen reported back to back rebound years in percent cold-water taxa in 2021 and 2022 since a recent low in 2019. Nevertheless, in 2021 and 2022, numbers and percent of warm-water taxa at Hebgen rose after being reduced or held steady during 2018. Substantial shifts in the relative abundances of cold-water versus warm-water taxa were evident during the past decade. However, normally the dam releases cold water during the summer, yet due to construction at the dam, warmer surface water was released from 2009 through 2015 and again in 2017. This likely didn’t help matters, but even still, cold water taxa percentage has not since equalled the 2008 (pre-dam repairs) except for the increase observed in 2018.

I hope that summary is helpful. Below are a series of graphs collected from the 3 NWE reports on macroinvertebrates. 




Special Section On Salmonflies

For all those nerds that love everything to do with salmonflies, this is for you. Over the course of the season I'll be writing on different science topics related to the Upper Madison. Salmonflies are my favorite topic. So I'll start with a summary of some of the latest research on them specifically on the Upper Madison.

I could easily argue that no other bug means more to another town than the salmonfly does to Ennis, Montana. Both ecologically and economically, this single stonefly species is massively important to the local health of the watershed as a food source and the health of the economy bringing anglers from all over the world for the most famous hatch on the most famous trout stream. So, naturally, it’s rather important we understand this bug. How is the population doing, what are the trends, and how can we ensure we don’t lose it?

The salmonfly (Pteronarcys californica) is a large-bodied aquatic insect that lives in rivers for 3 to 5 years before emerging onto land as an adult. Some streams’ salmonflies hatch at 3 years some later. No one really knows how long our nymphs take to age, but likely on the latter end of that scale for the Madison. During the hatch, salmonflies are a critical food source for trout, birds, and many other predators. Mega protein available in those bugs. However, salmonflies are very sensitive, and considered an indicator species. Indicating good stream health and water quality with relatively denser populations and worse stream health with relatively lower densities. Threats to salmonflies and other stoneflies include warming water temperature, increasing fine sediment, and changes to suitable flows and food supply. 

Recently, much research has been conducted on salmonflies in the Upper Madison. We’re seeing baseline data on our population of bugs and much of it is validating what we see as anglers on the water. But also what we’re learning is concerning. 

One publication by researchers at Montana State University (MSU) coupled contemporary and historical datasets documenting physical and ecological variables over four decades to quantify changes in the abundance, emergence timing, and body size of salmonflies in the Madison. They found water temperature was the main driver of salmonfly abundance, emergence timing, and body size. Salmonfly densities were negatively correlated with summer water temperature, whereas substrate type played a negligible role. Emergence occurred 20 days earlier, and male and female shuck lengths were 13.8% and 11.3% shorter, respectively, at the warmest site relative to the coolest site (4°C difference). These patterns were supported by historical data as well. For example, a 1.2°C increase in mean annual water temperature in the Madison River between 1977 and 2017 coincided with evidence for upstream range contraction. Salmonflies, formerly abundant on the Lower Madison, are no longer present in enough abundance to find regularly. Between 1973 and 2017, emergence timing varied among years, occurring up to 41 days earlier in years when spring water temperatures were relatively warm. 

Using this data they modeled into the future assuming climatic changes continue. They predict that salmonflies could disappear from an additional 28 km of currently occupied river, representing a 22.6% reduction of river. Most at risk of the next range contraction is the water from Ennis to Ennis Lake. Indeed, most would say that the hatch is not very reliable down there already…Here’s some figures from this research.

Bug density in relation to water temperature:

Nymph/Shuck length in relation to water temperature:

Emergence date in relation to water temperature:

Range contraction modeling:

Another interesting publication by the same researchers was on the topic of how we all are obsessed with “where is the hatch today?” We all know on the Upper Madison that the hatch slowly “moves” upstream over the course of a few weeks in June/July. In reality, it’s just that there’s so much thermal variation in water temperatures from low to high on the river that conditions for hatching are achieved earlier on the lower reaches than they are achieved on the upper reaches and as those conditions warm more and more upstream, the bugs in those respective reaches hatch. It’s not as if the bugs themselves are moving. Just that bugs in Ennis hatch earlier in the season than bugs at Lyons because the water temps hit their cue sooner down there. 

Well, that’s not necessarily the case on other rivers, like the Gallatin. Look at the graph below. The Gallatin salmonfly hatch is more synchronous. Meaning they all seem to hatch at the same time from the upper reaches of the river to the lower reaches, owing to the consistent thermal water temp regime up and down on the Gallatin. When the bugs come off up top, they’re also coming off near Gallatin Gateway. Water temp drives emergence and water temp doesn’t vary all that much from top to bottom like it does on the Upper Madison. 

Madison sites are 1 - below Hebgen, 2 - Three Dollar Bridge area, 3 - Palisades area, 4 - Varney, 5 - Ennis. The hatch on the Madison is totally asynchronous. We all knew this, but very interesting to see it displayed in the data. All thermal regime related from water temp. As a result, the total duration of the hatch on the Upper Madison is pretty long. About 27 days on average. This compared to around 13 days on the Gallatin.

What this publication also found was that at any given site on the Madison, bugs only hatched for 4-8 days compared to 6-11 days on the Gallatin. So while salmonflies can be found somewhere on the Madison for a longer time frame than the Gallatin (27 days vs. 13), the bugs can be found at individual sites on the Gallatin for more days. Cool stuff. 

These same researchers at MSU are planning on being here for the hatch this year to document emergence timing and densities as well as habitat preferences for salmonflies. They have some preliminary data to indicate that salmonflies prefer to emerge on vegetation (or other structures) that have some height to them, rather than sand or cobble. This summer, researchers will work on an experiment where they put out vegetation “mimics'' of different heights (e.g., 10 cm, 20 cm, 40 cm) at two sites on the Madison River (Hebgen Dam and Varney Bridge) and count shucks on them to test the hypothesis that salmonflies prefer to crawl upwards before casting aside their shucks and flying away, and that their emergence success (dead vs. alive inside the shuck) is higher when the vegetation is taller. If the data support this hypothesis, then the findings could point toward riparian plantings as a management tool to increase salmonfly emergence success, which then could potentially indicate that more eggs are produced for the next generation.

The salmonfly research lab can be found here and you can view their research publications:


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