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Salmonflies: A Madison Keystone

Updated: May 23

Pteronarcys californica

The greatest event of the natural world on the Madison River annually is the emergence of the king of all aquatic insects: the salmonfly. But do we appreciate how important they really are, both to the ecological and economic communities? Which begs another question, are we paying attention?


Salmonflies (often exceeding 6 cm in length) are incredibly popular among flyfishers. Spending 2-5 years as larvae living among gravels in the riverbed, they are cued by water temperature in early summer to emerge in spectacular hatches that draw anglers from around the world. Hatches generally last for around a week at any given location, rising and lowering in density.


The most thorough research on salmonflies on the Madison was a Master's thesis by Heidi Anderson at Montana State University in 2018 titled (Environmental Drivers of Salmonfly Ecology in Southwest Montana). In addition to her own research, she was able to utilize citizen science data from a fellow who somehow managed to survey Varney for first emergence date of salmonflies every year since 1977. Pretty incredible. Another study on salmonflies in Colorado last year (2020) is also fascinating ("Quantifying the habitat preferences of the stonefly Pteronarcys californica in Colorado").


Salmonfly emergence on the Madison, as we anglers already know and Heidi documented in detail, are locally synchronized but throughout the entire reach of the Madison River, the hatch is more broadly, asynchronized. For example, the maximum difference in median emergence dates among surveyed sites in 2016 and 2017 averaged 22.5 days across the two years (21 days in 2016 and 24 days in 2017). River wide emergence is considered asynchronous when emergence timing does not overlap among sites on the same river. Hence, when the hatch is on, you may see fishing reports on the upper Madison referencing that the hatch has "moved" upstream to a certain area. And, that's an important distinction from many other rivers where salmonflies are present. For example, on the nearby Gallatin River, just over the Madison Range, Heidi documented that salmonflies are far more synchronized in their hatch throughout the river. Meaning, emergence dates at the bottom of the river are closer to the emergence dates at the upper end of the river, than observed on the upper Madison. This is further exemplified in that emergence of salmonflies on the upper Madison have been documented to last on average 2.25X longer than the Gallatin. This is one of the biggest reasons the upper Madison is such an iconic location for anglers worldwide chasing salmonflies. They're around for quite a while.


This asynchronicity is hugely important to the larger ecosystem as this variable resource pulse can extend the duration of resource availability, and thereby stabilize and boost consumers’ (like birds, snakes, spiders, etc.) seasonal energy intake. Fat cheeseburger bugs served up for a longer period of time.


In addition to being large, iconic, indicative of clean water, and charismatic, drawing people from around the world to the region to experience, the salmonfly indirectly contributes to local economies through eager anglers traveling to the region en masse for the opportunity to cast a dry fly as long as their fingers. The salmonfly also plays a critical ecological role including processing detritus, dominating aquatic subsidies to terrestrial ecosystems, and providing an important carbon resource pulse for over twenty species of aquatic and terrestrial consumers.



The authors of the study in Colorado on habitat preferences for salmonflies found that salmonflies (larvae and adults) made up between 65-75% of mean annual stomach contents for trout on the Colorado River! I would bet that number could be even higher on the upper Madison. And look no further for a reason to always fish a girdle bug when nymphing on the upper Madison.


Put simply, there's a lot of weight on their tiny shoulders. Which is why, I at least, view them as a "keystone species." A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance. And the upper Madison might be the world's most well known river for this charismatic keystone aquatic insect.


But here comes the rub.


Given the exponentially increasing temperatures predicted by climate models, the historically high rate of water temperature increase on the Madison River, evidence of a salmonfly population primarily constrained by above-optimal water temperatures, and the fact that salmonfly larvae take 2-5 years to emerge, salmonfly populations on the Madison River are very susceptible to habitat alterations and are likely to experience further upstream range contraction and reduced body size into the future. It has been predicted that the historic range of salmonflies on the Madison River could be halved by the end of the century.


In fact, we've already seen major contraction in salmonfly presence, abundance, and even size of the bugs themselves on the Madison River. All indicating responses to changing climatic conditions. Since 1977, salmonflies in the Madison River have experienced a 34.0% upstream range contraction and 11.8% reduction in adult dry mass at the most downstream extent of their current distribution.


Save Our Sammies?

This includes a loss of salmonflies from 40 river miles below Ennis Reservoir since 1977. Back then salmonflies were found throughout the entire length of the Madison River. In 2017, salmonfly larvae were below detection at all sites below Ennis Reservoir despite more rigorous sampling efforts, with twice as many replications per site using a sampler with 2.5X the sampling area in 2017 compared to 1977. Put simply, they looked way harder for salmonflies in 2017 than 1977 and found nothing for their efforts. Further, salmonfly larvae were relatively abundant directly below Ennis Reservoir in 1977 (35.3 larvae/square meter at one site), but have not been observed emerging at this site in 13 of the last 20 years!


These findings are corroborated by biomonitoring data collected on the Madison River by Northwestern Energy; salmonflies have not been detected since 2012 at their 3 annual monitoring sites below Ennis Reservoir. Thus, while salmonflies may persist in portions of the Madison River below Ennis Reservoir at very low densities, they no longer fill the functional, ecological roles (e.g., organic matter processing or prey for fish and terrestrial predators) that they likely did when they comprised a larger part of the benthic community.


In addition to range contraction, adult male and female salmonfly dry mass (size) decreased 14.5% and 10.6%, respectively, between 1977 and 2017 at Varney Bridge, the only site for which we had both historical and current body size records. So in review, both salmonfly range and physical size is shrinking. I doubt anyone thinks it's a coincidence that these changes are paralleled by an observed 2.16°F increase in mean annual water temperature.


Interestingly, for the 2 sites where emergence timing data is available, salmonfly emergence timing showed no evidence of advancement between 1973 and 2017 likely because spring water temperatures that cue emergence remained consistent over time. But, curiously, emergence dates varied widely across years, ranging over 39 days at one site and over 41 days at another. Densities of salmonfly larvae at 5 different sampling sites on the upper Madison in 2017 showed high densities, with the highest density of larvae noted at the Varney site. Center of the salmonfly universe. So at the moment, salmonflies are holding their own on the upper Madison. For now.


To help provide context for how these changes in salmonfly populations may continue throughout the 21st century, Heidi used predictive spatial models of temperature change and extrapolated historic water temperature trends. Way back about 15 years ago, I did a similar Master's thesis, except with wolves and livestock depredations in Michigan. I developed a predictive spatial model to help biologists forecast where depredations on livestock may occur in the future based on wolf population modeling and landscape changes. Modeling isn't exact, but it's very effective at helping understand where things are headed.


For example, much of the relevant literature regarding distributional shifts for freshwater macroinvertebrates is predictive, using current conditions and climatic models to forecast future conditions. Similarly, most evidence of reduced aquatic insect body size and advanced emergence timing at warmer temperatures comes from experimental studies or field studies utilizing a current spatial gradient of water temperatures. Examples of biological change in natural settings with long-term records of shifting environmental conditions could help better understand the realized impacts of climate change and human disturbance on freshwater critters like salmonflies.


Assuming observed warming trends continue, mean August water temperatures will increase approximately 3.6°F by 2100. A warming of this scale will result in further loss of 18 miles of suitable salmonfly habitat due to mean maximum water temperatures exceeding 66.2°F, a critical threshold above which salmonflies are unlikely to persist. Remember, though they're conspicuous for a few weeks above water, they still spend the vast majority of their lives under water. This would add up to a total of 50.1% habitat loss on the Madison River since 1977. Salmonfly populations directly above Ennis Reservoir are the most at-risk.

If Heidi's modeling is even remotely accurate, the next generation of anlgers may not see a salmonfly crawling on a picnic table at the park in Ennis ever again just like we have lost them on the lower Madison. That's kind of depressing.


It really puts into perspective the importance of monitoring and water regime flow management. Here on the upper Madison our river habitat is at the mercy of the dams above and who's controlling the knobs to a certain degree. Hebgen outflow and the management by Northwestern Energy are critically important to the two major limiting factors for salmonfly emergence on the upper Madison: water temperature and percent fine sediment. As Heidi showed, salmonflies start to get smaller and we actually begin seeing salmonflies dissapear when mean maximum August temps rise above 66.2°F or when sedimentation fills the gaps between cobbles along the bottom of the river.


Some researchers have described salmonflies as "sediment sensitive" having a very specific range of ideal percent fine sediment in cobble substrate (around 2.6% for fines ≤0.06mm). This is good information to keep in mind with the upcoming sediment mobility study on the upper Madison.


On the upper Colorado River near Granby, salmonfly distribution has been significantly reduced due to impacts of a mainstem reservoir related mostly to sedimentation. And once they're gone, we can't bring them back. Officials in Colorado and Utah have tried. On the Logan, Gunnison, and Arkansas Rivers. In all three cases, attempts to re-establish salmonflies from areas once occupied all failed.


What would the upper Madison River be without its most celebrated star? What would that mean for local economies?


Northwestern Energy will be conducing a sediment mobility study on the upper Madison this year. While it's a bit like the fox guarding the henhouse as a friend said recently, it's important that we pay attention and ask questions. These studies are required by law, so it's not as if Northwestern Energies is doing this out of the goodness of their hearts. And these studies aren't exactly widely known or publicized either, making it difficult to be aware or get involved, but more and more of us are doing what's necessary to pay attention and ask questions.


If conservation of salmonfly habitat is a goal of resource managers, then temperature and flow management should focus on optimal temperatures and reducing the input of fine sediment in rivers. Being proactive is the best approach. We still have salmonflies on the upper Madison. Let's keep it that way.


So many conservation issues highlight the need for monitoring. Monitoring keeps the science honest and accountable and allows us to adapt. Someone put it to me this way recently. It's like inventory management to a business. Without inventory monitoring, it's pretty difficult to manage a business. And we all want to know what's going to happen with our resources, that's called forecasting. And you simply can't do forecasting without a standardized monitoring program over time. Ideally, this monitoring is open, transparent, and involves everyone helping to gather data. Hopefully that's something we can improve in the future. I'm working on this with a few partners and will have some exciting news to share soon.