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Salmonfly Science

Updated: May 28


Arguably, no other bug means more to another community than the giant salmonfly does to Ennis, Montana. Pteronarcys californica. The "Big Bug." Sammy. Both ecologically and economically, this single stonefly species is massively important to the local health of the watershed as a high density resource pulse 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 with a changing climate leading to changing thermal trends and regimes in the water we fish, how can we ensure we don’t lose it? After millennia of seasons, we enter another of the big bug. As we do, I thought it'd be good to review the "State of the Salmonfly," which I hereby propose to be the Montana State bug.


The giant salmonfly (Pteronarcys californica) is a large-bodied aquatic insect that lives in the upper Madison (notice I didn't say lower...more on that in a bit) for 4 years in total. Sometimes you see written, 3 to 5 years. Sometimes you see 3 years. The truth is, researchers tell us that it's 4 years on the Upper Madison. The first year is completely as an egg. Eggs are laid by females during each hatch in late June and early July. Those eggs go down to the bottom of the river and sit. What cues them to start developing? Interestingly enough, super cold water. Those eggs need temps to get pretty close to freezing to cue development. And by the end of their first year, they hatch from the egg and start growing as baby sammys. You can find them if you look carefully enough. Take a seine next time you go to the river. Do a little San Juan shuffle and collect bugs in your net as their dislodged. All age classes of salmonflies are easily distinguishable from other stoneflies. They're jet black (unless freshly molted, in which case they look copper toned). Sift through and you'll see representatives from each age class ranging from miniature salmonflies, maybe 1/2" long, to the mature big ones around 2 inches. Thoraxes massive. Antennae arching long and forward. Now these ones are gonna be popping in a month, migrating to the banks very soon in preparation for departure.


During the hatch, salmonflies are a critical food source for trout, birds, and many other predators. What scientists would call a "resource pulse." Mega protein available in those bugs. However, salmonflies are very sensitive, and considered an indicator species. Indicating relatively good stream health and water quality with denser populations and relatively worse stream health with lower densities. Threats to salmonflies and other stoneflies include warming water temperature, increasing fine sediment, and changes to suitable flows and food supply. In my opinion, rivers which once bursted with salmonflies but are now absent have lost their soul. And yet, this threat faces many of our cherished streams in the west, the Madison included.


Recently, much research has been conducted on salmonflies in the Upper Madison. And some is still ongoing. 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. 


Water Temperature Drives the Bus


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. Makes sense. Salmonfly densities were negatively correlated with summer water temperature. 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 (7.2°F difference). These patterns were supported by historical data as well. Additionally, a 1.5°F 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, if at all. Damn. That leaves Ennis on the front lines...


Between 1973 and 2017, emergence timing varied among years, occurring up to 41 days earlier in years when spring water temperatures were warm. Interestingly, in this research, despite seeing a 1.5°F increase in mean annual water temperature in the Madison River between 1977 and 2017 emergence timing showed no evidence of advancement between 1973 and 2017 at either of their study sites on the Upper Madison. The hatch doesn't show any trend in occurring earlier year over year for the last 50 years outside of normal variation. So water temperature seems to be more important as a within year factor on emergence timing. But if water temps get too warm, salmonflies can't survive.


Using this data they modeled into the future assuming climatic changes continue. The model suggested that salmonflies could disappear from an additional 17 miles of currently occupied river, representing a 22.6% reduction of river presence. The lowermost stretch of river on the Upper Madison with salmonflies currently is right here in Ennis down to Ennis Lake. And that makes it the most at risk of the next range contraction. Indeed, most would say that the hatch is not very reliable in the few miles of river between Ennis and Ennis Lake already. Water temperature may be playing a huge role in this section. Lower densities at higher mean annual temps. This tracks with what I see flipping rocks at Valley Garden compared to Windy Point. Fewer salmonflies down there. So as water temps creep up, salmonflies apparaently get less dense in population and smaller. And interestingly, mean water temps within the preceeding month of a hatch seem to be highly correlated to emergence timing. On the Upper Madison that means the month of May. Cooler mean May temps and the bugs come off earlier compared to higher average May temps. We're talking small changes too. A degree centigrade or two makes a big difference. Here's some interesting figures.


Bug density in relation to water temperature:

Nymph/Shuck length in relation to water temperature:


So as we're now about a month away from when we traditionally see salmonflies somehwere on the Upper Madison, let's do some bar room back of the knapkin analysis using this research to make our best guess as to when we'll see first emergence this year.


Take Varney Bridge on the Upper Madison for example. The epicenter of the Madison. We've got a USGS gauge located at the bridge keeping track of water temps, stage, and flow 24/7 365.


I downloaded the water temp data, and the mean May temperature at Varney is 8.43 degrees Celcius. So let's take the researchers data and see where we fall for a predicted first emergence at Varney this year. Like a salmonfly forecasting tool.



That put's Varney's predicted first emergence date this year right around 177 in Julian days. That's June 25th. Probably a few days earlier for Ennis. Sounds about right. It'll be cool to see how that matches up.


Range Contraction Modeling


Imagine a time when we no longer see salmonflies on Main Street in Ennis. That time may be sooner than we think. By 2100, these researchers project near total loss of the big bug around Ennis. And although this is predictive modeling, and not exact, the evidence is clear that we're facing challenging times for water temperatures which will have upstream effects on our bug populations and one of the first to be majorly affected is our flagship species.


Progression of Emergence


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. Or as dramatic. Take the Gallatin River, one sub basin to the east. 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 can also be coming off near Gallatin Gateway. Water temperature drives emergence and doesn’t vary all that much from top to bottom like it does on the Upper Madison. 


Emergence was measured with daily exuvia counts. Shucks. Designated stretches of bank were searched for shucks and cleared everyday. This provides a great measure of relative abundance since we know salmonflies almost exclusively crawl to streamside vegetation to hatch, and are big enough to see easily. For reference in these figures, 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 it's very interesting to see it displayed in the data. This data is from 2016 and 2017. It's curious to me the double peak of the hatch at each site on the Upper Madison, but not on the Gallatin. 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:



Two other researchers with the consulting group Save The Hatch are also working on the Upper Madison. I recently had a chance to tag along and learn more about their work. Back in the late 70s, Montana FWP conducted research documenting abundance of salmonflies throughout the Madison River. These researchers are duplicating that research to see where our bugs stand over 50 years later at the same sites. Should be real interesting seeing their findings.


Good luck out there this salmonfly season!


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