Researchers probe threats to salmonfly, a foundational bug | Local News

Each year in late May and early June, trout feast and anglers flock to waterways for the giant salmonfly hatch, when the ecologically vital bugs emerge from streams as nymphs and hatch out of the water into winged adults before reproducing and dying, leaving behind only the “shucks” they emerged from clinging to brush on the shoreline.

Growing up to 3 inches long, adult giant salmonflies are the largest of the stonefly family. They are so substantial that osprey, which generally consume fish, have been observed snatching up airborne salmonflies. As giant salmonfly populations decline in some streams across the West, Birrell and James Frakes, both researchers at University of Montana, are trying to understand what factors affect a bug that is essential to trout and the economies of fishing towns.

In February, the pair launched The Salmonfly Project, a nonprofit that aims to better understand the causes of the insect’s decline and push for increased environmental monitoring and remediation where the species is most threatened. 

“They’re, I would say, the most economically important aquatic insect,” said Frakes, 27. “Because the influx of people from all around the world who come to Montana to fish, they come for the salmonfly hatch because it’s like the most world-renowned fly-fishing opportunity that you could imagine.

“It’s inevitably good for these small-town economies because it brings all that money into the fishing community.” 

John Staats, manager of the Rock Creek Mercantile, said on Saturday that “it’s highly anticipated and people will book years in advance to try to get a float trip during salmonfly season because the fishing is just that good. It bumps the number of fisherman and, in turn, the amount of cash flow coming in because of it.”

Studies have shown the insects are equally foundational for trout themselves. A recent study found that salmonflies accounted for up to 70% of trout stomach contents during a hatch in Colorado. Giant salmonflies are “massively important,” Birrell said, noting Rock Creek’s still healthy salmonfly population. “The bugs sustain the whole ecosystem.” 

But in other places, like Utah’s Provo River, salmonflies have disappeared altogether. Birrell, 29, completed his undergraduate degree in conservation biology at Brigham Young University in Provo and conducted one of the first surveys of salmonflies in the river since the mid-20th century. Building off of baseline data from the 1950s and ’60s, he “went back and found, ‘Woah, they’re not here anymore.'”

“If you look at the collective research over the past decades or so, you can see that the trajectory is downward for salmonflies across the West,” Frakes said. “More research needs to be done on specific watersheds, like for instance Rock Creek. There’s no real evidence that the population here is declining. But there’s definitely clear evidence of that in other drainages. For instance, stretches of the Madison River, there are documented declines of salmonflies there.” 

Although populations of the bug remain healthy in many places – they were crawling all over Frakes and Birrell along Rock Creek – Birrell said a greater decline could be looming: “We have published declines, or at least reported declines, in at least 10 really important trout [streams] and fisheries in the Western Unites States, all across Montana, Utah and Colorado, and there are some suspect streams elsewhere that haven’t been completely confirmed.” 

Paired up in Missoula, Birrell, who is working on a doctorate in ecology and evolution, and Frakes, who graduated from UM with a bachelor’s in aquatic wildlife biology and a master’s in ecology and evolution, set out to determine which factors most affect giant salmonflies’ ability to survive. Through research together and separately, which they’re now in the process of writing for peer review and publishing, they discovered that a combination of factors including streamflow, water temperature and heavy-metals pollution may determine whether the insects could survive in a given waterway. 

Frakes specifically studied the effects of lead and copper pollution on giant salmonfly nymphs’ ability to survive warming water. Lead pollution, he found, didn’t affect the insects much, but exposure to copper pollution reduced the nymphs’ tolerance for warm water by 4 or 5 degrees Celsius, meaning that giant salmonflies in copper-laden water could die off from less severe warming than those in cleaner water. 

“As climate change worsens, they’re going to have to deal with warmer temperatures and also high levels of heavy metals.” he said. “The main result that came out of that experiment is that if they’re exposed to copper, they have a harder time dealing with high temperatures. The Upper Clark [Fork River] is much more affected by temperature [than Rock Creek] and that’s because it’s a much more open riparian area … and it runs and meanders through fields, so the sun beats down on it. Also, a huge thing is that there’s tons of water withdraws for agriculture up in the Clark, so in the summertime it’ll be 4 or 5 degrees warmer in the Clark than it will ever be [in Rock Creek].” 

Birrell’s research focused on other factors he believed could affect the insect’s ability to tolerate warming water: oxygen levels and flow rates. Salmonflies live up to four years as nymphs before hatching into adults, and Birrell and Frakes said that the insects are ideal for research because they can survive just as long in captivity, allowing researchers to observe the species over weeks or months, rather than the hours- or dayslong studies common with other insects. Birrell said his research involving temperature, oxygen and flow over 100 days was “one of the first long-term tolerance study on any insect.” 

Birrell slowly increased the median water temperature for salmonfly nymphs in low and high flows, and with low and high oxygen levels, all while simulating daily temperature swings for night and day. Like Frakes, he discovered that the species’ temperature tolerance isn’t absolute, but rather it’s dependent on other environmental factors. Oxygen level, however, wasn’t one of them: In low oxygen, the nymphs simply developed bigger gills. 

“The gills are about twice as big on the ones that were warmed up and had low oxygen than individuals we take directly from the river,” he said. “Their physiology, their morphology, changed to the point where they could handle more stressful conditions.” 

That didn’t hold true for streamflow; Birrell observed about a 10-degree Celsius drop in the nymphs’ maximum survivable temperature in water with almost no flow versus fast-flowing water. Overall, though, he found that “they’re actually really resilient to prolonged temperatures. We expected that we would get 100% mortality around, you know, when the median temperature of the water was 25, 27 degrees Celsius—that’s in the 80s [Fahrenheit], so warm water. But we didn’t get full mortality until the water was in the 90s. And the warmest water that’s ever been recorded on this river is 21, 22 degrees Celsius. On the Blackfoot, there’s a lot of salmonflies over there, maybe 23, 24 [degrees Celsius]. They can handle, at least for one summer, temperatures much, much greater than that.” 

Exactly what salmonflies can and cannot handle is the focus of The Salmonfly Project, Frakes and Birrell’s new nonprofit, which they hope will be a vehicle for funding and performing further research and, eventually, monitoring and remediation. The aim, Birrell said, “is to really figure out what the deal is and then do restoration once we know what to do. But the first step is research and it’s monitoring, so we can know why they’re disappearing from different rivers, and then we can hopefully do some kind of eventual restoration. Or we can tell managers of these streams, where declines haven’t occurred, what the thresholds are that we need to protect them against.”