What happened to Utah's salmonfly?

An undated image of salmonfly, which is a large stonefly.

An undated image of salmonfly, which is a large stonefly. (Robert Williamson)


5 photos

SALT LAKE CITY — Even if you are not a fly fisher, there's reason to be concerned with the loss of salmonfly in Utah rivers and streams.

This giant stonefly, a large aquatic insect, is an indicator of water quality. It can only live and thrive in cold, clean, well-oxygenated water. Its dwindling numbers and possible extirpation from river systems could have detrimental effects on other aquatic insect species, fish and birds that share the river environment, and even a river's aesthetic value decrease for people.

Provo and Logan rivers

Salmonfly extirpation from rivers such as the Logan and sections of the Provo has been studied and documented.

Despite some speculation that exists, the truth is no one knows the reason for the Logan River salmonfly demise. Attempts to repopulate the Logan River with salmonflies from neighboring Blacksmith Fork River were unsuccessful in the early 2000s.

In 2018, BYU professor Riley Nelson and a colleague wrote an article published in the BYU Journal of Undergraduate Research that stated they found a decline in present salmonfly numbers in the Provo River compared to historically documented numbers. Their Provo River samplings indicate that the number of salmonfly nymphs, which live underwater in the rocks of the river bottom, have declined from historic sampling numbers ranging in the hundreds to present samplings of single digits.

Ogden and South Fork of Ogden rivers

A decline of stonefly numbers in the Ogden River below Pineview Reservoir and a possible decline in the South Fork of the Ogden River below Causey Reservoir is now suspected.

About 10 years ago, fly fishers began to notice a decline in the number of adult salmonflies typically observed around the Ogden River. At first, it was thought that they just missed the emergence, otherwise known as the hatch. The large salmonflies typically emerge sometime in the month of May; yet, the tell-tale sign of emergence — the exoskeletons on the rocks, tree trunks and bridge abutments — was minimal.

Some fly anglers look forward to the salmonfly hatch on the Ogden River every year. Gene Castellano of Ogden is one such fisherman.

"I have fished the Ogden River for too many years to admit," said Castellano. "In the last 30 years, I have witnessed some great salmonfly hatches. There used to be so many bugs; they would land on you while fishing. The old wall and more recent cement barriers along the road that follows the Ogden River in the canyon always had salmonflies climbing on them when the hatch was on. Add to that all the salmonflies that used to be in the willows and other streamside vegetation, and we are talking hundreds of salmonflies every few hundred yards or so."

"The salmonflies hatch sometime in May on the Ogden River," Castellano continued. "I have gone to the river in May a couple times each week the last five years and have only found a few salmonflies. Something is definitely wrong. Most of the bugs have disappeared. I'm not even seeing very many of the nymphal shucks on the rocks."

Tom Doxey is another local regular who looks for the salmonflies every year. "The salmonflies are just not in the Ogden Canyon section of the river like they used to be. I suspect it could be from pollutants and warm water in Pineview Reservoir," he said.

Doxey also fly fishes above Pineview Reservoir on the part of the Ogden River known as South Fork.

"I fished a decent salmonfly hatch this year up on the South Fork, so there are still a few up there," Doxey said," but I have also noticed that some stretches of the river where I used to see them on South Fork, they seem to be gone. There is reason for concern."

Possible reasons for salmonfly decline

Silt from reservoir dams

At Ogden River's canyon section, Doxey observed that the river bottom had changed. The rocks in the river bottom have more silt around them. Salmonflies live in the cracks and underside of river-bottom rocks. Silt will fill in these areas and take away the habitat salmonflies need for survival, he said.

Research documented in a 2018 master's thesis by Heidi Elise Anderson indicates that siltation is a possible cause for the decrease in salmonfly numbers. And while there's much anecdotal evidence that siltation may be depleting the salmonfly habitats, further research is needed to find out if the dams somehow capture the silt and then allow more silt to enter the river when released from the dams.

Have the dams on the Provo River system and Ogden River system allowed more silt to be flushed down the rivers? It's possible. But the Logan River seems to be an anomaly as there are no major dams along its course.

Warmer reservoir water

Warmer water will kill salmonflies as well as other aquatic insects. Is the water in the reservoirs heating up to levels too warm for salmonflies? This is a real possibility for the Ogden and Provo river systems, yet it would not explain the extirpation of salmonflies in the Logan River since it has no major dam structures.

Continued drought in the West may be a factor in a decrease in salmonfly numbers. Typical snowpack years would provide larger and longer amounts of cold water into the Ogden and Provo river systems, thus providing the cold, clean well-oxygenated water for salmonfly survival. Perhaps less snow and less and quicker runoff over the last several years is detrimental to salmonfly populations.

Fabled western rivers

According to a report from the Montana Natural Heritage Program, there is a concern for a decrease in salmonfly numbers in sections of many fabled rivers of the West. Rivers such as the Big Hole, Madison, and Yellowstone are showing dwindling numbers of salmonflies in areas where large numbers were historically present. The same conditions that Utah rivers have been struggling with are suspected culprits to salmonfly population decrease in these waters: drought, thermal tolerance, siltation and pollution.

Without help from Mother Nature providing heavier snowpack, prolonged runoff and cooler seasonal temperatures, and human efforts to clean the environment and stop pollution, continued extirpation of the salmonfly may continue.


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