Arkansas River: Restoration of Giant salmonflies (Pteronarcys californica) hasn’t had the hoped for results — yet

Giant salmonfly (Pteronarcys californica) By Walter Siegmund (talk) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10470719
Giant salmonfly (Pteronarcys californica) By Walter Siegmund (talk) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10470719

From The Colorado Springs Gazette (Lance Benzel):

…on the Arkansas River, Pteronarcys californica has been missing from the menu for upwards of a century, the casualty of a toxic past.

One state aquatic biologist hypothesizes they suffered a localized extinction, or extirpation, during an era when Leadville mine waste flowed unchecked into the Arkansas. That was before new water treatment measures initiated a turnaround that began in the early 1990s and eventually spawned one of the state’s most popular fisheries.

Now an effort to re-establish the bug, also known as the giant stonefly, seeks to add a fresh chapter to the unfolding success story. It has anglers sitting up and taking notice, even as it puts the river’s vaunted recovery to the test.

In 2012, Colorado Parks and Wildlife launched a three-year effort that scooped up an estimated 135,000 giant salmonfly nymphs from the Colorado River near Kremmling and deposited them at eight test sites near Salida.

After mounting what the agency calls the largest insect transplantation on record, a problem emerged at a critical juncture.

In 2015, a year after the last of the salmonfly deliveries to the Arkansas, state wildlife workers went back to the test areas to gauge their progress, searching the riverbank in 100-foot swaths, from the water’s edge to the willows.

After 58 man hours, they found no evidence that transplanted salmonflies had crawled out of the river to shed their exoskeletons and sprout wings, the culminating change in their roughly three-year lifecycle.

Further searches this spring and summer turned up no adults and little more than a “handful” of exoskeletal chucks, said Greg Policky, the state aquatic biologist who devised the experiment…

After mining came to Leadville in 1859, heavy metals began filtering into the Arkansas and ravaged its ecosystem, killing all fish around Leadville. Further downstream, near Salida, trout for decades lived for no more than two to three years – long enough to spawn, but too brief to acquire significant size.

“The Arkansas was a dead river,” said Jean Van Pelt of the Southeastern Colorado Water Conservancy District.

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The river’s fate began to change in 1992, when two treatment facilities were constructed near Leadville to remove heavy metals like cadmium and zinc, generated on mine runoff, before they reached the river.

The effect was nearly immediate.

“It turned things around,” Policky said. “By 1994, we had a self-sustaining population of brown trout here.”

Better water quality cleared the way for two decades of piecemeal improvements, including efforts to restore the Arkansas to a natural state in areas where it had been straightened or otherwise modified, a common occurrence in the developing West.

One recent project in Hayden Flats south of Leadville, for example, created a new bend in the river by installing a subterranean structure of latticed logs and timber at the river bank, then burying it under transplanted willows. The result is a veritable “fish condo” that creates optimal flow conditions while giving trout a place to hide from predators.

Downstream near Salida, the effect of the river’s rehabilitation was profound, fattening up trout and extending their life expectancy to up to a decade.

In theory, it should have created trophy conditions for the giant salmonfly, too…

During all three years the bugs were stocked, they hatched in mid-May, fueling hopes it would be a matter of time before they took off like “gangbusters,” he said.

Back-to-back years without hatches suggest the process will be slower than expected; it could also indicate the experiment has been a failure.

Policky urges patience.

Any number of factors could explain the bugs’ apparent absence, Policky said, including competition from other bugs, or the large amount of sediment that washes into the river from the overgrown forests cloaking the Collegiate Peaks.

But he acknowledges the problem could also be environmental. For that reason, the search to explain the bugs’ failure to take wing is centered on water quality data measured by sensors by the river’s headwaters near Leadville.

So far, the data show no evidence of heavy metals in the water, but the monitoring isn’t continuous, raising the possibility that some level of contamination could be finding its way back in.

Standing at the river bank, he mulled the possibilities.

“Did heavy metals rear their ugly head again? Did we have a release that we don’t know about? This is the canary in the coal mine.

“If indeed I can track it to water quality, Pteronarcys is how I’m going to get there.”

On the other hand, fish and other bug communities appear to be thriving, an indication that perhaps some other cause is to blame. Policky chalks it up to another unknown in an unprecedented effort to revive a bug species through transplantation.

“Frankly, there’s never been anything like this, for sure not to this magnitude.”

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