What’s next for the Snake River after Idaho’s unprecedented invasive mussel treatment?

The rapid timeline from when the Idaho State Department of Agriculture found invasive quagga mussel larvae in the Snake River in September to when it deployed a chemical treatment two weeks later was unprecedented in the world of invasive mussel control. Yet the unparalleled effort also brings several uncertainties about what happens next.

In two rounds of post-treatment surveys, agency staff did not find any mussels in the six-mile river stretch in Twin Falls. However, the state won’t know if the treatment was effective until next spring. During the cold winter months, mussel reproduction decreases, making it harder to detect them when they’re not releasing millions of larvae.

It’s also unclear how long it will take for the river ecosystem to recover after tens of thousands of gallons of a copper-based chemical were poured in to kill the mussels. As expected, the treatment also killed most fish in the area. Idaho Fish and Game recovered over 3,000 dead fish from this stretch, weighing six to seven tons.

Most of the dead fish were suckers, pikeminnow and perch. Fewer largemouth bass and sunfish died, Fish and Game said. It’s possible those fish swam out of the treatment area to avoid the copper. Some species might return naturally, but sturgeon will need to be reintroduced through hatcheries. Fish and Game estimated that all roughly 50 sturgeon in this part of the river died from the treatment.

During a Fish and Game Commission meeting, Eric Stark, the fishery program coordinator, stated the agency would likely delay reintroducing sturgeon until it’s clear the river won’t need further treatment.

To better understand how the river might recover, some scientists are studying the bottom of the food chain: aquatic insects.

“We collect them and preserve them in ethanol, and then we send them to a professional taxonomist to look at them under scopes,” said Christopher Mebane, the Deputy Center Director of the USGS Idaho Water Science Center in Boise.

The USGS will count the species and number of insects in the river and compare that to the pre-treatment numbers. Mebane highlighted the lack of precedent to predict the river’s recovery.

“I’d say when they’ve done similar things, but no one’s done similar things,” he said.

Mebane examined the scientific literature on events like Wyoming intentionally poisoning the Green River in the 1960s to kill carp or accidental train derailments. Some species returned in a year, while others took longer. For aquatic invertebrates floating downstream, navigating through dams and pools in the Snake might be challenging, but Mebane is optimistic the species will eventually recover.

“Nature abhors a vacuum,” he said.

Find reporter Rachel Cohen on Twitter @racheld_cohen

Copyright 2023 Boise State Public Radio