BOISE, Idaho — Idaho Fish and Game now has an idea of the effect of the state's quagga mussel treatment's impact on fish populations in the mid-Snake River.
In October, the Idaho State Department of Agriculture (ISDA) treated a 6-mile stretch of the Snake River near Twin Falls to kill off the invasive quagga mussels, first detected in the river in September.
Throughout the treatment process, Idaho Fish and Game has been surveying and monitoring fish mortality on the river.
High fish mortality from the quagga mussel treatment was expected because the aggressive copper-based formula prevents the fish from taking oxygen out of the water and into their bloodstream.
Before the treatment started going into the water, Idaho Fish and Game sampled the section of the Snake River that was impacted - between the Twin Falls Dam and Centennial Waterfront Park - to help develop population estimates.
The department continued surveying fish populations throughout treatment and now has an idea of how it has impacted fish in the river.
"What we identified is that a lot of the game fish - your largemouth bass, bluegill, sunfish - there was essentially no mortality associated with those species," Mike Peterson, the Magic Valley regional fisheries manager for IDFG said. "We did see high mortality in...large scale suckers, northern pikeminnow, yellow perch - and white sturgeon."
Fish and Game counted 48 sturgeon had died during the treatment. All sturgeon deaths were hatchery-origin, and about half of the sturgeons that died were between two and four feet long, while the other half were between five and six feet long.
There were no sturgeon deaths downstream of Auger Falls.
"The sturgeon mortality that occurred occurred between Pillar Falls and Centennial Park," Peterson said.
As for what's next, the ISDA will continue to monitor the stretch of the Snake River to determine how effective the quagga mussel treatment was.
Fish and Game will be playing the waiting game for a while longer. The department said they need to ensure there won't be another round of treatment on the river before translocating fish into the impacted area.
When the weather warms up, IDFG said they will sample more fish populations on the river to better understand how the treatment impacted smallmouth bass populations. Ultimately, Peterson expects they will translocate smallmouth bass back into the river - while the sturgeon population will take longer to rebuild.
"We have some tools in our toolbox that should allow us to help rebuild that population fairly quickly," Peterson said. "We have fish in the hatchery system that potentially we could grow to a larger size for a longer period of time in the hatchery before we stock them into the river. When they shut the canals off each year in the fall, we get routine phone calls about stranded sturgeon from other sections of the river, and we could potentially move those fish into this reach to try to rebuild that population."
The Snake River treatment is part of the state's plan to eradicate the invasive quagga mussels, which threaten the Snake River's food chain and Idaho's waterways.
"The largest component to the unchecked quagga is the potential costs for the infrastructure changes that would have needed to happen in terms of hydropower, in terms of irrigation diversions, and anybody that pumps water out of the system," Peterson said. "We're talking about potentially hundreds of millions of dollars of change."
A complete summary of Idaho Fish and Game's survey on quagga mussel treatment's impact on fish populations can be seen here.
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