Consequences of invasive mussels could prove costly

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A boat propeller encrusted with invasive mussels.

A tiny mollusk could have massive ecological and economic implications in Montana, which earlier this month became the latest state in the U.S. to confirm the long-feared presence of invasive mussels.

On Nov. 9, the Department of Fish, Wildlife and Parks announced that Tiber Reservoir, located south of the Hi-Line near Shelby, had tested positive for free-floating invasive mussel larvae known as “veligers.” Montana now joins a growing number of Western states engaged in a multi-billion-dollar effort to combat the presence of fast-spreading mollusks known for re-shaping freshwater environments while wreaking costly havoc on infrastructure and recreational areas.

ZEBRA AND quagga mussels, two similar species belonging to the genus Dreissena, originally made their way to the U.S. in the 1980s, inadvertently introduced by ocean-going ships entering the Great Lakes from Eurasia.

Within five years of the mussels’ detection in the Great Lakes they had made their way more than 2,300 miles to the mouth of the Mississippi River, according to Erik Hanson, an environmental consultant and member of the Flathead Basin Commission with more than a decade of experience in aquatic invasive species prevention.

In the years since, the bivalves have dispersed throughout the Midwest and Southwest. Only Idaho, Wyoming, Washington and Oregon have remained free of any known mussel introductions.

Shawn Devlin, an assistant research professor at the University of Montana’s Flathead Lake Biological Station, said once they gain a foothold on a new body of water, they multiply prolifically and are typically there to stay. An adult of either species can produce up to 1 million eggs per year.

“A tabletop could have hundreds of thousands of those organisms,” Devlin said. “If we have a local infestation that goes unchecked, it’s not going to be local for long.”

After an egg is fertilized in the water column, the larvae grow as free-floating veligers for two to three weeks before settling onto a surface. The juvenile mussel’s bissel threads adhere it to a substrate, where it typically takes about one year before becoming an adult capable of reproduction.

Both larvae and adults are capable of contaminating a new body of water, although Hanson said the latter is the most common method of spread.

“The highest likelihood is of boats that have been colonized by adult mussels being put into a new water body,” Hanson said, referring specifically to the confirmed veligers found in Tiber Reservoir. “Typically, the amount of veligers transmitted is very low. ... However, if you had a wakeboard boat that was full of water, you could transport a lot more veligers that would pose a greater risk.”

THE UNWELCOME news places the threat of mussel infestation at the doorstep of the Columbia River Basin — the only major watershed in the West still believed to be free of quagga and zebra mussels.

So far, the contamination has only been detected in the Missouri River Basin. But it hits dangerously close to home for those working to protect the relatively pristine waters of Flathead Lake and the surrounding system of rivers and creeks.

For the past couple years, Devlin has been creating a comprehensive computer model of Flathead Lake, mapping out the system of currents, seasonal temperature shifts and nutrient loading throughout the largest body of fresh water west of the Mississippi. Connecting those physical processes to the living organisms in the lake is trickier, but he said it can still provide an educated estimate of how changes to the system ripple up through the food chain.

“The potential ecological impacts are really high, meaning it’s going to change the whole dynamic of how the system works,” Devlin said Friday. “What zebra mussels and quagga mussels do is they eat the phytoplankton, and the food web changes dramatically because there’s no food source left for the zooplankton.”

The scarcity of zooplankton would in turn impact organisms farther up the food chain.

“All that energy that was fueling fish growth in the water body is now fueling mussel growth,” Devlin said.

That could mean clearer water, but far less fish. And while Flathead Lake is renowned for its already clear, glacier-fed waters and relatively in-tact ecosystem, the lake’s nutrient availability is already low.

“There’s not much margin for error before an effect is felt. Zooplankton would crash because it’s a very unproductive, low-nutrient system,” he added. “Zebra mussels can filter a ridiculous amount in a given day. … It’s not going to take much to say, drop the population of phytoplankton by 50 percent.”

THE LAKE’S underwater denizens aren’t likely to be the only ones who feel the effects. Economists at the University of Montana have estimated that Flathead Lake provides billions of dollars to the area in the form of tourism spending, property values and ecosystem services.

“From a recreational perspective, when these mussels infest, the shells are extremely sharp and basically limit where you can walk in the lake without injuring yourself,” Devlin said. “Within a few years, you’ll start seeing shells of the mussels washing up on shore, limiting the ability of people to use the beaches.”

In many lakes and reservoirs in the Southwest, mussel infestation has become a constant battle for operators of hydropower, irrigation and municipal water infrastructure. The thick buildup of mussel colonies can completely clog a six-inch-diameter pipe and require more frequent replacements of water screens. Pumps for irrigation or hydroelectric dams often become encrusted, causing overheating and ultimately requiring replacement.

“One of the first things you’re going to start seeing is the mussels start colonizing, impacting water removed from the lake for irrigation or any homeowner that has a water supply in the lake,” Hanson said. “Their pipes might start getting encrusted and losing their flow.”

A 2010 report commissioned by the Northwest Power Conservation Council estimated the additional cost to federal hydropower dams at $16 million per year, and $5 million per year for privately operated dams in the Columbia River Basin. Throughout the river basin, the report estimated the overall additional cost to taxpayers at $100 million per year.

STATE REP. Mike Cuffe, R-Eureka, is a member of the Northwest Pacific Economic Region, a coalition of representatives from the Pacific Northwest in the U.S. and Canada. He said Montana’s mussel detection has rattled natural resource experts and policymakers throughout the region, and the news took center stage during the group’s meeting last week in Boise.

“The folks in Washington, Idaho and Oregon desperately want to keep it out of their areas, and so do the folks in Alberta and British Columbia,” he said. “What Montana has to do is plan for the worst and pray for the best.”

While containment efforts have seen mixed results in other parts of the country, it’s typically just a matter of time before mussels begin colonizing downstream lakes and rivers, as the veligers can easily float to mussel-free bodies of water and quickly establish new populations.

In recent years, Montana has promoted an aggressive public-information campaign to encourage boaters to thoroughly clean their vessels before launching in a new lake or river.

Fish, Wildlife and Parks spokesman Rob Aasheim said the state spent nearly $1 million on AIS prevention efforts last year, which in addition to the “Clean. Drain. Dry.” campaign also helps fund the state’s 22 mandatory boat-inspection stations operating during the main boating season. In 2015, those check stations conducted 37,000 inspections, according to the state agency’s records, and turned up five boats contaminated with zebra or quagga mussels.

Cuffe has been one of the Montana Legislature’s leading advocates for increased vigilance to keep invasive mussels outside the state’s borders, but said the news this month has proved that those measures will need ramping up.

“The way we’ve been doing it is playing Russian roulette. We don’t have the funding to run boat-check stations as many months as we would like to, or as many hours as we would like,” Cuffe said Thursday. “I have a great fear that we are going to see an ecological catastrophe unfold in slow motion.”

Reporter Sam Wilson can be reached at 758-4407 or by email at

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