Invasive zebra mussels disappear from St. Croix River

A new survey of the river has given boaters, native mussels and swimmers reason for hope.




3 minute read

Zebra mussels cover native mussels
Zebra mussels cover native mussels (Photo courtesy U.S. Army Corps of Engineers)

The National Park Service’s annual search for one of the most problematic invasive species that has infested the St. Croix River came up empty-handed for the first time last year. The news bodes well for boat-owners, native species, and swimmers.

The Star Tribune’s Kevin Giles reports:

A St. Croix River menace that’s waged war on the healthiest native mussel population in Minnesota has retreated, bringing hope of a turning point in what had seemed a relentless invasion.

The fingernail-sized zebra mussels that crept into the river’s wider, deeper pool known as Lake St. Croix more than a decade ago diminished in significant numbers at eight monitoring locations last year, said Byron Karns, a National Park Service biologist who issued a report this week detailing his findings.

“When we went out in August to do these collections, there weren’t any [zebra] mussels to count anymore,” said Karns, who had noticed changes since 2006 in the 25-mile stretch of river between Stillwater and Prescott, Wis. “It suggests at some point later in the summer something was happening that was wiping out a whole class of mussels.”

Higher, turbulent water that persisted into August might have flushed zebra mussels out of the St. Croix last year, Karns said. A carpet of crushed shells on the river bottom also suggests that common carp have been feasting on them.

Zebra mussels represent a significant threat to the St. Croix and other Minnesota rivers and lakes because they multiply rapidly and attach themselves to native mussels, killing them and altering ecology and food chains. Their hard shells also cut swimmers’ feet and fishing lines and inundate docks.

Continue reading…

The sharp decline in zebra mussels is seen in the chart below from the most recent National Park Service study (PDF). You have to look pretty hard for the green bar which indicates the mussels, or lack thereof, that were found in 2011:

Chart showing decline in zebra mussels in the St. Croix River from 2009 to 2011

Good News for Natives

Winged mapleleaf mussel in the St. Croix River
The disappearance of zebra mussels is great news for the St. Croix's native mussels, including the endangered Winged Mapleleaf mussel. (Photo courtesy U.S. Fish and Wildlife Service)

Anybody who has had a boat engine ruined, or pump lines clogged, or docks covered in the mussels can breathe easier, for now. So can the river’s native mussels, two species of which are endangered and found in few places other than the St. Croix.

The Winged mapleleaf mussel and Higgins’ eye mussels are both important native species, defining the St. Croix’s waters with their presence. The mapleleaf, for instance, was once found in 34 rivers in 12 states. Today, it is found only in the St. Croix and four other rivers; the U.S. Fish and Wildlife Service says the Winged mapleleaf’s decline was largely caused by zebra mussels.

What You Can Do

Nobody is saying the zebra mussels are entirely gone, or gone for good. It’s possible that a unique combination of factors last year, including abnormally high water for much of the season, caused this temporary reprieve.

But, river visitors can help slow the spread in a few important ways:

  • Do NOT travel upstream of the Arcola Bridge from below it. This important regulation is meant to prevent zebra mussels from spreading up the river. Adult mussels do not typically travel on their own, and especially not upstream. The Park Service prohibits travel above the High Bridge from below it. Please follow this important rule.
  • Clean your boat and trailer. Drain your entire boat, including live wells and bilge, before leaving the landing in a zebra mussel-infested waterway like the St. Croix. If your boat was in the water for more than a day, wash it with either a high-pressure washer, or dry it for at least five days.
  • More from the Minnesota Department of Natural Resources.

As always, enjoy the river … without the company of these troublesome creatures for the time being.


You may republish this article online or in print under our Creative Commons license. You may not edit or shorten the text, you must attribute the article to St. Croix 360 and you must include the author’s name in your republication.

If you have any questions, please email


Creative Commons License Attribution-ShareAlikeCreative Commons Attribution-ShareAlike
Invasive zebra mussels disappear from St. Croix River