Zebra Mussel Decline Continues in the St. Croix

For two consecutive years, almost none of the invasive species have been found in the river.




3 minute read

Zebra mussels
Zebra mussels cover a native mussel.

Two years of monitoring has found drastically reduced numbers of zebra mussels in the St. Croix River. The findings are beginning to raise hopes that the invasive species might be reduced indefinitely.

When 2011’s reduced numbers were reported last summer, biologists warned that it was probably a fluke. But now they are saying the numbers were way down again in 2012.

“Twenty-two live mussels”

Population surveys the last two years have found hardly any of the tiny, destructive mussels where they would have once blanketed the bottom or underwater structure.

Byron Karns, an aquatic biologist with the National Park Service who led the research, explained the extent of the decline, “During our annual sampling at eight long-term monitoring sites, we found only 22 live zebra mussels. In the past, we typically found thousands.”

While the St. Croix’s population is certainly higher than the 22 specimens they found, it’s “not millions, as might have been the case as recently as 2008,” Karns told the Pioneer Press, which first reported the story.

A Park Service report states that the findings represent an average of less than one zebra mussel per square meter, a “dramatic drop” from more than 12,000 in 2007 at one sampling site.

The million mussel question

Why have the mussels suddenly disappeared? There are a couple possible explanations.

The big one is that exceptionally high water in the river in June the last two years is harming mussel reproduction, as it washes larvae downstream.

Zebra mussels were originally lake-dwelling species, and thus don’t fare well in fast-moving rivers, where the larvae, which don’t swim, float in the water for weeks before hatching.

But the high water is likely not the only reason. Common carp are eating lots of the mussels, and high temperatures the past couple years might have also caused die-offs.

Good news for the river and people

Anyone with a boat on the river, or who swims in it, or simply appreciates its natural biodiversity, ought to be heartened by the news. Zebra mussels clog boat engines, cut swimmers’ feet, and hurt native species, including the St. Croix’s world-class population of 40 native mussels.

The health of native mussels is an indicator of the overall health of a water body. “They’re sort of the coral reefs of the healthy water systems,” Rich Baker, endangered species coordinator at the Minnesota Department of Natural Resources, told the Star Tribune.

Zebra mussels scooped from the Mississippi River's Lake Pepin . (Photo by Minnesota DNR)
Zebra mussels scooped from the Mississippi River’s Lake Pepin. (Photo by Minnesota DNR)

Rick Chapman, marina manager at Sunnyside Marina in Bayport, says what marina staff have seen corroborates the research.

“We saw our first Zebra Mussels in the fall of 2008, but they were limited to boats we knew traveled at least as far as Treasure Island Casino,” Chapman tells St. Croix 360. “2009 was the worst year then it dropped off considerably in 2010. We did not spot a single one in the fall of 2011 or 2012.”

And stay out!

With the zebra mussel population so reduced, it might be hard for the species to grow again alone. But people brought the mussels to the St. Croix once before, and they could do it again.

It’s important to note that no zebra mussels have ever been found above Stillwater. The upstream travel restriction at the Arcola High Bridge is meant to help that. But anyone taking a boat from the lower river or another water body and putting it in the upper river needs to exercise extreme caution.

River users should observe a few methods for helping keep the mussels out of the waters where they recreate:

  1. Clean visible aquatic plants, zebra mussels and other prohibited species from watercraft, trailers, and equipment before transporting from any water access.
  2. Drain water from bilge, livewell, motor, ballast tanks, and portable bait containers before leaving water accesses or shoreline property. Keep drain plug out and water-draining devices open while transporting watercraft.
  3. Dump unwanted bait in the trash.
  4. Wash your boat with high-pressure water and rinse with very hot water and/or dry boats and water-related equipment for at least five days.

More information is available on the Stop Aquatic Hitchhikers website.

“Clean, Drain, Dry”

This Minnesota DNR video explains how to ensure you don’t transport invasive species with your boat.


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If you have any questions, please email greg@stcroix360.com


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Zebra Mussel Decline Continues in the St. Croix