Zebra mussel numbers soar in lower St. Croix River

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Non-native zebra mussels cling to a native pocket mussel found in the St. Croix River in Prescott, before the invasive mussels are removed and the native species is released. (Greg Seitz, Sainte-Croix 360)

The number of non-native zebra mussels in the last 22 miles of the St. Croix River increased sharply in 2020, and again this year. The population explosion comes after nearly a decade of shrinking to the point where they have been rarely seen.

While the river is home to 41 world-class native mussel species, zebra mussels threaten boats, people and the ecosystem.

The invasive mussels were first found in the United States in 1998 and on boats at St. Croix in 1995. Breeding populations were found in 2000, and the population has fluctuated since then.

To reduce the risk of invasive clams moving up the river, boaters have been banned from traveling north past the Soo Line High Bridge since 1997. To help monitor the population, the National Park Service has started taking an annual census. zebra mussels in 2004. (Find out more on the page zebra mussels of the national panoramic river of Saint Croix.)

The agency has placed cinder blocks on the bottom of the river in several places. Divers collect them every spring and scientists count the number of zebra mussels. In the fall, the agency visits marinas along the lower reaches of the river and inspects boats that have been pulled out of the water, again counting the number of zebra mussels they find.

Cinder blocks and boat controls help scientists estimate the level of infestation.

Highs and lows

Number of zebra mussels at lower Sainte-Croix River monitoring sites 2014-2020. (Courtesy of the National Park Service)

In 2012, there was a sudden drop in numbers, a trend that continued over the next two years. Very few, if any, invasive mussels have been found on the blocks year after year.

The reprieve reduced the chances of a boat engine being destroyed by mussels clogging its water intakes, or that someone wading in the water cut their foot on the small, pointed shells. It left more food for native fish and other aquatic creatures, helping the entire ecosystem.

A watch-block of the Sainte-Croix river covered with zebra mussels. (Courtesy of Marian Shaffer / National Park Service)

Then, last year, the blocks were again covered with small striped seashells. The numbers have increased by up to 1,000 percent.

“The conditions had to be perfect,” said Marian Shaffer, aquatic biologist for the Saint Croix National Scenic Riverway. “We have higher temperatures, much lower water levels, lower flow than normal in previous years.”

Boaters and users of Prescott Beach and Lake St. Croix Beach also reported to St. Croix 360 that they had seen the numbers increase this summer.

The working theory is that juvenile zebra mussels are washed downstream when the water is high on the river, before they can cling to something solid. During years of high water, the river pushes zebra mussels, especially their helpless larvae, out of Sainte-Croix before they can grab anything.

But the reverse is also true: periods of low water can facilitate the proliferation of invasive species.

Possible protections

Zebra mussels cover a rock found in the Sainte-Croix River at Prescott. (Greg Seitz / Sainte-Croix 360)

Unlike native mussels, which are large and heavy, have a muscular ‘foot’ to hold on to the bottom and move around, and generally avoid the strongest currents, zebra mussels are native to Eurasian lakes and are not well. adapted to rivers. Native mussels use fish to transport their juveniles until the larval-like organisms (called “veligers”) are old enough, but the zebra mussel veligers float freely in the water for a few weeks before s ‘hang somewhere.

Although numbers have increased in the area of ​​known infestation, zebra mussels have still not been found on cinder blocks upstream of the Boom site landing stage. Preventing their spread is the reason why boats are not allowed to ascend the upper deck of Soo Line Arcola. Once the voracious and prolific mussels move upstream, there’s not much anyone can do but watch their numbers increase and decrease.

The river below Stillwater is unlikely to ever be totally zebra-free, unfortunately, but there is still hope for the hundreds of miles of river upstream. Although humans don’t know how to eradicate them, people have figured out how to prevent their spread. The only way they move to new waters is with human help.

This is why boaters must respect the ban on driving upstream from the Pont Haut. This is also why it is imperative that boaters clean and drain their boats before and after using the river. Zebra mussel larvae can travel in bilge water or wells, and adults can hide in nooks and crannies of a hull.

This spring, Park Service scientists decided to add another cinder block to the monitoring sites. The new block is located near the High Bridge, and this fall they will be checking for any zebra mussels attached.

With relatively low numbers at sites downstream to the I-94 bridge, the hope is that the river above Stillwater is still safe. But scientists will continue to check year after year, hoping no one is bringing passengers north.

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