The number of non-native zebra mussels in the final 22 miles of the St. Croix River rose sharply in 2020, and again this year. The population explosion comes after nearly a decade of dwindling to the point they were rarely seen.
While the river is home to a world-class 41 species of native mussels, zebra mussels threaten boats, people, and the ecosystem.
The invasive mussels were first found in the United States in 1988 and on boats in the St. Croix in 1995. Reproducing populations were discovered in 2000, and the population has ebbed and flowed ever since.
To reduce the chances of the invasive clams from getting farther upriver, boaters have been prohibited from traveling north past the Soo Line High Bridge since 1997. To help monitor the population, the National Park Service began taking an annual zebra mussel census in 2004. (Learn more on the Saint Croix National Scenic Riverway zebra mussels page.)
The agency has placed cinder blocks on the river bottom at several points. Divers retrieve them each spring, and scientists count the number of zebra mussels. In the fall, the agency visits marinas along the lower river and inspects boats which have been removed from the water, again counting how many zebra mussels they find.
The cinder blocks and boat checks helps scientists estimate the degree of infestation.
Ups and downs
The reprieve reduced the chances a boat motor would be destroyed by the mussels clogging its intakes, or that someone wading in the water would cut their foot on the sharp little shells. It left more food for native fish and other aquatic creatures, helping the whole ecosystem.
Then, last year, the blocks were covered in the little striped shells again. The numbers increased by up to 1,000 percent.
“The conditions must have been just right,” 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 beach users in both Prescott and Lake St. Croix Beach have also reported to St. Croix 360 about seeing the spike in numbers this summer.
The working theory is that zebra mussel juveniles get washed downstream when there is high water on the river, before they can grab onto something solid. During high water years, the river pushes zebra mussels, especially their helpless larvae, out of the St. Croix before they can grab hold of something.
But the reverse is also true: periods of low water may make it easier for the invasive species to proliferate.
Unlike native mussels, which are large and heavy, have a muscular “foot” to hold onto the bottom and move around, and usually avoid the strongest currents, zebra mussels are native to lakes in Eurasia, and not well adapted to rivers. Native mussels use fish to carry their juveniles until the larvae-like organisms (called “veligers”) are old enough, but zebra mussel veligers float free in the water for a few weeks before latching on somewhere.
Even though numbers have risen in the known infestation zone, zebra mussels have still not been found on any cinder blocks upstream of the Boom Site boat landing. Preventing their spread is why boats are not allowed to travel upstream past the Soo Line Arcola High Bridge. Once the ravenous and prolific mussels get carried upriver, there’s not much anyone can do except watch their numbers rise and fall.
There is little chance the river below Stillwater will ever be totally free of zebra mussels, unfortunately, but there’s 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.
That is why boaters need to respect the High Bridge upstream travel prohibition. It’s also why it’s imperative that boaters clean and drain their boats before and after using the river. Zebra mussel larvae can travel in bilge water or live wells, and adults can hide in a hull’s nooks and crannies.
This spring, the 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 check to see if there are any zebra mussels attached.
With numbers relatively low at sites downstream until about the I-94 bridge, the hope is the river above Stillwater is still safe. But the scientists will keep checking year after year, hoping no one brings any passengers north.
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