REGIONAL— Water samples from four of northern St. Louis County’s largest and most popular fishing lakes have shown the presence of zebra mussel larvae, a discovery that could have major repercussions for the lakes’ fisheries.
The presence of the larvae, known as veligers, was detected through routine zooplankton testing by DNR fisheries staff who had gathered samples through test nets on Lake Vermilion, Lake Kabetogama, and both Crane and Namakan lakes.
The significance of the find is difficult to predict, but it certainly is not good news for frequent users of the four lakes. In some lakes in Minnesota, the mussels have exploded in population, creating a major nuisance for boat and dock owners. They are also known to have a significant impact on fish populations where they grow in abundance, as they consume large amounts of algae that would otherwise sustain larval fish and other native animals.
On Vermilion, Matt Hennen, the DNR’s large lake specialist based in Tower, captured the veligers in just one of five “tows” of his plankton net, which is part of his routine biological sampling on the lake.
Rich Rezanka, the DNR’s regional AIS specialist in Grand Rapids, said Hennen’s plankton sampling, which is done throughout the ice-free season, has consistently found single digit numbers of the veligers in just one of his five standard sampling locations, which is located on Big Bay. The persistent presence of the veligers at the one location would seem to suggest an adult population somewhere in the vicinity, said Rezanka, but none have been found in any of the four lakes identified this week.
The veligers are microscopic and they initially float like algae. Eventually, as they grow, they become too heavy to remain in the water column and settle out, where they attach to hard surfaces, like rocks, dock posts, or boat bottoms, and begin to form shells.
While the presence of zebra mussels has had significant impacts on many Minnesota lakes, Rezanka said the impact on the four lakes cited this week is harder to predict. All four are located on the Canadian Shield, where shallow soils and the existing geology have very limited amounts of calcium, a mineral that’s critical to shell development for zebra mussels.
Rezanka noted that the discovery of veligers in Rainy Lake in 2021 has not led to the discovery of any adults, nor has their presence appeared to alter the lake’s ecosystem in any detectable way. The DNR and the National Park Service have both sent divers into the waters of Rainy Lake looking for adult mussels, but so far those searches have come up empty.
Rezanka said, ideally, the low calcium levels in the affected lakes will prevent the establishment of a thriving population. That’s a view shared by Keith Reeves, area fisheries manager in Tower. “We’re optimistic that we won’t see many environmental changes in the lake,” he said.
For now, members of the Vermilion Lake Association, who have undertaken extensive efforts to address the threats posed by aquatic invasive species, are waiting for more information before formulating a response. “It was a shock to me,” said Gary Haugen, co-leader of the association’s AIS fight. “We’ve been very diligent to try to keep out the zebras.”
Haugen said he’s advising the public not to panic, but to wait until there’s more information. “We’ve got to get the facts and sort this all out.”
Haugen noted that the lake association has undertaken its own sampling for veligers and, to date, its testing has come up negative. He noted that the association’s water testing consistently demonstrates that calcium levels in Lake Vermilion are low enough to make it difficult for the mussels to establish themselves.
Rezanka said the DNR will now increase its monitoring for veligers, juvenile, and adult zebra mussels on the four lakes. He noted that the Vermilion Lake Association has been monitoring for juvenile and adult zebra mussels for years, using special plates suspended in the water, that are checked regularly for the presence of attached juvenile or adult mussels. To date, that sampling effort has found no zebra mussels on Vermilion.
Classified as infested
While the presence of veligers appears to be limited in scope and concentration, the four lakes will nonetheless now be classified as infested with zebra mussels. The DNR is also adding Sand Point Lake, a connected water located between Crane and Namakan, as infested despite the lack of any confirming test results.
Foreign invader
Zebra mussels, native to Eurasia, are named for their striped appearance. They are filter feeders that feed on algae in the water. The larvae eventually attach to hard surfaces such as rocks or boat lifts, where they can accumulate in dense populations. They made it to the U.S. several decades ago in the ballast water of ocean-going ships entering the Great Lakes and have been accidentally spread by humans to many other lakes in the Midwest.
Preventive steps
Whether or not a lake has any invasive species, Minnesota law requires boaters and anglers to:
• Clean watercraft and trailers of aquatic plants and prohibited invasive species,
• Drain all water by removing drain plugs and keeping them out during transport, and
• Dispose of unwanted bait in the trash.
Some invasive species are small and difficult to see. To remove or kill them, take one or more of the following precautions before moving to another waterbody:     
• Spray with high-pressure water.
• Rinse with very hot water (120 degrees for at least two minutes or 140 degrees for at least 10 seconds).
• Dry for at least five days.