The number of zebra mussels in Lake Mille Lacs has declined this year for the first time since they were discovered in 2005, Department of Natural Resources researchers say.
Zebra mussels per square foot dropped from about 1,270 per square foot last year to roughly 1,070 per square foot this year, a survey completed this week shows, said Tom Jones, treaties manager in the DNR's Aitkin Fisheries Office.
"Any invading species tends to ramp up to very big numbers and then fall back some degree," Jones said. "In zebra mussels, that's been observed in other places, where they've gone up to some outrageously high density and then backed off by maybe a third or so."
The DNR counts the mussels as a destructive invasive species. They filter plankton from water, making it clearer and causing more vegetation to grow deeper in lakes, potentially damaging the fish food chain. They can also attach themselves and clog motors and water intakes, and their sharp edges can be hazardous to swimmers.
The discovery of zebra mussels often generates anxiety and frustration. Stopping the spread has been a state priority.
Researchers will need more data next year to determine if this year marks the beginning of a decline or a one-time result of cold spring weather or other factors, Jones added.
This is the first time the agency has documented how long it takes for the mussels to fully spread around a lake.
"There's really no other lake where they were found at such low densities, found so early in the infestation," Jones said. "This is really the first time that we've documented how long it takes a mussel population to go from nothing to maximum."