Less than a year after they were first discovered in Lake Minnetonka, troublesome zebra mussels have settled into most of the bays of one of Minnesota's outdoor jewels, officials said on Wednesday.

In the first of monthly surveys by the Minnehaha Creek Watershed District, the invasive species was detected at most testing sites across the west metro lake.

The district began a three-year study this spring by placing monitoring devices at 32 sites from Grays Bay to Halsted Bay. In the first monthly check in late June, zebra mussels were found at 24 locations.

There was no evidence of zebra mussels at five sites, including Halsted Bay, Harrisons Bay and Coffee Cove. Monitoring devices were lost at three locations and will be replaced.

Zebra mussels were first found in the lake's east end, in Wayzata Bay, last summer. They had spread by last fall across much of the lake's eastern half.

"It's no surprise the zebra mussel infestation is spreading west across the lake," said Kelly Dooley, the district's water-quality technician, citing their reputation for fast growth. "We're committed to doing everything we can to prevent its spread to other waters, but we need the public's help."

Left to their own devices, zebra mussels will continue to spread, littering beaches with their sharp shells. They damage boats and equipment and destroy the health of lakes, rivers and streams.

A public-awareness campaign to fight the spread of zebra mussels began this summer, reminding people to clean, drain and dry their boats and equipment before entering new water. For more information, visit the "Save Our Summers" page at www.minnehahacreek.org/saveoursummers.php.

"It is extremely likely that zebra mussels will be introduced into Christmas, Minnewashta and other lakes surrounding Lake Minnetonka unless something is done to inspect boats and manage access at these lakes," Eric Evenson, director of the watershed district, said last week.

Zebra mussels also have infested Lake Mille Lacs and at least 20 other Minnesota lakes, along with the St. Croix and Mississippi rivers.

Paul Walsh • 612-673-4482