With 14 lakes covering 14,000 acres and 119 miles of shoreline, the Whitefish Chain of Lakes near Brainerd is one of the most popular water recreation areas in Minnesota.

Which makes the recent discovery of zebra mussels there all the more heartbreaking.

"It's unfortunate," said Dan Swanson, Department of Natural Resources aquatic invasive species specialist based in Brainerd. "It's a premier watercraft recreation site."

Zebra mussels — adults at least 2 years old, capable of reproducing — were found in Cross Lake and Lower Hay Lake — bodies on each end of the chain, meaning the likelihood is great that all the lakes either already are infested, or will be shortly, including Whitefish, Swanson said.

Divers will search the lake this summer, looking for more zebra mussels. No one knows what impact the mussels will have on the chain, but they threaten to alter the lakes' ecosystems, and at the very least the razor-sharp shells eventually could litter the chain's famed sand beaches.

Dave Fischer, president of the Whitefish Area Property Owners Association, said he was "deeply disappointed" over the news, and said the popularity of the Whitefish Chain and the infestations of other nearby popular lakes — including Gull, Mille Lacs and Pelican — made the chain more vulnerable.

"I think we've been fortunate to avoid it as long as we have," he said.

Fisher said his group, which represents 1,000 lakeshore owners, will continue to monitor the seven public accesses on the chain, checking boats leaving and arriving the lakes, hoping to slow the spread to other area lakes.

"I don't think it's hopeless," Fischer said. "We need to do as much as we can to slow the spread until controls maybe come along."

Meanwhile, Swanson said the Whitefish chain is at an early stage of the infestation. "You really have to work to find them right now," he said.

Making it all the more amazing that two sisters, age 10 and 12, discovered the infestation.

"They are home-schooled and had a chapter on invasive species," said Swanson. "They thought they spotted one from their dock [on Cross Lake], so they put on their dive masks and looked hard for 2 ½ days, and they found four or five."

Their dad called the DNR, which confirmed the girls' discovery.

"It's really quite impressive," Swanson said. "If you picked 1,000 people off the street, I don't know that they could do what these kids did. They did everything by the book."

The DNR searched and found more, then expanded the search to Lower Hay Lake, where it discovered others.

Doug Smith • dsmith@startribune.com