ST. PAUL, Minn. (AP) - Zebra mussels have been found in a fourth lake in Minnesota this year.

The state Department of Natural Resources said Friday the invasive species has been discovered in Forest Lake in northern Washington County.

DNR biologists inspected the lake after a citizen found a zebra mussel on a rock. They found additional mussels and say the population is likely well established and reproducing. For that reason, the DNR doesn't plan to treat the lake.

The DNR says boaters and anglers need to be vigilant about following the law and cleaning plants and debris from their boat and remove drain plugs while traveling.