Waupaca’s Shadow Lake has joined the list of area waterways dealing with the invasive zebra mussels. A city resident discovered the mussels clinging to an anchor on a mooring buoy, and the DNR confirmed they are zebra mussels. They’ve also been found by lifeguards at the swimming beach, where the mussels were found on the underwater structure of the diving dock. Zebra mussels were first discovered in the Great Lakes in 1988, and in Wisconsin waters in 1990. Biologists say the female mussel begins reproducing within about 6 to 7 weeks of settling in a water body.