The Maryland Department of Natural Resources says mute swans are destroying vital underwater grasses of the Chesapeake Bay. Fewer than 500 of the birds are still living on the bay and its tributaries. Biologists say adult swans eat up to eight pounds of underwater grasses daily, eliminating critical vegetation that filters bay water and controls erosion. The swans are native to Europe and Asia, averaging 25 pounds and a wing span of almost seven feet. Debate has raged over the state’s population-control effort, which has involved shooting the swans or snapping their necks and also coating eggs with vegetable oil to suffocate embryos.