In Mosquito Lagoon, where oysters enjoy relatively pristine water, Walters discovered a different threat.

Wakes from boats on the Intracoastal Waterway had shoved the shells off the muddy lagoon bottom into piles that were no longer submerged at high tide the way the filter-feeders need to be, she said.

By 2000, about 9 percent of the lagoon’s oyster beds had died, evident by the bleached-white piles of heaped shells along the channels. That hurts not only the oysters but the dozens of fish, crab, shrimp and algae species that live on the shells.