But it is acutely toxic to crustaceans and many other marine organisms. The Washington state study by toxicologist Cynthia Stonick found that 60 days after the initial spraying, water squeezed from sediments showed carbaryl present at 0.57 to 1.15 parts per billion. Those concentrations are nine to 19 times higher than the water quality levels recommended by the National Academy of Sciences for marine life protection. The study documented how tidal action in the bay picks up the carbaryl and pushes it into areas hundreds of feet beyond the target oyster beds. In one of those areas, the insecticide level the day after spraying was more than 2,000 times higher than the National Academy of Sciences guidelines.
Oysters grown in shrimp-infested ground can sink, smother and die.
Crab and many other species that live among the shells might then leave the tidal flats. The spray would kill off crabs and other vulnerable species for a short time. But the studies found that crab and other marine populations would rebound, for a substantial overall net increase. Stonick is more cautious. Stonick recommended better monitoring of the spray's effects and developing a plan to reduce its use.