http://newrepublic.i2p/article/160790/can-oysters-save-seas
Even in the late 1800s, at the height of the oyster trade, there were billions of oysters in the bay. Watermen harvested about 20 million bushels of oysters a year, compared to a few hundred thousand now. Until the turn of the twentieth century, oysters could filter the entire bay in a day or so; now, it takes about a year.