Pesticides, manure, and fertilizers run off farm fields and enter the food chain of the marsh. Chemicals from factories are also added to the rivers and end up downstream in the salt marsh, where they are concentrated in the bodies of salt marsh animals. In some areas, the marsh is filled in and built on. We cannot simultaneously use the salt marsh as a dumping ground for chemicals and expect to eat clean oysters that come from it. Nor can we fill in the marsh for beautiful homes and expect to catch the same number of shrimp from that marsh.