The San Francisco Bay looks and smells a lot better these days than it did 30 years ago, in part because municipal sewage and industrial waste are no longer dumped directly into the water. But many fish and wildlife species in the Bay are still in steep decline. The problem now is numerous small sources of pollution — oil and gas spilled on streets, pesticides applied to lawns and farm fields, dioxin and PCB contamination in soils, mercury from mining and even dental offices. Meanwhile, on the other coast, scientists and citizens are working in the Chesapeake Bay to restore populations of oysters, a keystone species in the bay.