Oysters are delectable, and serve many useful environmental functions, but a new global survey of the status of native populations compared to historical numbers confirms what many people have suspected — the number and size of oyster reefs are just a tiny fraction of what they once were.
Overall, populations are down about 85% from their levels around 130 years ago, according to a large international team of researchers from the US, China, Italy, Uruguay, and Australia. They published their findings in the February 2011 issue of the journal BioScience. The decimation has largely been caused by overharvesting, disease, and introduction of exotic species.
- "Oyster Reefs at Risk and Recommendations for Conservation, Restoration, and Management," BioScience, February 2011, by Michael W. Beck, et al.: release  (includes link to full study).
In the United States, populations along all of the Atlantic and Pacific coasts are considered either poor or functionally extinct. The only somewhat bright spot is the Gulf of Mexico's US, Mexican, and Central American waters, which are the source of the great majority of the world's remaining wild-harvested oysters (though the total, globe-leading Gulf catch today is just a tiny fraction (one-tenth) of what the catch once was in 1890 in just one setting — Virginia waters — in one country) . The current status of Gulf of Mexico oysters is classified as fair; the survivors face threats from oil drilling and many other activities.
Canadian waters in and around the Gulf of St. Lawrence are also classified as fair. All European and Mediterranean waters are classified as either poor or functionally extinct, and the Yellow Sea between China and the Koreas (which is the second largest global source of current oyster wild-harvesting) is considered poor. All Australian waters fall under the functionally extinct category. The only other global bright spots are "fair" populations off some New Zealand shores, and several stretches of "good" or "fair" populations off certain South American shores.
While wild oysters are both a contributor to healthy ecosystems and an indicator of ecosystem health, farmed oysters have become a more important source of food than wild ones in many places. There are also plenty of local stories about oyster cultivation.
- "Wild Oyster Reef Death Doesn't Equal A Bivalve Shortage,"  NPR's Shots health blog, February 9, 2011, by April Fulton. Based on a post in Julie Qiu's In a Half Shell blog. 
When covering this topic, you'll need to dredge up local sources to get the specifics on local threats, conservation opportunities, and links with aquaculture operations. This report can provide useful context and data.