Recent travel inspired me to do a little research. Oysters are a form of carbon sequestration. They accomplish the same kind of permanent carbon storage as technology costing millions. The oyster shell is calcium carbonate (mostly) and lasts for many millenia.
I found that your typical oyster has 10gms of carbon, (or 30 gms of CO2). At current prices of 10-20 Euro/ton for tradable CO2 it's not much per oyster. The fisheries commision did investigate getting Kyoto credits.
Turns out you don't get credit for revenue generating activities (like growing and selling oysters). If you grew the oysters and threw them away you might get credit. But activities that generate revenue don't count as CO2 reduction, even if they do reduce the CO2 levels. Welcome to Kyoto bureaucracy rules.
So you just get to feel good about helping the environment when you eat an oyster. It's helped the water quality and helped the climate.