https://en.wikipedia.org/wiki/Billion_Oyster_Project - Different effort, this one is about putting out living oysters, not just shells as GP was talking about.

And to be clear, in GP's link, living oysters do show up, but they weren't put there deliberately (the shells and every other development provided the larvae with places to grow and establish themselves). The NY project started with oyster larvae placed on discarded shells to deliberately create the new environment with an oyster population.