Quickly breeding oysters to combat oceanic pollution?
According to @AllInOne from the comments: This is currently what a project in New York City is trying to do.
Their listed goal on wikipedia is to get one billion live oysters in New York Harbor by 2035.

According to their website, they do this because of the following reasons:


  
  Oysters have a remarkable ability to filter nitrogen pollution from water as they eat. This is a heroic feat, because excessive nitrogen
  triggers algal blooms that deplete the water of oxygen and create
  “dead zones.”
  “Once all but extinct in New York City’s waters, the whales are undeniably back.” –Popular Science Oysters play a key role in
  attracting life. They earn their nickname as “ecosystem engineers”
  because we see biodiversity levels increase dramatically surrounding
  oyster reefs.
  Oyster reefs can help to protect New York City from storm damage—softening the blow of large waves, reducing flooding, and
  preventing erosion.
  


the website also carries on to state that 1 million pounds of shells have also been recycled as a result of this initiative.

Other comments suggest that in order to prevent invasive species, one would want to do this with oysters that already exist in the region.