After that oyster slides into your mouth, and you’ve tossed another empty shell onto your plate, do you ever think about where those shells end up?
Probably not. Yet that pile of mollusk shells contributes to more than 7 million tons of “nuisance waste” discarded every year by the seafood industry that mostly winds up thrown into landfills or dumped into the ocean. Researcher James Morris calls the practice “a colossal waste of potentially useful biomaterials.”
He and his colleagues have a better idea, one that could reap huge ecological benefits. They propose using shells to restore damaged oyster reefs or crushing them for applications in agriculture and engineering. Both require little time and money.
“Reusing shell waste is a perfect example of a circular economy, particularly as shells are a valuable biomaterial,” Morris said. “Not only does it improve the sustainability of the aquaculture industry moving forwards, but it can also provide secondary economic benefits to shellfish growers and processors as well.”
Morris and a team of CACHE (Calcium in a Changing Environment) scientists from the Royal Belgian Institute of Natural Sciences have been studying environmental and economically sustainable options for mollusk shells, and recently presented their findings to a meeting of the Society for Experimental Biology.
Mollusk shells are rich in calcium carbonate — which makes up about 95 percent of their composition. The shells are the perfect material for repairing damaged oyster reefs, according to the scientists.
Oyster reefs have been under great stress in recent years from climate change, over-harvesting, and a decline in water quality. Healthy shellfish soak up heavy metals and other pollutants, including nitrogen, which can lead to harmful algal blooms. And, their abandoned shells provide homes for other organisms. Once destroyed, the benefits shells provide the ecosystem disappear.
Old oyster shells also provide an excellent surface for oyster larvae to settle. “For oysters, particularly, overfishing and disease have reduced the amount of living and dead shells present, and subsequently the amount of hard surfaces available for the young to settle on,” Morris said.
Several projects already are using waste shells to create reefs for oyster larvae to settle on. “If done properly, these structures quickly get covered in living oysters, which in turn attracts other species,” he said. While acidic carbon pollution has been harmful to oyster shells, “this does not affect their ability to be used in reef structures or other calcium carbonate applications at present,” Morris said.
Most of the world’s calcium carbonate comes from limestone mining, which the researchers describe as “ecologically harmful and unsustainable.” Oyster shells can supplant limestone as a source of calcium carbonate, which is a common ingredient in cement and can be used to treat wastewater. Crushed shells also can be fed to hens as a calcium supplement or spread on farmers’ fields to control soil acidity.