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More than 85 percent of the world’s shellfish have been lost—which is why the Conservancy is rebuilding oyster reefs in places like the Gulf Coast. One eco-minded reader has a question: can this important work be done using recycled or scrap concrete?

Read the answer from coastal projects manager in Alabama. And don’t forget to send us your questions on any conservation subject for one of the Conservancy's 550 staff scientists. (Note: We regret that we can only answer one or two questions each month and that we cannot answer the others offline.)

Fred Litt of Sagamore Hills, OH, writes:

Jeff DeQuattro, coastal projects manager for The Nature Conservancy in Alabama, replies:

Thank you for such a great question! Before I give you an answer to this question, I think it might help if we step back for a moment and explain that oysters prefer hard surfaces in which to attach themselves. An oyster's favorite thing to attach to is another oyster. And this is why we often use oyster shell when we rebuild oyster reefs. Oyster shell, however, is a commodity and is not always available or the most cost-effective material to use in rebuilding an oyster reef.

There are hundreds, if not thousands, of products and methods on the market today that are touted as reef building materials. The question of using recycled concrete for building oyster reefs in Alabama has come up before, and at first glance it sounds like a wonderful idea.

However, upon further exploration, we have encountered even more questions and hurdles to making something like this happen.

Here are some problems with using recycled concrete:

1. Concrete is porous, and it often retains or traps oil and grease or other residue that might change the physical characteristics of the water quality in the immediate area of an oyster reef. These chemicals could alter the biological performance of the reef, which is something we like to avoid.
2. Concrete from bridges and roadways often has rebar or other reinforcing materials that are encapsulated into concrete, but then are exposed upon demolition. While we encourage very little physical interaction with our restored oyster reefs by people, these sharp components of concrete can create a safety hazard that we would like to avoid.
3. The logistics of accepting scrap concrete can be challenging. For instance, if I get a call from the Department of Transportation saying, "Hey Jeff, I have this recycled concrete from an old bridge. There is no metal or residue from oil and grease. Can you use it for an oyster reef?" Well, thank you Jane DOT person for thinking of us, but we might not have a project that is shovel-ready, or the resources to store it until we do, and if we do, we might not have the resources to transport it again to the dock-side where the barge is waiting to deploy it.

That being said, we do use concrete in some of our oyster reef restoration work. For example, we often use Reef Balls, which are concrete domes that range in size and weight that we choose based on the site-specific conditions. The contractors that make the Reef Balls use a specially formulated additive in the concrete mix to ensure that normal mixtures, which can sometimes be harmful in the marine environment, are not used.

Thanks again for your question!