A Sticking Point

Oysters must find their footing in the first few weeks of their lives. Where they choose to attach their foot, a strong muscle that anchors their shell to another hard surface, dictates where they’ll spend the rest of their lives.

Along the bottom of the Chesapeake Bay, like attracts like—the best place for an oyster to settle is on top of another oyster. Over time, the layers of oysters form a thick, rocky network as young oysters attach to older oysters.

Oysters grown through aquaculture also find their footing. For approximately 90 percent of Maryland’s oyster aquaculture acreage, oysters are grown directly on the bottom of the leased area, and must still attach to a hard surface.

In this method of growing oysters, known as bottom culture, the oysters grow loose on leased beds. The bottom of the leased area must be hard to begin with, or growers can spread oyster shell to create a hard surface that keeps their oysters out of the mud. Usually, the leased area that’s available needs to be groomed first, according to Nick Hargrove of Wittman Wharf Seafood.

To prepare the oyster larvae for life on the leased area, oyster growers use a process called remote setting, where the oyster larvae are placed in tanks full of oyster shell. Once they attach to these oyster shells, which begins the phase of their life known as “spat,” the spat-on-shell is distributed onto the bottom of aquaculture leases.

According to Maryland regulations, only oyster shell can be used to harden the bottom of a lease, and oyster growers are required to spread a minimum of six inches of oyster shell on their lease as a base to grow their oysters. But for growers like Hargrove, who can barely get enough shell to cover the bottom of his lease, alternatives like recycled concrete could allow oyster growers to plant more oysters on their leases. 

The cost of oyster shell compounds the problem. In the last five years, the cost of shell has risen substantially, with the current market value between $4 and $6 a bushel. It takes nearly 17 bushels to create just one cubic yard of shell. The low quantities and high prices can be prohibitive for oyster growers who plant directly on the bottom of their leases. As a result, researchers like Matt Gray at the  University of Maryland Center for Environmental Science  are investigating alternatives to oyster shell.

“There are a number of groups out there that have been trying to figure out what oysters like, and what they can make out of concrete or some kind of mixture with concrete and shell and stuff like that, and see if that works,” Gray says. “And then, even if you've made the material, the next question is: What shape do you make? Do you make a shell? Do you make a slab? Do you make some kind of weird, reef-looking thing? Do you coat surfaces like bulkheads with it?”

Although oysters often build upon one another to form reefs, they aren’t too finicky about the hard surfaces they settle on. Along the East Coast, some oysters can be found growing on dock pilings, or even on the frames of sunken cars or boats.

In some parts of the East Coast, specially constructed blocks of concrete that mimic the surface area of an oyster reef are used for oyster settlement along the shoreline. Similar concrete materials could be used in oyster aquaculture in Maryland. Pieces of concrete could be used either as a material for settling oysters onto directly, or as a base layer underneath oyster shell on the Bay bottom.

“My idea would be, in early April or May, we put out recycled concrete. Then, we take whatever shell that we have and we sprinkle it out on top,” says Hargrove of Wittman Wharf Seafood. “Why put six inches of the shells on the bottom, when only one inch of it is [needed] for oysters to strike on?”

Exploring alternative substrates, Maryland’s Aquaculture Coordinating Council is creating a preliminary report considering alternatives to oyster shell that could be incorporated into aquaculture practices, and the benefits and drawbacks of each. The report will focus primarily on alternative materials that could be used for directly setting oysters in the spat-on-shell process. Maryland’s Governor Wes Moore has also created an Oyster Shell and Substrate Task Force to identify solutions for retaining oyster shell and increasing availability of oyster substrate in Maryland.

“Maryland has a real opportunity to be a leader in embarking on new technologies; new best practices to produce oysters,” says Ward Slacum, director of  Oyster Recovery Partnership . “Alternative substrate is just one of those opportunities, but it's a pretty significant one.”

Finding enough oyster shell, either for the setting process to produce spat-on-shell or to spread along the bottom of the Bay for an aquaculture lease, is one of the main constraints for the expansion of oyster farming as an industry.

“There are two barriers to growth within Maryland’s aquaculture industry,” says Ferry Cove Shellfish President and CEO Stephan Abel. “One is larvae, and the other is substrate.”

An oyster reef may have the unchanging appearance of a pile of rocks, but the environment of a healthy reef is one of simultaneous growth and breakdown. There are generally two main zones on the reef: the outermost layer exposed to an oxygen-rich environment, and a deeper, inner layer where dead oyster shell is preserved and becomes the underlying framework of the reef.

Oyster shell, which is made mostly of calcium carbonate, naturally breaks down in the reef environment, in part from pests like sponges and worms that bore into the shell and weaken it. At a molecular level, water chemistry plays a significant role in how quickly shell breaks down into calcium and carbonate ions—and how quickly oysters can grow shell by taking in these building blocks from the water.

Although people usually think about water chemistry in terms of salinity, which ranges from salty seawater in the mouth of the Bay to fresher water up in the Bay’s tributaries, the water’s acidity also varies widely throughout the Bay. This important aspect of water chemistry is typically measured by pH. On the pH scale, a pH of 7, or a neutral pH, is the pH reading for pure water with nothing dissolved in it. The Bay’s brackish waters are slightly basic, usually measuring in the range of pH 7 to 9, and their chemistry is influenced by factors such as salinity and temperature. Research indicates that oyster larvae can develop normally in an environment with a pH ranging from 6.75 to 8.75, with an optimal pH range of 8.25 to 8.5.

More acidic waters contain less of the carbonate needed to grow shells and cause oyster shells to break down more quickly, but researchers are still working to understand the balance between reef breakdown and build-up processes.

One way researchers measure the effects of chemical and biological processes on oyster shell is by calculating the saturation state of calcium carbonate in the water. Hakai Institute Postdoctoral Researcher Iria Giménez describes saturation state as a “corrosivity index” that indicates whether water conditions will dissolve the calcium carbonate that makes up oyster shell, and how fast.

“It’s the variable that defines where we are in that continuum between the dissolution of shell or making more shell,” Giménez says. This measure depends on the water’s pH, which is affected by temperature, salinity, water depth, and the amount of carbon dioxide dissolved in the water. “Related to all those variables are processes that also [influence] the carbon dioxide levels or the pH.”

Like oyster shells, the concrete proposed as substitute for oyster substrate may seem inert, but its components, crushed materials like granite gravel and a binding agent to hold them together, change over time. Freshly poured concrete has a pH value as high as 13 due to the alkaline quality of Portland cement, the common binding agent of many industrial concretes. For comparison, this is the pH of bleach. Concrete’s pH lowers as it comes into contact with the environment and ages, but the best concretes for oyster recruitment need to have a near-neutral pH soon after they are poured. To attract oyster larvae, the surface of the concrete should be chemically neutral or slightly basic.

Specialized add-ins can make concrete a better substrate for oysters. Larvae don’t settle as well on granite, a common ingredient in industrial concretes, compared to other substances like limestone, which contains calcium carbonate. Adding calcium carbonate, the main chemical found in oyster shell, into concrete mixes can help attract larvae to settle on the concrete.

Traditional concrete used for roadways and buildings is designed to last. It must weather storms and wind, and withstand structural stress. But to mimic the natural breakdown of shell on an oyster reef, researchers think a concrete-based product would need to break down gradually over time. Research shows that dead oyster shell has a half-life of 3–10 years in an estuary like the Chesapeake Bay. In comparison, traditional concrete mixtures used for buildings are designed to last for many decades.

Asking concrete manufacturers to make a product designed to break down may seem counterintuitive, explains Dave Bushek, director of the  Haskin Shellfish Research Laboratory at Rutgers University .

“Their first reaction is usually, ‘Why would you even want to do that?’ So we have to talk about ecology, natural systems, and resilience,” Bushek says.

The other major difference between oyster substrate concrete and traditional concrete, apart from durability, is texture. While many concrete products are designed to be as smooth as possible, oyster larvae prefer a rough surface to attach to. A rough surface, similar in texture to the ridges on an oyster shell, gives the oyster’s foot a better hold, and predators can’t pry them off as easily, Bushek says.

Other types of shell, like clam shell, have been used as a substitute for oyster shell. Similar to oyster shell, they contain calcium carbonate. However, there are drawbacks to using other shells as substitute: clam shell is more fragile than oyster shell and will start to break down after it has been handled more than once. The more uniform shell structure also causes clam shells to stack together, leaving only the outermost layer available for settlement. Other shell types may be most suitable for hardening the bottom of an oyster lease.

Ultimately, Hargrove would like to see materials like crushed concrete used in Maryland oyster aquaculture, similar to what Louisiana’s commercial oyster growers do in the Gulf of Mexico. There, growers use oyster shell if available, but when oyster shell is too costly or supply is short, they often use limestone or crushed concrete on their oyster leases. Many private leaseholders in Louisiana waters favor crushed concrete, since it is typically the least expensive option, according to Earl Melancon Jr., professor emeritus and oyster scholar with Louisiana State University.

“You can get as much of it as you want—there’s no bottleneck there,” Hargrove said. “I see that as the next upcoming thing. Even if we don't put it on top of our existing [oyster] bar, and put them around the edges, now we're using the bottom that we have, and we're making more available for oyster recruitment. We can do better than just trying to preserve the bars we have; we can grow the industry.”