Clam Fever: Testing New Ways to Protect South Carolina’s Clam Farms

Clam farmers along the South Carolina coast are catching a “fever,” but this one has nothing to do with being sick. During the summer months, schools of migratory cownose rays (also known as a fever of rays) can move through South Carolina’s estuaries looking for their favorite food—clams and other shellfish—creating a serious challenge for clam farmers.

Principal Investigator Josh Kim, shellfish aquaculture specialist for the S.C. Sea Grant Consortium (Consortium) is leading a project team of South Carolina hard clam farmers researchers from the SCDNR), and Clemson Agribusiness Extension Specialists to look for a solution. The team is testing different types of clam bags and coatings to determine which designs can better protect farmed clams from predators while remaining practical to deploy and affordable for farmers.

Growing Clams in South Carolina

Hard clam aquaculture is an important part of South Carolina’s coastal economy and culture. Approximately one dozen clam farmers across the state produced more than

Many farmers use a method known as bag culture. Small seed clams are placed into mesh bags, which are secured on the estuarine bottom where the clams grow until they reach harvest size.

Clam bags provide an important layer of protection, but are not impenetrable. Farmers must contend with a variety of environmental conditions, organisms, and predators throughout the growing process. Of these challenges, many growers report that predation is among the most damaging.

When the “Fever” Arrives

During the warmer months, cownose rays migrate along the South Carolina coast and congregate in areas where they encounter abundant bottom-dwelling prey.

Cownose rays are well equipped for finding and feeding on animals buried in the sediment. They possess specialized electroreceptors called ampullae of Lorenzini that can help detect prey, as well as powerful jaws capable of crushing hard shells. Their broad, modified fins disturb the seafloor and expose prey buried beneath the sediment.

For clam farmers, this combination can be particularly problematic.

Cownose rays feed on clams through grow-out bags by damaging the bags themselves. Farmers often recognize ray damage by the shattered clam shells left behind, which can resemble pieces of broken dinner plates.

Once a bag is damaged, the remaining clams can become vulnerable to other predators. Crabs, for example, take advantage of openings in the damaged bags. Their feeding  leave distinctive V-shaped marks on clam shells.

The result is more than just lost clams. Damaged grow-out gear may also need to be repaired or replaced, adding labor and material costs for farmers.

Finding a Better Clam Bag

South Carolina clam farmers have developed several strategies to reduce predation. Polyester mesh bags are commonly used because they are relatively affordable, easy to handle and harvest, and provide protection compared with some other types of gear.

Many farmers also use a These coatings, made up of different variations of water, acrylic, and latex, reinforce the mesh and provide additional protection from predators. However, there is a trade-off.

Net dip can be expensive, and coated bags are stiffer. That increased stiffness may encourage more organisms to attach to the gear, a process known as biofouling. Heavily fouled gear make farm maintenance more difficult and may affect the conditions experienced by the clams inside the bags.

Stiffer bags also interfere with the clams’ ability to burrow into the surrounding substrate. Because clams naturally bury themselves in sediment, anything that interferes with that behavior could potentially affect growth and survival.

For farmers, choosing the right gear means balancing several factors at once: predator protection, cost, durability, biofouling, and the health and growth of the clam crop.

Putting Clam Bags to the Test

To better understand which gear designs provide the best balance, the project team is conducting a field study with South Carolina clam farmers and Clemson Agribusiness Extension Specialists

The study will compare four clam bag treatments:

1. Untreated mesh bags.
2. Thicker mesh bags.
3. Mesh bags treated with a latex-based net dip.
4. Mesh bags treated with an acrylic net dip.

The different bag designs will be tested at four commercially active hard clam leases in South Carolina. These sites represent different environmental conditions and farming practices, allowing researchers to see how the gear performs under real-world conditions.

Researchers are working alongside farmers to monitor several factors, including clam survival, growth, biofouling, and damage to the bags.

Clam growth will be measured through both shell height and tissue weight, providing a more complete picture of how the different gear types affect the crop.

The team will also conduct a cost analysis for each treatment. A bag that provides excellent predator protection may not be practical if it is too expensive or negatively affects clam growth. Understanding those trade-offs is an important part of developing recommendations that farmers can actually use.

Research to Farm

The goal of the project is not simply to identify which clam bag survives the most encounters with a cownose ray. Researchers are looking for gear that provides an effective balance between predator protection, clam performance, durability, and cost.

Because the study is being conducted on active commercial farms, the results can be evaluated in the same environments where farmers are already growing their crops. The different bag designs are also intended to be practical options that can be incorporated into existing farming operations.

Once the study is complete, the results will be shared with the broader hard clam aquaculture community through a peer-reviewed scientific publication, an extension one-pager for farmers, two YouTube videos, a workshop, and webinar.

For more information on hard clam and other shellfish aquaculture or mariculture research, contact Josh Kim, Shellfish Aquaculture Specialist.

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