Wild harvest of oysters begins October 1st in Maryland, so this month we are focusing on the other side of oyster harvest, aquaculture. Oyster farming is one of many tools available in the field of oyster restoration. The idea here is that oyster aquaculture can reduce fishing pressure on wild oysters while still meeting the commercial demand for oysters. In addition, demand for oyster spat for aquaculture helps support oyster hatcheries that are also producing oyster spat for restoration planting and reef building.
Oyster aquaculture has been around for centuries, at least since ancient Greece. In those days, wild oysters were harvested and moved to salt marshes and basins where they could be removed and consumed on demand [1]. Today, oysters are the dominant species of shellfish produced and this form of aquaculture is practiced throughout the world. In fact, global aquaculture production of oysters overtook wild harvest in the 1950s, though the US is an anomaly and still relies heavily on wild harvest.
Advancements in oyster hatcheries are why oyster aquaculture is now complementary to wild harvest. In France in the mid 1800s, a mason named Hyachinte Boeuf decided to try his hand at oyster farming [2]. He built a low stone wall along the coast as a way to create a reservoir in which to grow oysters. Instead of needing to buy young oysters to grow out in the reservoir, he found the reduced current allowed oyster spat to settle and grow spontaneously on the stone wall he had built. Later, oyster farmers were able to place tiles covered with lime and sand out near oyster reefs. Baby oysters would attach to these tiles and the coating would allow them to be more easily removed and placed into containers where they could grow larger.
In the 20th century, oyster hatcheries began harvesting adult wild oysters to use as broodstock. By manipulating water temperature, the broodstock oysters could spawn on demand, allowing for the creation of vast quantities of oyster larvae which would then be set on a variety of materials depending on their use. For restoration, oyster larvae are typically provided with empty oyster shells (aka cultch) to set on. Once the larvae have settled into spat, they can be planted directly on wild oyster reefs or on a foundation of substrate to create a new reef. For aquaculture, oyster larvae are provided with ground up oyster shells (aka microcultch) which are about the size of a grain of sand and produce single adult oysters which are ideal for consumption. Unfortunately, we are currently experiencing a shortage of oyster shell in the Chesapeake Bay region, so researchers are working on alternate substrate so that oyster hatcheries can continue to grow their production.
In the 1980s, oyster hatchery science was further advanced by the development of triploid oysters [3]. Triploids are organisms with 3 copies of each chromosome, instead of the usual 2. The majority of triploid oysters are unable to reproduce, which means that they have more energy for growth and metabolism so they reach market size faster and can be harvested year-round. Typically, wild oysters are not eaten in warmer months of the year now because this is when reproduction is happening. During this time, oysters are either full of reproductive cells or thin and watery after having released those cells – either way, they are not the fat, delicious oysters we would expect to be on our plates.
Here in Maryland, the University of Maryland Center for Environmental Science created what would become the Oyster Culture Facility at the Horn Point Laboratory in Cambridge shortly after Tropical Storm Agnes damaged vulnerable oyster populations in the Chesapeake Bay [4]. In the 1990s concerns about the low oyster populations through the Chesapeake had resource managers working together to come up with a restoration plan. Eventually, the Oyster Recovery Partnership was formed and they partnered with the hatchery to help produce oyster spat for restoration. Today, you can purchase spat from this hatchery for a variety of purposes, from large scale aquaculture, to small scale oyster gardening, as well as for restoration.
The first permits for private oyster aquaculture leases became available in the 1990s in Maryland [5]. At this time, the Maryland Department of Natural Resources released 10 experimental permits for shellfish aquaculture. By 2023, there were over 475 shellfish leases in Maryland, comprising around 165 unique aquaculture businesses, the majority of which produced oysters [6]. 78 of those leases were right here in Talbot County. In 2022, Maryland aquaculture businesses produced just under 95,000 bushels of oysters worth just over $7 million USD.
Oyster aquaculture here in Maryland typically utilizes floating cages and mesh bags and would be considered inshore-nearshore aquaculture as it takes place in the Chesapeake Bay. Farmers can purchase oyster spat of varying sizes, though the larger they are, the more expensive they are. As the oysters grow, an oyster farmer must constantly sort them, moving oysters along into larger mesh bags. Aquaculture oysters reach market size at 2.5 inches, unlike wild oysters which must be a minimum of 3 inches. Again, this has to do with the fact that aquaculture oysters do not reproduce so we aren’t worried about leaving them in the water long enough for them to do so.
Even though aquaculture oysters aren’t reproducing, while they are in the water they are still providing habitat for other animals, providing other animals with food, and filtering the water, which are the three major ecosystem services that oysters provide. And once they’re harvested, they provide humans with food, with the luxury of being fat and delicious year round. The one complaint about aquaculture oysters is that because they grow so fast, their shells tend to be more brittle than wild caught oysters. This is really only a problem for the casual oyster shucker, as it likely doesn’t make much difference to a professional.
If you’re interested in trying your hand at oyster aquaculture, we are partnering with the Chesapeake Bay Maritime Museum to put on an Oyster Gardening workshop on Saturday, November 1st from 10 to 1 pm. You can learn more and find the link to register by clicking here.
References:
[1] https://www.sciencedirect.com/science/article/abs/pii/S0308597X19305433
[4] https://www.umces.edu/oysters/oyster-culture-facility
[5] https://www.mdsg.umd.edu/sites/default/files/2019-12/Maryland%20Aquafarmer%20-%20Spring%201995.pdf
[6] https://extension.umd.edu/resource/maryland-shellfish-aquaculture-industry-2022-glance-fs-2023-0702/