Heart of the Chesapeake
Monday, August 1, 2005
By: Michael Bhargava

As keystone species, oysters fundamentally shape the environment in which they live. Without them, the Chesapeake Bay's health is at serious risk.

It is a rite of spring. As estuaries on the East Coast begin to warm each spring, a few adventurous American oysters expel millions of eggs into the surrounding water. Taking the cue, billions of other oysters quickly follow suit and release their sperm and eggs, clouding the water with potential new generations of oysters.

Those watery clouds that mark the coming of spring, however, have been noticeably smaller in recent years. American oysters, also known as eastern oysters, are at an all-time low in the Chesapeake Bay and elsewhere on the east coast of North America. Historical overfishing, pollution, and two deadly diseases have all taken their toll on oyster populations.

As a result, oyster populations have fallen 99 percent from historic populations. The absence of American oysters (Crassostrea virginica) has dramatically affected the Chesapeake Bay, which is the epicenter of concern for oyster populations. A keystone species, oysters fundamentally shape the environments in which they live. They protect shorelines from erosion. They provide structure on which other animals can settle and find shelter. And, most critically, they filter the water of algae and other nutrients, keeping the water clean and clear. Without the oysters, the environment of the Chesapeake has become critically degraded.

Full Service Ecosystem

Like clams and scallops, oysters are bivalves. They feature two protective shells tightly connected by a single adductor muscle. After passing two to three weeks in the larval stage floating in the water, the oysters settle and search for a hard surface on which they can make their homes. The newly settled oysters are known as spat. The oysters tend to gather together in clumps, creating oyster reefs that provide texture to the estuary floors.

The Swiss writer Francis Louis Michel commented on the remarkable size of those oyster reefs in the Chesapeake Bay in 1701.

“The abundance of oysters is incredible,” he wrote. “There are whole banks of them so that the ships must avoid them. A sloop, which was to land us at Kingscreek, struck an oyster bed, where we had to wait about two hours for the tide.”

Oysters are filter-feeders. To feed, they relax their adductor muscles to slightly open their shells. Specialized hair-like cilia pump water into the shell, where other cilia trap particles of algae and other nutrients and push them toward the oyster’s mouth deep inside the shell.

As they age, American oysters often change gender from male to female. During their first year of life, American oysters generally release sperm into the water. But as they grow bigger and develop more energy reserves, they become female and release eggs, which are more complex and difficult for oysters to grow.

Like other oysters, the eastern oyster can create pearls if a grain of sand becomes wedged between its shell and the fleshy layer of tissue called the mantle, which secretes the shell. The oyster covers the intruding sand with layers of new shell and creates a pearl. However, unlike the stunning Tahitian pearls that come from black-lipped pearl oysters, the pearl of the American oyster has little luster and no commercial value. Although they have little value as pearl-producers, eastern oysters are invaluable for the ecosystem services that they provide.

Imagine a house that cleans itself. That is essentially what oysters do for their ocean habitat. Oyster beds provide shelter to dozens of species that hide in the three-dimensional reefs. The complex nooks and crannies of oyster reefs can provide 50 times more surface area than a flat bottom can. Barnacles and sea anemones root themselves onto the shells of oysters. Meanwhile, worms, sea squirts, snails, sponges, and crabs all find shelter within the reefs, hiding from predators. That proliferation of life attracts dozens of fish species that find plentiful food around the oyster reefs.

At the same time as they provide shelter, oysters also clean the surrounding water. When they were at peak abundance, American oysters could filter all the water in the Chesapeake Bay every 3.3 days. In their search for food, oysters remove algae and other nutrients from the water, making it visibly cleaner. The clear water allows sunlight to penetrate to the estuary bottom, where sea grasses and other plants can grow in the light and create complex ecosystems where none could grow before.

Historical Decline

Native Americans harvested oysters for thousands of years across their range from the Gulf of St. Lawrence in Canada down to Florida. Colonial settlers quickly followed suit and began harvesting the oysters, creating a thriving industry.

Harvests of oysters peaked in the Chesapeake Bay in the 1870s and 1880s, as oystermen from New England streamed south seeking new oyster beds after depleting those in the north. By the late 1880s, the Chesapeake Bay was the world’s leader in oyster production, with more than 10 million bushels of oysters being taken from the Bay every year. One fifth of all Americans employed in fishing harvested oysters in the Chesapeake.

This extraordinary level of fishing, combined with the introduction of new technologies, soon ruined the fishery. Unsatisfied with using hand tongs or rakes to harvest the oysters, fishermen soon began using dredges (which were legalized in 1865) and hydraulic machine tongs. The dredges not only collected vast numbers of oysters, but also destroyed the reefs on which the oyster spat settled. As a result of this overfishing and habitat destruction, oysters in the Chesapeake began a steady decline, plummeting by 60 percent between 1880 and 1930. In the 1920s, Maryland and Virginia began experimenting with artificially planted oysters in the Chesapeake Bay in order to resurrect the dwindling oyster supply. The programs had limited success, in part because fishing pressures continued, and in part because of the emergence of two other threats to the oysters.

Water pollution was one such threat. As the land surrounding the Chesapeake developed, its waste got dumped directly into the watersheds. Agricultural fertilizer, sewage, and animal waste entered the Chesapeake, creating algal blooms that overwhelmed the remaining oysters. Toxic chemicals from industry killed free-floating larvae before they could settle on the estuary bottoms. And erosion from farms buried oyster beds in silt.

The second threat came in the form of two parasites that devastated already compromised oyster populations. In the 1950s, a protozoan parasite carrying a disease known as MSX began spreading through oyster populations across the East Coast. The parasite began in Pacific oyster populations in Asia, but was accidentally introduced to the United States, where it devastated American oyster populations. By the 1970s, it had significantly reduced oyster populations in the mid-Atlantic. Scientists discovered another parasite carrying a deadly disease known as Dermo in 1949 in the Chesapeake. The parasite originated in the Gulf of Mexico but had spread as far north as Delaware Bay by 1990. As the climate warmed, the parasite continued to migrate north and infect more oyster populations. It can now be found as far north as oyster beds in Maine.

Because of all these threats, oyster populations hit their low point in 2002, by which time 99 percent of the oysters that once lived in the bay had vanished. Whereas in the 1870s oysters could filter the entire Chesapeake in 3.3 days, by 2002 it took them more than a year.

Bringing Back the Oysters

Environmental managers currently face a catch-22 in conserving oysters. If oysters returned to the Chesapeake in large numbers, they could help clean pollution in the Bay by filtering out excess algae and other particles. However, American oysters cannot return in part because the Bay is so polluted. As a result, environmental managers have been forced to aggressively reintroduce oysters into the Chesapeake. The Chesapeake 2000 agreement—signed by Virginia, Maryland, and Pennsylvania—committed those states to increase the number of oysters in the Bay tenfold by 2010.

To meet that goal, scientists have created sanctuaries to encourage oyster growth and to prevent harvesting. The sanctuaries restore the estuary habitat by providing oyster spat with hard substrate on which to settle. Environmental managers have built artificial reefs in the Chesapeake with discarded oyster shells and other suitable materials in areas that traditionally had been productive oyster grounds.

Both Maryland and Virginia are actively encouraging oyster farming. Oysters lend themselves well to aquaculture, in which protected beds are artificially seeded with larvae. Aquaculture allows managers to select the oyster for size, gradually increasing the amount of meat that can be harvested from the oysters through selective breeding. In 2002, Maryland instituted a tax credit of $500 for anyone who purchased an oyster float to farm oysters. The floats allow the oysters to grow in the middle of the water column rather than on the estuary floor where MSX and Dermo may lurk. As a result, the float-farmed oysters may be less susceptible to those diseases.

Managers are also now experimenting with importing non-native oysters into the Bay, although this approach is controversial. Many hope that imported oysters will more aggressively recolonize the Bay and jumpstart a future profitable harvest of oysters. In the 1990s, Virginia first began importing the Pacific oyster from Asia (the original source of MSX) for implantation in the Bay. However, results were disappointing, since the oysters showed only modest growth and high mortality. More recently, scientists have been studying the more prolific Asian Suminoe oyster. In studies so far, the oyster has shown less susceptibility to MSX and Dermo, as well as more rapid growth than the native American oyster. As a result, commercial oyster harvesters have pushed hard to force the states to increase the importation of Suminoe oysters for implantation into the Chesapeake.

Introducing non-native species, however, brings tremendous risks. Many scientists fear the introduction of further disease. Although scientists have taken measures to prevent an invasion, a lethal pathogen could easily slip through. Second, many fear that Suminoe oysters will become an invasive species that crowds out native oysters and other species. This could lead not only to the extinction of the American oyster in the Bay, but also to the fouling of boats, pipelines, and marinas with millions of zealous oysters. Zebra mussels area a prime example, having created enormous headaches in the Great Lakes and Mississippi River drainage since their introduction in 1986.

So far, scientists have introduced only sterile Suminoe oysters into selected areas of the Chesapeake, but since the process is not 100 percent effective, fertile oysters could mistakenly be introduced and breed on their own. In 2003, the National Research Council of the National Academy of Sciences completed a study on the introduction of non-native oysters into the Chesapeake. It concluded that introducing sterile Suminoe oysters was not a panacea to the oyster industry or Bay restoration, and that further study was necessary to determine the impacts of introducing non-native species before they enter the Bay.

Environmental managers still have a long way to go before they revive the Chesapeake Bay, reducing pollution levels and siltation from upstream erosion. But they now know oysters are a critical component. In the future, the clouds of sperm and eggs that the oysters release every year will be not only a rite of spring, but also a sign of the Chesapeake’s rebirth.

Michael Bhargava is a freelance writer based in California.