How to Serve Oysters
Improving water quality by restoring habitat
By David C. Richardson
In the summer of 2012, researchers and staff at Horn Point Laboratory on the Choptank River did more to serve oysters on the Chesapeake Bay than perhaps anyone has ever done before.
For eons, back to a time long before some of the earliest human pioneers trekked across the Bering Strait to set foot upon North America, the Chesapeake Bay has provided ideal habitat for one of nature’s oldest and oddest critters: the eastern oyster (Crassostrea virginica). By the 1600s, when Captain John Smith sailed furtively up the Chesapeake’s many tributaries and backwaters, the eastern oyster had already established itself in the briny waters, providing a rich source of protein to the original settlers of the mid-Atlantic region.
According to 18th century sources, oyster shoals in various places broke the surface, and natural oyster reefs were considered a navigational hazard in the bay. Some scientists today argue that up until the recent past, oyster reefs may have been the dominant ecosystem on the coastal waterway, but not anymore.
Following the over-exploitation and demise of New England’s oyster fisheries in the years following the Civil War, the mid-Atlantic’s Chesapeake Bay assumed the burden of supplying oysters to the country’s burgeoning seafood canning industry. By the end of the 1800s, fishermen on the bay were harvesting 15 million bushels of oysters annually from the Maryland portion of the bay alone. And harvesting in the Chesapeake Bay accelerated through much of the next century with very little oversight. The oyster population collapsed.
Mike Naylor, who heads Maryland Department of Natural Resources shellfish program says, “It’s inconceivable to imagine how many oysters there must have been to sustain multiple millions of bushels of harvest over many decades—but the same thing that played out in the Chesapeake Bay played out in New York’s estuaries, and other estuaries around the world. You had prolonged exploitation, until you reached a level where the population just crashed, and harvests declined dramatically.”
Over-harvesting, habitat destruction, and diseases introduced by commerce and travel took their toll. Scientists say what remains of the oyster population is a remnant amounting to less than 1% of its 17th century norms.
Authorities expected the 2012 harvest to bring in at maximum just a few hundred thousand bushels, a figure that by today’s standards, contrasting with the millions of bushels annual harvest in the past, is considered a large haul.
Roger Newell, professor at University of Maryland Center for Environmental Science (UMCES) who studies oysters intensively, says with oysters so conspicuously absent, the bay is not at all what it used to be. Newell, along with experts from the UMCES Horn Point Oyster Hatchery and the Oyster Recovery Partnership, believe it might be a good time to start putting some oysters back in.
|Photo: David C. Richardson
Oysters ready to spawn in the lab at Horn Point Hatchery
Needless to say, over four centuries a lot has happened to the bay besides the loss of oysters. The largest marine estuary in North America, the Chesapeake Bay drains the Susquehanna River watershed, stretching from New York State to the coastline of southernmost Virginia. It’s an area that over 400 years has seen an exponential increase in human population, booming from a few hundred thousand when Smith arrived to 16 million residents today. And it’s still growing.
Along with the population growth, urbanization, large-scale farming, and industry have reshaped the waters flowing into the Chesapeake in almost every way imaginable: altering the surface contours of the watershed, the plant and animal communities along its banks, and the chemical makeup of the water it receives. Introduced species, ranging from fishes to microbes, have wriggled their way into valuable niches, often with devastating impact on native flora and fauna. Yet, Newell and the members of the Oyster Recovery Partnership believe restoring the oyster to its once prominent position in the ecosystem could be a major step in guiding the bay toward a much healthier future.
As in many aquatic ecosystems, one of the central problems facing the Chesapeake Bay is eutrophication due to an overabundance of the nutrients nitrogen and phosphorus. Aside from the natural sources of these inputs, such as decaying leaf matter deposited from forests in the fall, and atmospheric deposition, the bay receives an extra share of these nutrients from anthropogenic sources, including fertilizer-laden runoff from agricultural lands and lawns, sewage and septic discharges, and even exhaust fumes from motor vehicles.
According to the Chesapeake Bay Foundation, each year, roughly 300 million pounds of “polluting nitrogen” reaches the Chesapeake Bay, generating a cascade of environmental problems.
“Nutrient enrichment has contributed to widespread changes in coastal habitats, including loss of sea grasses, and depletion of dissolved oxygen in bottom waters,” writes Roger Newell in a 2005 article titled “Eutrophication of Chesapeake Bay: Historical Trends and Ecological Interactions.” These habitat effects have altered the mix of species and interfered with the natural food chain and composition of fish and invertebrate communities through a range of ecological mechanisms, threatening to turn a thriving, productive, and economically sustaining ecosystem into a virtually moribund monoculture of stress-tolerant, but less than desirable, flora and fauna.
Phytoplankton, on the other hand, thrive on the excess nutrients. According to Newell and colleagues, in some instances, eutrophication-induced shifts in phytoplankton communities involve “enhanced growth of algae species that cause direct harmful effects, including production of toxins, noxious discoloration, and floating mucilage.”
Newell explains, “Historically, there was not much algae in the Chesapeake Bay—there was not much nutrient coming into the bay. But now, we’ve put a lot more nitrogen and phosphorus in the bay, so now the bay is producing way too much phytoplankton.”
As the huge masses of phytoplankton algae die off at the end of their life cycle, the bacteria that break down this decaying plant material can draw so much oxygen from the water that they generate suffocating dead zones. In the deeper channels of the bay, these hypoxic dead zones are capable of snuffing the life from fish and other aquatic organisms that venture near.
Although Newell says the dead zones have been observed from time to time since the 1930s, in recent years they seem to be occurring over ever-larger areas of the bay and lasting longer. He says the bay’s difficulties with nitrogen may be on track to intensify as time goes on, explaining that the increasing frequency of hypoxic zone events may indicate “the bay has become less able to assimilate nitrogen inputs without developing hypoxia, a change that may have arisen from the degradation of key ecological processes sensitive to eutrophication effects.”
Along with restoring habitat such as sea grasses, he says, oysters could play a key role in mitigating that trend.
Oysters occupy themselves by eating. They dine almost continuously, feeding for up to 23 hours per day. They feed on virtually anything small enough to get past their gills and cilia. An individual oyster, Newell says, is capable of filtering a gallon of water every hour. Filtering these huge volumes of water daily, a healthy population of oysters can play a major role in controlling phytoplankton in the bay.
During the late 19th century, oyster stocks in summer months were capable of filtering the entire water column of the bay, from surface to bottom, in three to six days. The depleted populations of today would require approximately 300 days to accomplish the same feat.
However, Newell says under contemporary conditions in the bay, the oysters “actually filter a lot more food than they need, because they’re eating from the buffet, whereas before they were rooting around in the field for a few nuggets of food.”
Nonetheless, they do provide an immediate ecological benefit by incorporating some of the nitrate and phosphorus nutrients from the phytoplankton they consume in their tissues, removing it temporarily from the water column. Of course, when they die and decompose, they release the nutrients again. However, if some predator consumes the oyster, then its quotient of nitrogen or phosphorus might be removed from the water on a more permanent basis, winding up near a sea bird’s roost, an animal den, or perhaps on the dinner plate. But Newell says, “I think it is important that people recognize that it is sometimes more valuable to leave them in the water than to actually eat them.”
That’s because other than nourishing themselves, oysters provide a second way of removing excess nutrients from the water
Newell says oysters “are actually rejecting a lot of material they filter from the water column without digesting it, because they don’t need it.” This half-eaten material, called pseudofeces, is concentrated in tiny pellets by the mollusks and drifts down to the bay floor, where it comes into contact with the biology and chemistry in the lower depths. There, natural biological processes—similar to those designed by engineers for sewage treatment plants—reduce the nutrients in the pellets to the relatively inert nitrogen gas (N2). Once it has been converted to N2, the nitrogen is no longer available to stimulate phytoplankton growth, interrupting the eutrophication process and potentially slowing down the cycle of aquatic degradation. Newell says, “It’s a secondary means by which a healthy population of oysters can remove the nitrogen from the aquatic environment.”
According to Newell, the potential benefit of this denitrifying process may hinge on where currents carry the pseudofeces from a particular oyster bed, a topic he and colleagues plan to explore in future studies.
|Photo: David C. Richardson
Workers with the Oyster Recovery Partnership deliver oyster shells collected from restaurants
and processors to age in the summer sun.
But repopulating the bay with oysters will take another level of understanding—that between the oysters themselves. And it generally means sparking a little commitment from the oysters, be they male, female, or something in between (individual oysters of some oyster species have been observed to change their gender several times over a lifespan. Eastern oysters, scientist say, begin life as females, with some portion of the population transforming into males as they mature).
Dr. Don Meritt, the aquaculture expert managing the oyster breeding operation at the Horn Point Hatchery in Maryland, says the facility tries to keep pace with the gonadal cycle of the oyster. Although it may sound tricky, according to Meritt, the oysters’ mating behavior “is largely controlled by the water temperature.”
“During the winter, oysters essentially hibernate,” he says. “They don’t even pump, they don’t filter, they don’t feed, they don’t do much of anything—they just lie there.” But as the water warms in the spring, Meritt says, the oysters “wake up, they start to pump and filter and feed—and as it gets a little bit warmer, they start to produce either eggs or sperm.”
When the water reaches the optimum temperature for spawning, from 28 to 30°C, the oysters broadcast those eggs and sperm out into the water. Triggered by pheromones released by the first few oysters to spawn, the neighboring oysters follow suit, and fertilization occurs external to their bodies.
At Horn Point, technicians use temperature-controlled baths to prime oysters gathered from the bay for breeding. Once conditions in the indoor sluice simulate the outdoor environment of early to mid-summer, technicians pluck the oysters from their wash and submerge them neatly in rows among their ilk, in a set of stainless steel tubs. Although the setting might lack some of the charms to which oysters in the wild might be accustomed, Meritt says they soon begin to spawn, releasing their sperm or eggs into the warming waters.
At that point it is possible to distinguish between oysters that are spawning as males and those spawning as females, and for the sake of efficiency, the two sexes are sorted accordingly. Later, technicians mix the sperm with the eggs under more controlled conditions and transfer the resulting zygotes to large oyster larval tanks to “grow up.” There, the progeny of Horn Point’s pairings find their first home swirling through algae-stocked pools in the warehouse-sized hatchery.
For their first few weeks of life, the oyster larvae pass through the stage of free-swimming zooplankton. Bobbing under the influence of the tank’s aerators and their own microscopic cilia, without need for prompting, the larvae begin the characteristic feeding behavior that will carry them through life.
|Photo: David C. Richardson
Technicians count oyster spat on a sampling of shells to estimate hatchery production.
After three to four weeks gorging themselves on algae, the larvae reach a size of between 70 to 300 microns, signaling that it’s time for them to find a permanent home. And Meritt says finding new homes for the hatchery’s young wards constitutes the first “major bottleneck” in the oyster recovery effort.
Their favorite habitation is clean oyster shell on the bay floor left behind by a previous generation of shellfish. Attaching themselves to these older shells to spend the rest of their lives, the young oysters, known at this stage as spat, build up the oyster reefs that provide habitat and nurseries for many aquatic organisms over many generations. However, due to haphazard harvesting, and the dredging and exploitation of native shells for use as raw materials for pavements and other products in historic times, naturally occurring reef habitat is in short supply.
To overcome this obstacle, Horn Point Hatchery has partnered with several organizations to obtain clean shells in the large quantities needed for restoration projects. Along with the Oyster Recovery Partnership and the Shell Recycling Alliance, the hatchery has forged ties with seafood processors, restaurants, and shuckers. While there may be some competing demand for shells to supply commercial oyster farming operations, Meritt says the partnership members are happy to participate in restoration efforts by contributing shells that would have otherwise gone to waste.
Once the spat glue themselves to the shell to begin their transformation into miniature oysters, it’s time to set the shell out in the bay. That step requires careful planning.
If there’s one thing oyster spat don’t like, it’s a squishy bottom. Meritt says there’s little hope for survival among oyster spat that land face down in a silted creek.
Members of the Oyster Recovery Partnership go out before all the plantings and pre-survey potential sites “to make sure the bottom is rocky or covered with shell” rather than silt or mud, which would suffocate the newly planted oysters, explains Naylor of the Maryland DNR.
|Photo: David C. Richardson
These flasks of algae will help provide a balanced diet for the hatchery’s oyster larvae.
According to Naylor, at times the project goes to great measures to target prime spots for planting. He said at Harris Creek, a tributary of the Choptank River and one of the most promising candidates for restoration, the state has established an oyster sanctuary. To support the effort, the National Oceanographic and Atmospheric Administration (NOAA) provided precise sonar surveys to help select the key sites for planting. Using GPS as a guide, the US Army Corps of Engineers restoration division carefully laid down a pervious substrate of granite and shell in shallows of the channel to provide a base for plantings. “Having the bottom as good as you can possibly get it is the key, so when the shells do fall down the mortality is minimized,” he says.
Naylor is appreciative of the support the program has garnered from the community. “There is not a lot of large-scale restoration going on in the United States because of the funding situation. We are fortunate that the state legislature has been able to put forward $7.5 million for the effort.”
And he is optimistic. “Two years of funding is likely to result in complete restoration of the Harris Creek tributary to the Choptank River, making it possibly the first tributary in the Chesapeake Bay that’s been restored, ever. It has the potential to be extremely significant and create a blueprint for how future projects are done,” he says.
In one of its biggest spawning seasons to date, in 2012 Horn Point Hatchery successfully introduced over 880 million oyster spat to the Chesapeake and its tributaries. Meritt says these colonies of oysters are destined to be more than just filter feeders. If successful, they anchor a community, creating habitat for blue crabs, rockfish, and other economically and ecologically important species, and over successive generations will build up a substantial living reef.
But Naylor says the payoff will continue far into the future and far afield, when descendants of hatchery-spawned oysters themselves begin breeding. According to Naylor, models generated by the University of Maryland suggest that oyster larvae from Harris Creek will eventually “make their way across the bay to settle on the western shore, and all the way up the eastern bay and pretty much throughout the entire Choptank River.”
Although he acknowledges the waters of the Chesapeake Bay have been so dramatically altered that “no one even to dreams of restoring the population to those historic levels,” he says, “We have a hope of restoring the population back to what it was a few decades ago, which would be a massive improvement over what exists today.”
Nonetheless, Newell says he would like to see a continued emphasis on preventing nitrogen from getting into the receiving waters by cutting back on nitrogen fertilizers, through public education and stormwater quality controls. He said, however, bivalve restoration projects can provide a “useful supplement to these broader activities.”
“You can’t blame all the current problems in Chesapeake Bay on the loss of oysters. Obviously, there are a lot of inorganic nutrients that come in there, but oysters were an important component in terms of consuming phytoplankton and removing inorganic silt from the water column and keeping it down on the sediment surface,” he says.
He continues, “It’s really difficult to tease out these sorts off effects, because you’ve got so many thing happening simultaneously—you’ve got the decline in oysters, you‘ve got this increasing population of people in the watershed, and then you’ve got this increasing use of fertilizer from the Green Revolution after the Second World War, when the use of fertilizer started going up dramatically. You’ve got more nutrients coming into the bay producing phytoplankton, and you’ve got the loss of the oysters. It’s truly impossible to say what the magnitude of change in water quality is that can be associated with oysters. It certainly is an important component, whether it’s 15% of the problem or 20% of the problem.”
He says reducing the nitrogen inputs to the bay would be essential even if oysters were restored. “Even if you put 100% of the historical oyster population back in the bay, there would still be a water-quality problem, because we get a lot of phytoplankton blooms in the spring before the oysters become active and feeding, but at other times during the year they can be an important control on the phytoplankton. It’s a really complicated issue, and there has been a lot of debate about this in the literature.”
Restoring the oysters to the Chesapeake should be seen as just a part of a much bigger picture. “This is not a magic bullet that’s going to improve everything instantly; it’s part of a balanced ecosystem,” says Newell.
He wants people to take a closer look at the oyster and its value to society.
“For many generations, people have looked at them only as a food source, but they are in fact a central part of a balanced ecosystem—and that’s true of most animals. If you take an animal out of the ecosystem completely, there are changes in that ecosystem associated with the loss of that animal. And oysters were, at one point in time in Chesapeake Bay, an important consumer of phytoplankton.”
Author’s Bio: David C. Richardson is an award-winning science writer.