Restored oyster reserves host more marine life despite parasites, biologists find

In the campaign to restore the Chesapeake Bay, oyster sanctuaries are among the most contested strategies. But new research suggests these no-harvest areas work, and not just for oysters.

In a new study published on July 4 Marine Ecology Progress SeriesSmithsonian biologists have found that oyster preserves contain more abundant populations of oysters and other animals—and the presence of two common parasites doesn’t stop that.

Oysters form the backbone of the Chesapeake Bay. In addition to injecting millions of dollars into the regional economy each year, they also act as vital habitat and filter feeders that clean the water. But their population has dwindled to about 1% of historical levels. Disease, overharvesting, habitat loss and pollution have all reduced their numbers.

In the past two decades, Maryland and Virginia have worked to restore their oysters by creating extensive networks of refuges where oysters are protected from harvest. This led to a recovery in oyster growth, habitat quality and biodiversity in the refuges, a new study found.

“Refuge programs appear to be working to facilitate the regrowth of oyster reefs after so many decades of overharvesting,” said lead author Zofia Anchondo, who conducted the research as part of her graduate residency at the Smithsonian Environmental Research Center and the University of North Carolina at Chapel Hill.

But at the same time, the revival of marine life went hand in hand with an increase in oyster parasites. The study looked at two parasites in particular: the boring fungus and the earthworm.

Boring fungi drill holes into oyster shells to find shelter. Blisterworms make U-shaped burrows inside the shells. The presence of either can make oysters unsightly or even unsalable. Both are considered likely to have originated in the bay, and neither is harmful to humans. For this study, the researchers did not look at dermo or MSX, two non-native parasites responsible for some of the worst oyster crashes, although they collected data for a follow-up study.

Parasitism is the most common lifestyle on Earth, so the presence of a parasite is not necessarily a sign that the environment is unhealthy, the authors noted.

“Parasites have been overlooked as an important part of biodiversity,” said co-author Allison Tracy of the University of Maryland, Baltimore County and the University of Maryland School of Medicine. “But they are a natural part of ecosystems… The way ecosystems function naturally depends on the effects of parasites.”

“It doesn’t prevent high oyster densities,” said Matt Ogburn, co-author and senior scientist at the Smithsonian Environmental Research Center. They may even be good for long-term oyster development, Ogburn added, although that question requires more detailed research. “Restoring oyster reefs is likely to help the oyster population become more resistant to the parasites that are there now compared to what would be possible if everything was harvested all the time.”

The new study focused on three tributaries of the Chesapeake: the Choptank River, the Great Wicomico River and the James River. Each had its own oyster reserve and, for comparison, another harvest area where watermen and watermen could freely harvest oysters.

The researchers used footage from underwater GoPro cameras to assign each reef a habitat “score” (one to four, based on percentage of oyster cover and vertical structure). The GoPro videos also allowed them to record other animals visiting the oyster reefs. Divers, working under a state research permit, later collected some of the oysters from each reef to estimate oyster density and look for parasites.

Overall, oysters fared better in the sanctuaries. All three tributaries had higher densities of harvest-size legal oysters in their refuges than in their harvest sites. In two tributaries—the James and Great Wicomico rivers—oysters of all sizes, including juveniles and juveniles, were more abundant in refuges.

The sanctuaries also scored higher for habitat quality, and videos captured plenty of underwater animals taking advantage of them. Blue crabs, rockfish and summer flounder were just some of the species that flocked to the sanctuary’s reefs. In the James and Great Wicomico rivers, researchers estimated that refuges host 10 times more animals and nearly twice the number of species than harvest reefs. The only exception was the Choptank River, where animal life was low in both refuge and harvest sites.

Yet as the oysters thrived, so did two parasites. More than half of the oysters at all six sites—refuge and harvest—had telltale holes from the boring fungus attack. However, the boring fungus was more prevalent in the sanctuaries than in the harvest sites in two of the three tributaries. Mudworms were much less abundant—infecting 2–10% of oysters—but still more abundant in refuges than collection sites in the two tributaries.

The authors pointed out that none of the parasites are dangerous for humans. Both parasites only infect oyster shells, not the oyster tissue that humans eat.

“It’s more of a concern for fisheries because they can make the shells less attractive,” Tracy said. “It may reduce the value of oysters for the half-shell market. But they have no effect on our health.”

In the unlikely event that a person accidentally slurps one of these parasites with their raw oyster, Tracy said, it would simply pass through their system without impact: “There’s no shell for it to find to burrow into your stomach. , it would be uninteresting.”