Biologists discover restored oyster sanctuaries harbor more marine life despite parasites

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

In a new study published July 4 in Marine Ecology Progress SeriesBiologists at the Smithsonian have discovered that oyster reserves support larger populations of oysters and other animal life. And the presence of two common parasites is no obstacle.

Oysters are the backbone of the Chesapeake Bay. In addition to pumping millions of dollars into the regional economy each year, they also serve as vital habitat and filter feeders that clean the water. But their populations have dwindled to about 1% of historic levels. Disease, overfishing, habitat loss, and pollution have all decimated their numbers.

Over the past two decades, Maryland and Virginia have worked to restore their oysters by creating extensive networks of sanctuaries where oysters are protected from harvest. This has led to a rebound in oyster growth, habitat quality and biodiversity within the sanctuaries, the new study reports.

“The reserves appear to be working, allowing for oyster reef regrowth after decades of overexploitation,” said lead author Zofia Anchondo, who conducted the research as part of her graduate fellowship 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 has gone hand in hand with an increase in oyster parasites. The study looked at two parasites in particular: the boring sponge and the mud blister worm.

Boring sponges bore holes in oyster shells to find shelter. Blister worms form U-shaped burrows in the shells. The presence of both can make oysters unsightly or even unmarketable. Both are likely native to the bay, and neither is harmful to humans. For this study, the researchers did not look at dermo or MSX, two non-native parasites that have been blamed for some of the worst oyster crashes, although they did collect 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 not in order, the authors emphasize.

“Parasites have been ignored as an important part of biodiversity,” said Allison Tracy, a co-author from 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 naturally function is dependent on parasite effects.”

“They’re not preventing high oyster densities,” said Matt Ogburn, a co-author and senior scientist at the Smithsonian Environmental Research Center. They may even be good for the long-term evolution of oysters, Ogburn added, though that question needs more definitive research. “Restoring oyster reefs is likely to help oyster populations become more resilient to the parasites that are there now, compared to what they might be 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 a different harvest area for comparison, where fishermen were free to harvest oysters.

The scientists used footage from underwater GoPro cameras to give each reef a habitat “score” (one through four, based on percent oyster cover and vertical structure). The GoPro videos also allowed them to record other animals visiting the oyster reefs. Divers, working under state permits for research, later collected a few oysters from each reef to estimate oyster density and search for parasites.

In general, oysters fared better in the sanctuaries. All three tributaries had higher densities of legally harvested oysters in their sanctuaries than at their harvest sites. In two tributaries, the James and Great Wicomico, oysters of all sizes, including young and baby oysters, were more abundant in the sanctuaries.

Reserves also scored higher on habitat quality, and the videos captured many marine animals using them. Blue crabs, rockfish, and summer flounder were just a few of the species that moved to reserve reefs. In the James and Great Wicomico rivers, researchers estimated that reserves supported 10 times as many animals and nearly twice as many species as harvest reefs. The only exception was the Choptank River, where wildlife was low in both the reserve and harvest sites.

But while the oysters thrived, so did the two parasites. More than half of the oysters at all six sites (sanctuary and harvest) had the telltale holes of a boring sponge attack. However, boring sponge was more common in sanctuaries than in harvest sites within two of the three tributaries. Mud bladder worms were much less abundant (infecting 2-10% of oysters), but still more common in sanctuaries than in harvest sites for two of the tributaries.

Neither parasite, the authors pointed out, is dangerous to humans. Both parasites infect only oyster shells, not the oyster tissue that people eat.

“It’s more of a concern for the fisheries because they can make the shells less attractive,” Tracy said. “It can reduce the value of the oyster for the half-shell market. But they don’t affect our health.”

In the unlikely event that someone accidentally slurped up one of these parasites with their raw oyster, Tracy said, it would simply pass through their body without impact: “There’s no shell it can find to bore into your stomach. So basically it wouldn’t be interested.”