There are more parasites in oyster sanctuaries. Here’s why that’s a good sign.

Oyster sanctuaries, like those in the Chesapeake Bay, are working, according to a new study — but not just for the bivalves.

Biologists with the Smithsonian Environmental Research Center found evidence that sanctuaries in tributaries of the bay have higher populations of oysters and other marine life, including two common parasites. Notably, the presence of those parasites is not preventing more abundant numbers of other species.

“It’s a good line of evidence to show the restoration efforts are leading to more oysters and more complex habitat,” said Allison Tracy, one of the study’s authors. The study was published July 4 in the journal Marine Ecology Progress Series.

The growing oyster population went “hand in hand” with an increase in the parasite population, according to the Smithsonian. The study looked specifically at the boring sponge and the mud blister worm. Both are considered “likely” native to the Chesapeake and neither is considered harmful to humans.

However, both make oysters ugly and perhaps unappealing to consumers. They infect the oyster shells, but not the tissue on the inside that people eat.

“They’re cosmetic from the perspective of the consumer,” Tracy said. “They don’t kill the oyster, we think.”

And though the parasites make the shells unappealing, researchers said they could ultimately be a good thing. Parasitism is “the most common lifestyle on Earth,” according to the study’s authors; ecosystems need parasites to function properly.

“The sanctuary programs appear to be working and facilitating oyster reef regrowth after so many decades of overharvesting,” the study’s lead author, Zofia Anchondo, said in a statement.

The study focused on three Chesapeake tributaries — the Choptank River, the Great Wicomico River and the James River — which each have an oyster sanctuary and harvest area. The researchers used GoPro cameras to evaluate and compare the oyster sanctuaries in each tributary to harvest areas.

The sanctuaries in each tributary had higher densities of “legal” or harvest-sized oysters than each tributary’s harvest site, according to the researchers. And in the Great Wicomico and James rivers, oysters of all sizes were more abundant in the sanctuaries.

The oyster population in the bay is, by some estimates, at just 1% of its historical levels, according to the Smithsonian. Oysters were once so abundant in the bay that their reefs became navigational hazards, according to the University of Maryland Center for Environmental Science.

In addition to boosting the economy, oysters filter the water and remove pollution. They also provide a habitat to other marine life, and oyster reefs can protect shorelines from erosion.

“For the average person who cares about the health of the Chesapeake Bay, it’s [this research] essential,” Tracy said. “We want to be restoring these populations, so these populations are sustainable into the future.”