Can Oysters Save Chesapeake Bay?
Not on their own, scientists say. Their numbers are too low and it’s not clear how much nitrogen they break down
Can oysters clean up the Chesapeake Bay? The question, posed by a group of reporters, resonated at a damp hatchery where new generations of baby oysters were preparing for their next mission: helping to rescue the United States’ largest estuary from decades of overfishing, runoff from manure and synthetic fertilizers, and diseases.
These iconic shellfish, appreciated by food lovers worldwide, have long been considered important contributors to a healthy habitat for fish, crabs, and aquatic plants.
But researchers at the Virginia Institute of Marine Science (VIMS) say that oysters alone may not be enough to clean up the 200-mile long bay, whose picturesque watershed is home to over 17 million people.
Oysters can filter up to 100 liters of water per day when they feed and help maintain the vital presence of light and oxygen in water bodies by removing algae and excess nutrients, especially nitrogen, flowing in from agricultural and industrial effluents that have affected the health of the bay.
Decades of pollution have reduced oxygen levels in the bay waters, creating “dead zones” that kill aquatic life. Earlier this week VIMS released a report based on a 10-year study of the bay’s fishes that says dead zones in the Chesapeake Bay are impacting several key fish species such as the Atlantic croaker, white perch, spot, striped bass and summer flounder.
VIMS scientists say the bay is too polluted for its dwindling oyster population clean up all by itself.
“Shellfish can do a lot in terms of reef and habitat for fish, but thinking that because they filter a lot they can clean the bay, well, I think that this has been a bit overblown,” said professor Mark Luckenbach, when we met him at the VIMS campus along the York River, one of several tributaries of the Susquehanna River that flows into the bay. Luckenbach is part of a team of researchers at VIMS who are studying ways to protect the bay habitat.
Part of the pollutants filtered by the oysters return to the water in quantities that vary depending on the conditions of their environment, the researchers said.
“Medium-sized oysters can filter around five liters of water per hour, or over 100 liters a day. But most of what they filter is put right back,” Luckenbach said. “The calculations don’t end with what they can filter, they end with how much nitrogen they actually take out and that is where the real problem comes from.”
Scientists are still working to estimate just how much nitrogen oysters can break down through a process called “denitrification,” which converts nitrogen, a pollutant, into harmless gas in the atmosphere. Recent research has shown that some parts of Chesapeake Bay, with different salinity and depths, can benefit more than others from the presence of oysters.
“When we calculate how much oysters can filter it sounds phenomenal: but when they filter that phytoplankton, only a relatively modest proportion, a 10 to 20 percent of nitrogen, goes in their tissues [and is converted to gas] and the rest of it is excreted,” Luckenbach said.
Around 50 million oysters per year would be needed to effectively reduce nitrogen loads in the water by just 1 percent, he said. That would require placing 100 million oysters in the bay every year (since the mollusks are harvested biannually), a level unimaginable today.
The oyster population in the bay plunged to around half a million a year when, after the late 1950s, two diseases known as MSX and Dermo began ravaging the mollusks. The combined impact of the disease, habitat loss, pollution, and overfishing has whittled down the bay’s oyster population. The oysters’ decline has threatened the survival of a region already harmed by fertilizers and chemicals. An old refinery, which has been shut down years ago, is still visible on the side of the York River opposite the VIMS hatchery.
“All the good oysters were taken to be sold,” said Tommy Leggett, owner and grower of the Chessie Seafood and Acquafarm, who also works for VIMS.
Since the late 1990s, federal and state-backed programs have been launched in an effort to rebuild the bay’s oyster population while promoting sustainable fishery needed for the local economy. VIMS researchers have been adopting different approaches to the problem, including removing lost fishing nets that continue to entangle and kill animals.
Back at the hatchery, Dr Anu Frank-Lawale and his team are breeding oysters that are genetically more resistant to diseases to repopulate reef sanctuaries and to help oyster farmers.
These laboratory-bred, sterile mollusks are called “triploids” because they have three sets of genes instead of the typical two. “We know who their parents are, we know who their grand-parents are and we know who their great-grand parents are,” Frank-Lawale said.
Triploid oysters are also appreciated for their lower mortality rate and for their full, sweet meat, which makes them a treat for both residents and tourists.
The annual “Shellfish Aquaculture Situation and Outlook Report” published by VIMS and Virginia Sea Grant shows that Virginia oyster farmers sold 28.1 million oysters in 2012, a 21 percent increase from the previous year.
“Since 2000 we have produced probably 70-80 million oysters, distributing them on sanctuary reefs throughout Virginia,” said Laggett, showing us some of the mollusks he will provide to the finest restaurants in the region.