85% of the World's Oyster Reefs Are Gone (and the Remaining 15% Are Imperiled)

Photo: James Noble / Istock

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Oysters are a true superfood, offering healthy portions of both iron and Vitamin B12, but overharvesting, contamination, ocean acidification and a host of other environmental threats make their place on our plates perilous.

According to a new study by the Nature Conservancy, 85% of oyster reefs have been lost worldwide. That's right: Only 15% of the world's oyster reefs remain. In most individual bays around the world, the rate of decline is even worse, exceeding 90%, and in many cases -- particularly in North America, Europe and Australia -- oysters are "functionally extinct."

Besides the obvious -- they're good to eat -- oyster reefs are important to shoreline ecology, filtering water, providing habitat for other fish and marine life, and buffering beaches from erosion.

"We’re seeing an unprecedented and alarming decline in the condition of oyster reefs, a critically important habitat in the world’s bays and estuaries," said Mike Beck, senior marine scientist at The Nature Conservancy and lead author of the report. "However, realistic and cost-effective solutions within conservation and coastal restoration programs, along with policy and reef management improvements, provide hope for the survival of shellfish."

The cause of this decline? The Nature Conservancy identified several important factors:

• Overharvesting and destructive bottom-trawling fishing practices
• Overdevelopment along coasts
• Declining water quality and altered water flow from upstream sources

A separate report, published today in PLos ONE by Smithsonian scientists, cites overfishing and disease as the prime causes of decline in shellfish -- but raises an alarm about a new and growing threat: ocean acidification.

Ocean acidification is caused by the absorption by the oceans of carbon dioxide pollution, which causes the pH to drop. The resulting carbonic acid inhibits the formation of carbonate shells and at highly acidic levels can even dissolve existing shells. Scientists have been trying to generate public awareness about this growing threat -- distinct from but related to global warming, because both issues share the common cause of burning fossil fuels. (A comprehensive report made this dire prediction: Oceans are returning to a primordial stew.)

Larval oysters -- particularly the Eastern oysters native to the east coast of North America -- are particularly susceptible to acidification, according to the report. Scientists grew oysters in water of varying acidity reflecting pre-industrial conditions as well as conditions expected in the next 100 years.

Under future acidic conditions, Eastern oysters experienced a 16% decrease in shell area and a 42% reduction in calcium content. Suminoe oysters, found in Asia, were more resilient.

"In the Chesapeake Bay, oysters are barely holding on, where disease and overfishing have nearly wiped them out," said Whitman Miller, the lead author, noting that native Chesapeake oysters have experienced a 98% decline since pre-Colonial times. "Whether acidification will push Eastern oysters, and the many species that depend on them, beyond a critical tipping point remains to be seen."

In addition to shellfish, ocean acidification also threatens plankton -- at the base of the marine food chain -- and coral. Added up, the threat could decimate the ocean ecosystem, particularly when coupled with overfishing, pollution, warming temperatures and other serious threats.

Last summer, scientists convened by The Nature Conservancy, agreed to the "Honolulu Declaration on Ocean Acidification and Reef Management." Here are some of its key elements:

1. Limit fossil fuel emissions
2. Build the resilience of tropical marine ecosystems and communities to maximize their ability to resist and recover from climate change impacts, including ocean acidification
3. Create new marine reserves that include those reefs less vulnerable to acidification, and consider climate change as a factor in managing existing reserves
4. Increase money spent on science and develop an international network to monitor and study acidification
5. Decrease other stresses to coral reefs, such as agricultural runoff and overfishing

Congress this year passed a bill that increases research support for ocean acidification. A separate efforts to limit carbon dioxide emissions to combat global warming -- the U.S. has been the top polluter for decades, with China only recently haven claimed the crown -- will also help the problem of ocean acidification.

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