Ready for an oysterpocalypse? We should be

Not even oysters will be safe from climate change. And that will include those on your plate.

Just as coral reefs in tropical waters, so coastal ecosystems around the planet are bound to end up suffering badly from the effects of climate change. And as a result the mortality rates of adult oysters will likely grow in years and decades, says a team of researchers from the University of Nantes, the Marine Environmental Science Laboratory in Plouzané, and the Cerfacs (European center for research and advanced training in scientific computing) in Toulouse.

The researchers, who published their findings in the journal Environmental Research Letters, say increased regional climate variability, including an increase in warm and wet winters triggered by global climate change, may well deal a blow to oysters and other residents of coastal marine ecosystems.

And that’s bad news. In addition to performing vital ecological services, marine creatures like oysters are key staples of people’s diets and thus comprise an important part of food security and socio-economic sustainability. Yet, the researchers say, “shellfish aquaculture and fishing are of primary importance but become more vulnerable under anthropogenic pressure, as evidenced by reported mass mortality events linked to global changes such as ocean warming and acidification, chemical contamination, and diseases.”

In order to track the possible future effects of climate change on oyster populations worldwide, the scientists investigated how climate variation impacted mortality rates among oysters along the French Atlantic coast. In so doing, they relied on data collected between 1993 and 2015. They found that mortality rates increased in months that followed warm and wet winters.

The researchers attributed this to the North Atlantic Oscillation (NAO), a fluctuating pattern of weather circulation in the North Atlantic that has been linked with winter storms across Europe. The NAO is a key driver of ecological variations such as individual growth rate in species, as well as their geographical distribution, phenology and survival.

“Benthic species like oysters are keystone species in coastal ecosystems. For example, they build reef habitats, which sustain a high biodiversity, and provide tremendous food source worldwide though fishing or aquaculture activities,” said Dr. Yoann Thomas, from the French National Research Institute for Sustainable Development who was the study’s lead author. “But they are very sensitive to changes in climate and water quality, because they cannot move if a location becomes inhospitable. In this sense, oyster populations are sentinels of long-term climate fluctuations and climate trends, and more broadly of the ‘health’ of coastal ecosystems,” Thomas added.

“We show that recurrent positive NAO episodes in winter, leading to milder temperatures along the northern European coast, have a long-lasting effect on the biological and environmental factors influencing oyster mortality,” the scientist said. “We also show that the cumulative mortality rate over a year significantly increases after winters dominated by positive NAO. From a practical perspective, this lagged relationship can be used for potential predictability of annual mortality at several months lead-time.”

Climate-related mortality risks affecting oysters can be traced back to such environmental factors as the unlocking of the cold-water barrier for pathogens in winter, the shortening of oysters’ resting phase because of warming water temperatures in winter, and increased metabolic rates that make them more vulnerable to diseases and environmental effects in spring and summer. Storms triggered by NAO also cause more fresh water to flow from rivers into the sea, which impacts oysters by lowering water salinity levels.

“What today are exceptional levels of mortality could become the norm by 2035, even if the global temperature increase is limited to ~2°C above the pre-industrial period as per the Paris agreement. Natural long-term climate variability on top of anthropogenic-induced warming could ever accelerate or delay the increasing risk by only a decade or so,” Dr. Thomas said. “The near-future looks bleak, but we show that this will be even worse without a clear reduction of the greenhouse gases emissions by human activities. We obviously need to take rapid action now to avoid further damage to very sensitive and vulnerable coastal ecosystems.”