Ocean Acidification

North America’s battle to save the oyster industry from climate change can inform a similar fight in Senegal

A Taylor Shellfish Farms worker harvests oysters in Oyster Bay, Washington. Like other growers along the US Pacific coast, Taylor Shellfish Farms was badly impacted by an upsurge in ocean acidity levels in 2007, and is now collaborating with researchers on monitoring and mitigation. (Image: Ted S Warren / Alamy)

Editor’s note: Ocean acidification is damaging shellfish from West Africa’s Atlantic shores to the Pacific coast of North America. In this the final instalment of a two-part series, Dialogue Earth looks at how lessons learned on the US west coast might help. Read part 1, about Senegal’s shellfish sector, here.

Oregon and Washington are at the centre of the shellfish trade on the west coast of the US. In 2007, the industry was valued at USD 111 million when it came close to collapse.

It began with a mystery: oyster larvae were dying in their millions in the coastal hatcheries that supply surrounding shellfish farms. Farmers and scientists initially believed they were being ravaged by a bacterial disease. It took a few months to discover the true culprit: the ocean water they had been pumping into their tanks had become more acidic.

Fully grown oysters being processed at an Oregon Oyster Farms facility in Newport (Image: Lynn Ketchum / Oregon Sea Grant, BY NC SA)

This change was due to the overlap of human-caused CO2 emissions and a seasonal upwelling phenomenon called the California Current system.

Winds blow towards the equator during the spring and summer, pushing surface water southwards and offshore. As these waters move, they are replaced from below by deep, cold water, rich in carbon.

At a more global scale, CO2 in the air gets absorbed by the ocean, acidifying it in the process. The extra carbon raised by the upwelling had increased the acidification tied to rising CO2 levels.

One-day-old Pacific oyster larvae from the Taylor Shellfish Farms hatchery in Dabob Bay, Washington. The larva on the left was raised in seawater with favourable carbonate chemistry. On the right, the water was more acidic with lower levels of aragonite, making it harder for the larva to form its shell. (Image: George Waldbusser and Elizabeth Brunner / Oregon State University, CC BY SA)

Oysters rely on the chemical compound calcium carbonate for their shells. Larvae use it in the form of aragonite, while juveniles and adults build their more substantial casings from the compound’s calcite form.

Carbonate levels in the ocean decrease when acidity levels rise. The California Current upwelling was therefore “making it more and more difficult for the oyster larvae to lay down their aragonite shells,” explains Jan Newton, co-director of the Washington Ocean Acidification Center, who was assisting with monitoring oyster hatcheries at the time.

Ocean acidification

More CO2 in the atmosphere means more CO2 taken up by the ocean. This leads to a series of chemical reactions that increase the concentration of hydrogen ions in seawater and make it more acidic.

In 2008, another powerful California Current upwelling almost destroyed the Whiskey Creek hatchery in Netarts Bay, Oregon – one of the largest such facilities in the US.

As well as nearly closing the hatchery, the event had “significant ripple effects on the availability of oyster seed [larvae that have settled and formed shells] for growers for several years,” says George Waldbusser, a researcher in ocean acidification at Oregon State University.

It galvanised the industry into action.

Acid tests ahead

In 2010, oyster growers teamed up with academics and state and tribal agencies to deploy buoys monitoring pH, salinity, temperature, oxygen and aragonite levels in water used by local hatcheries.

With industry, state and national funding, that network grew, and ultimately became part of the US Integrated Ocean Observing System (IOOS). This network of over a thousand ocean sensors spanning the entire US coastline is “like a national weather service but for coastal ocean data”, says Newton.

A researcher from the University of Washington prepares to deploy a buoy to monitor various aspects of the seawater’s chemistry off the Olympic Peninsula coast (Image: John B Mickett / University of Washington)

Real-time data about imminent upwelling events now enables hatcheries to react by “buffering” water in hatchery tanks – adding chemicals that reduce changes in acidity. At some sites, that has helped to significantly lower the levels of damage seen in previous upwelling years.

Newton believes fishers would otherwise have been left exposed to acidification’s invisible threat. “The advancement was made because people were talking to each other,” she says.

But technology to monitor and then treat water is not available in many places where ocean acidification is causing problems for shellfish farmers and harvesters.

Building capacity and resilience

Experts in the field increasingly draw attention to the discrepancy between countries in the Global South and North.

West Africa’s coastal waters are strikingly similar to those in the US Pacific Northwest. Both are highly productive, provide vital income to local people and are fed by upwelling systems that make them particularly vulnerable to acidification.

Fishers on Senegal’s coast have also been reporting problems with their oyster and other shellfish harvests.

“This is a real concern, because throughout the West African coast, shellfish occupy an important place in the household socio-economy,” says Malick Diouf, former director of the Cheikh Anta Diop University Institute of Fisheries and Aquaculture in Senegal. “[They are] a source of animal protein, and a source of financing for women with the establishment of the market economy.”

Fisher Clemencia Ndene checks mangrove roots in Senegal’s Saloum Delta for harvestable oysters and mussels. Shellfish are key to household economies in many West African coastal communities. (Image: Karo Zen / Dialogue Earth)

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Emma Bryce & Kebba Jeffang, Dialogue Earth, 3 May 2024. Article.