Saving Oysters from Ocean Acidification with Seaweed

Oyster farms are one of the most environmentally sustainable farms in the world. As filter-feeders, oysters and other bivalve mollusks like clams, mussels, and scallops, require no additional feed or fertilizers. They simply eat plankton from their surroundings, leaving cleaner shallows for the rest of the ecosystem to enjoy.

However, the late 2000’s brought a concerning trend of oyster farm mortality events. One culprit is higher levels of carbon dioxide absorbed into our waters, which creates a more acidic environment. As climate change progresses, the oceans grow more acidic and less hospitable for shellfish.

“It affects many ocean organisms, but especially animals that build shells and skeletons,” says graduate student Leah Wessler. “It’s a big barrier for aquaculture and wild fisheries.”

Wessler is in the MSc program Applied Animal Biology studying how ocean acidification affects shellfish farms and how we can mitigate or even reverse the damage. From coral reefs in the Caribbean to beluga whales in Alaska, Wessler has worked in marine conservation and research around the world. She was drawn back home to the Pacific North West by a growing sense of optimism and momentum in the shellfish aquaculture sector.

“Here’s an avenue of farming that’s sustainable, produces high-quality protein, and is already quite common in many areas of the world,” Wessler says.

“Here’s an avenue of farming that’s sustainable, produces high-quality protein, and is already quite common in many areas of the world”

– Leah Wessler

Shellfish farms operate along the coast line. Baby shellfish, known as larvae, are grown in land-based tanks that cycle in water from the ocean. The tanks allow for a more controlled environment for high density oyster larvae rearing. Mature oysters are grown in off-shore coastal farms. As a non-fed system, these farms produce no harmful runoff. In fact, the oyster farm structures decrease shoreline erosion and create new habitats.

Most forms of shellfish aquaculture use ocean water. While oysters are resilient, ocean acidification is a major threat that causes malformed larval shells in farmed and wild populations.

A Natural Solution to Carbon Dioxide

One potential solution to ocean acidification is farming seaweed and oysters together. The seaweed removes carbon dioxide from the water through photosynthesis, essentially ‘reversing’ the acidification in its local environment.

“This type of mixed growing is not new technology,” Wessler points out. “It’s an Indigenous practice that goes back thousands of years. For example, traditional clam gardens grow symbiotic ocean organisms together, allowing for healthier coasts and more abundant harvests.”

Wessler’s research focused on Pacific dulse, a species of seaweed native to the Pacific North West. She tested larval oysters and dulse in various pH levels, including more acidic environments to mimic future ocean acidification projections. While global average pH in the ocean has already dropped from 8.2 to 8.1, it is projected to drop by another 0.2 units before the end of this century.

Wessler observed the oysters under a microscope, as oyster larvae are under 100 microns long when they are most susceptible to ocean acidification. The oysters were checked for shell growth, abnormal hinges, and general ability to thrive.

On top of de-acidifying waters for shellfish, pacific dulse is also a high-protein food source, popular with First Nations on the west coast. While commercial production is not quite widespread in Canada, similar seaweeds are widely produced in other countries, such as Japan and China.

Wessler’s research is not unique to oysters. Dulse or other seaweed varieties can be grown with other bivalves to improve yield and water conditions in shellfish farms. This co-growing technique can also help with the conservation of native species, such as B.C.’s Olympia oyster.

Graduating this November, Wessler will return home to Washington state and looks forward to continuing her career in conservation and restoration of her local ecosystems.

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