Are farmed oysters, mussels and clams the ultimate green foods?

When you think of eco-friendly food options, your thoughts might lean towards staple items like lentils, but is it possible to achieve culinary luxury while remaining environmentally cautious? Bivalves such as oysters, mussels, clams, and scallops possess inherent green credentials, primarily due to their ability to filter and purify polluted water. Some new research posits that they might also have the capability to absorb carbon dioxide. So, could farmed bivalves take the crown as the ultimate eco-friendly food?

Food production accounts for approximately one-third of global greenhouse gas emissions, in addition to other environmental impacts. This makes it imperative to identify food sources that minimize environmental harm. Typically, farmed seafood is viewed as more sustainable than wild-caught fish, but farmed fish rely on wild fish for their diet. A study from last year revealed that the volume of wild fish harvested for fish feed has been significantly underestimated in the industry. Moreover, fish farming can lead to pollution from waste and uneaten feed pouring back into the oceans.

The primary advantage of bivalves is their unique feeding mechanism; they are filter feeders that thrive on microscopic particles in the seawater, including plankton. With many coastal areas suffering from pollution due to excess nitrogen and phosphorus, farming bivalves can serve as a method to cleanse the over-nourished waters and provide food at the same time.

While the wild harvesting of bivalves can be detrimental—dredging for scallops, for instance, disrupts ocean floor ecosystems—farming them is considerably less harmful when compared to traditional livestock farming practices, like the destruction of forests for cattle ranching.

But the plot thickens: in recent years, a substantial number of studies have suggested that farmed bivalves can sequester carbon—an assertion that could potentially allow farms to earn carbon credits, adding extra revenue.

However, skepticism remains. Fabrice Pernet from the University of Brest in France recently reviewed over 50 related studies and retains reservations about the conclusiveness of their findings. Many rely on theoretical models that suggest the process of forming calcium carbonate shells absorbs CO₂. Yet, as Pernet explains, this only occurs on a geological timeline. Over short periods—decades or centuries, which are crucial for global temperature control—the actual formation of carbonate tends to release CO₂ back into the ocean.

Moreover, Pernet points out that many waste shells are often incinerated, resulting in additional CO₂ emissions, rather than being returned to the ocean where they could otherwise mitigate atmospheric CO₂. As for the carbon content within the flesh of bivalves, it is released as CO₂ upon consumption.

Some researchers are proposing a different theory: that increased organic carbon accumulates within the sediments beneath bivalve farms, making these areas function as net carbon sinks. This claim emerges from recent studies on oysters cultivated in sizeable enclosures; however, Pernet indicates that these studies have shortcomings, like taking CO₂ measurements only during the day and summer when photosynthesis is active.

The research team insists their findings demonstrate that the accumulation and sequestration of carbon surpasses that which is released through shell harvesting. Shuang-Lin Dong from the Ocean University of China states, “The budget convincingly shows that carbon accumulation outweighs the carbon absorbed in shells.” Yet, Pernet remains unconvinced, stating, “Current evidence suggests that bivalve farming ultimately contributes to CO₂ emissions.”

But don’t hastily abandon your plans for a seafood feast just yet. While the consensus leans towards bivalve farming as being a net emitter, the volume is relatively minuscule, with only about 1.4 kilograms of CO₂ emissions or its equivalent for every kilogram of consumable weight, as revealed by a 2021 study. This is significantly lower than that of crops like wheat or maize, and bivalves require far less land and freshwater resources.

Ultimately, “Yes, farmed bivalves rank among the greenest food options available,” concludes Pernet. Enjoy your dining experience, guilt-free.