AsianScientist (Mar. 3, 2021) – Beyond their gastronomic allure, oysters also offer enormous benefits to the environment, find conservation researchers from Hong Kong. Their findings were published in Restoration Ecology, further supporting the need to restore Hong Kong’s lost oyster reefs.
Along with caviar, foie gras and truffle, oyster—a type of shellfish—ranks highly among the world’s most luxurious foods. However, up to 85 percent of the world’s shellfish reefs have been lost due to factors like over-exploitation, coastal reclamation and pollution. Hong Kong’s reefs are no exception, despite the 700 year-old oyster farming tradition of the bustling metropolis.
“Most people associate oysters with food, but less well-known is that oysters create reef habitats that support coastal marine life,” shared study co-author Ms. Marine Thomas, conservation project manager at The Nature Conservancy (TNC). “Only by restoring these lost habitats can we start to bring back some of the associated environmental benefits.”
But while the loss of oyster reefs is well-documented around the world, investigations into the state of Asian oyster reefs are far more limited. Before any restoration efforts can move forward, feasibility studies are needed, particularly within the unique biological and societal context of Hong Kong.
Accordingly, researchers from the University of Hong Kong’s (HKU) Swire Institute of Marine Science and TNC surveyed the distribution natural oyster reefs (Crassostrea spp.) in the shallow coastal waters surrounding Hong Kong.
Oyster reefs are built when young oyster larvae settle on older oyster shells or other hard, submerged surfaces. Once attached, the larvae eventually grows into a full-fledged oyster after three years. When these reefs are lost, however, adult oysters fit for reproduction are suddenly in short supply—leading to low levels of spawning and larvae. Because of this, oyster larvae are often cultivated in man-made hatcheries before being transplanted into the wild.
In Hong Kong, the team found that there were naturally high levels of oyster larvae. However, these levels were concentrated only in areas with existing reefs. Future restoration efforts could therefore focus on adding oyster shells and other hard surfaces for the larvae to settle on.
“We were excited to find high natural recruitment levels which suggests that oyster reef restoration is possible without hatchery intervention,” said corresponding author and HKU associate professor Dr. Bayden D. Russell. “We think that this recruitment is because traditional oyster farming in the Pearl River Delta has maintained populations of native oysters in the system…these farms could potentially act as a source of larvae.”
They also found that healthy oyster reefs primarily benefit coastal environments by acting as natural water purifiers. In fact, a single Hong Kong oyster (Crassostrea hongkongensis) can filter up to 30 liters of water per hour—among the highest filtration rates recorded of any oyster species.
Aside from making cleaner water possible, oyster reefs also provide habitat and nursery grounds for many native species. For instance, such reefs could house six times more species than bare muddy shores, including commercially and recreationally valuable fish and crabs.
“The human aspect remains our biggest challenge to bring these habitats back at scale. Shellfish habitats are still severely under protected in Hong Kong [and] wild harvesting is a huge problem—as soon as oysters or mussels are big enough to eat, someone will harvest them. We are working with the government on gaining more protection and recognition for these important ecosystems,” concluded Thomas.
The article can be found at: Lau et al. (2020) Restoration Potential of Asian Oysters on Heavily Developed Coastlines.
Source: University of Hong Kong; Photo: Kyle Obermann/The Nature Conservancy.
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