A recent study gives fresh insights into the value of restoring oyster reefs to protect the coastal wetlands by investigating the ecosystem around abandoned traditional oyster farms in Hong Kong’s Deep Bay.
Deep Bay, one of the most important wetlands in southern China, is located on the north-western border of Hong Kong and Shenzhen in the heart of the rapidly developing Greater Bay Area. The bay is also home to a traditional oyster farming industry that goes back 700 years and is listed as Hong Kong’s intangible cultural heritage. More than 37,000 birds visit the wetland each year, and the territory is identified as a priority area for joint efforts for protection and conservation by regional governments.
In a recent study, scientists from The University of Hong Kong and The Nature Conservancy (TNC) discovered greater biodiversity in Deep Bay’s abandoned oyster farms compared to neighbouring mudflats. The research paper published in Frontiers in Marine Science in May 2022 quantified some of the ecosystem benefits of both natural oyster reefs and traditional oyster farming methods and compared the areas with nearby mudflats.
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Abandoned oyster farm in Hong Kong. Photo: Tom Chan.
Hidden Habitat That Supports Ecosystems
Focusing on abandoned oyster farms near the mouth of Ha Pak Nai Stream in Deep Bay, scientists researched the potential ecological benefits of traditional “benthic” oyster farming techniques in the Pearl River Delta and how that could inform work to restore Hong Kong’s oyster reefs and benthic ecosystems.
Derived from the Greek word benthos – which stands for ‘the depth of the sea’ – the benthic ecosystem comprises the many organisms that make their home on or close to the seafloor and the shallow water habitats such as the marshes and mudflats along estuarine coasts.
Oyster reefs are historically a vital estuarine habitat providing multiple ecosystem functions and services. However, humans have over-harvested native oyster reefs worldwide, with an estimated habitat loss of 85%. Along many coastlines, native species of oysters and their reefs are functionally extinct.
People started cultivating oysters in the Pearl River Delta during the Song Dynasty (960–1279 AD) and continued as the harvest of natural oyster reefs declined. In the mudflats of Deep Bay, oyster farmers have nurtured magallana hongkongensis – the Hong Kong oyster – for centuries. Once an inexpensive food eaten by almost everyone, the shells were also an essential source of lime used in the construction and agriculture industry in the region.
The traditional method of oyster farming involved placing hard substrate materials such as rocks, concrete tiles, oyster shells, and posts in the soft mud to promote the settlement of oyster larvae. This approach is similar to the technique of using an artificial substrate that TNC practices in its oyster reef restoration projects in Hong Kong, Australia, and the US.
By providing hard substrate among soft sediment mudflats, these traditional oyster farms provided a habitat for native oysters and other species. However, in recent decades, oyster aquaculture in the region has declined and what remains now is the method of using rope-suspended raft culture techniques that enhance production.
TNC staff sifting through the mud to collect samples for the lab. Photo: Pak Nai (TNC).
Vibrant Life on the Seabed
The University of Hong Kong (HKU) and TNC researchers found that the abandoned oyster farms provided habitats for a rich diversity of fauna and were home to 61 species compared to 26 species found in the surrounding mudflats. In particular, the numbers of epifauna species such as crustaceans and molluscs were six and 18 times greater, respectively, in the abandoned farms compared to the mudflats.
Wildlife and birds are attracted to the wetland areas due to increased food availability by directly feeding on the cultured species or other estuary critters, from the tiny one-millimetre, micro-benthic invertebrates to rare marine worms, molluscs and mussels, crabs, and other crustacea.
The higher densities of crustaceans and worms provide essential sources of food for fish and birds such as waders, gulls, terns, egrets, the endangered black-faced spoonbill, and the vulnerable Saunder’s gull.
The nooks and crannies of the former farms and the build-up of old empty shells substantially increased the biodiversity and biomass of organisms around them. Scientific analysis also showed the abundance of species in Deep Bay’s abandoned farms is comparable to natural oyster reefs in the national marine park in Jiangsu Province, further north on the Chinese coast.
While oyster farms do not provide the same level of complexity as natural oyster reefs, they illustrate the potential for regenerative aquaculture to enhance the diversity and abundance of species by providing structures that resemble natural oyster reefs on previously structureless mudflats.
Learning From the Past and Nature
The long history of oyster farming in the Pearl River Delta means that not only have native oyster populations been prevented from functional extinction in the region, but the habitats left behind may also have provided us with the key to cost-effective local reef restoration.
“In many ways, through the traditional oyster farming method of building hard structures on the mudflats, Hong Kong oyster farmers are one of the first restoration practitioners as they maintained and brought back wild oysters and reefs where they would have otherwise completely disappeared,” said TNC Senior Conservation Program Manager Marine Thomas.
Could traditional farms be revitalised to complement restoration and showcase an example of ‘restorative aquaculture’? While seafood, lime, and even pearls were the original products of traditional oyster aquaculture in the Guangdong region, revitalising the farms may offer the potential for creating shared value by facilitating and sustaining biodiversity and future ecosystem services.
Further research could inform ecologically conscious aquaculture techniques. For conservationists and scientists working to protect our coastlines, Hong Kong’s abandoned oyster farms may also show new ways to speed up reef restoration and enhance the rebuilding of native oyster populations.
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