Guardians of the Pearl River Delta
Published on China Dialogue (October 16, 2020)

By Katherine Cheng

Nestled deep within one of the many bays of the Pearl River delta, a rare patch of mangrove forest can be found hidden between the towering skyscrapers on the Hong Kong–Shenzhen border. Egrets laze in the sun as small crabs scuttle at their feet, a mountainous skyline of construction cranes looming behind them.

Mangroves are small trees that grow along the coastlines of more than 100 countries in tropical and subtropical regions. They were once widespread on the Pearl River delta and around the inlets and islands of the neighbouring Hong Kong Special Administrative Region (SAR). Now there are only about 60 small patches remaining in Hong Kong – the largest is at Mai Po, at the head of Deep Bay (also known as Shenzhen Bay).

Protected by the Mai Po Nature Reserve, this mangrove forest and the surrounding mudflats has been designated as a Wetland of International Importance under the Ramsar Convention since 1995. It is part of a Hong Kong success story, albeit a limited one. After years of degradation, a recent survey conducted by Dr Stefano Cannicci from Hong Kong University’s Integrated Mangrove Ecology Lab found that mangroves are now making a recovery in the region.


Part of a project to compensate for mangroves lost during the construction of Hong Kong’s international airport on the northern side of Lantau island, the forest was planted in 2005-2007, and has brought many benefits to the village of Tai O. (Image: Katherine Cheng / China Dialogue)

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Although litter can often be found around the mossy roots of the Yim Tin mangroves, they remain a haven for wildlife, such as this crab. (Image: Katherine Cheng / China Dialogue)

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“Gei wai” shrimp ponds were introduced to Hong Kong in the 1940s by migrants from mainland China. A traditional aquaculture practice, the ponds were constructed by digging out the mud around stands of mangroves and creating embankments to keep the water in. Gates on the seaward side of the ponds allowed for the regular inflow and outflow of tidal water. The mangroves were kept to help harbour baby shrimp until they were ready to harvest. The Mai Po reserve is home to Hong Kong’s last remaining gei wai ponds – they are no longer in use, but are maintained by the reserve for their historical importance. (Image

Katherine Cheng / China Dialogue)
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The Mai Po reserve is surrounded by modern-day fish ponds. Aquaculture is a major driver of mangrove loss, not only through the removal of the trees to make way for ponds, but also due to pollution. If managed well, however, aquaculture and mangroves can be mutually beneficial to each other. Here, the fish ponds provide a buffer for the Mai Po reserve, protecting against creeping urban development. In their turn, the mangroves help clean the water and generally support the health of the local environment. (Image: Katherine Cheng / China Dialogue)

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This goes against the general trend. Over the past 50 years, 50% of the world’s mangroves have been lost. This is due to a combination of factors, including coastal reclamation, urbanisation, unsustainable aquaculture practices and pollution.
A decade ago, the rate of loss was between 1% and 2% every year. Things have improved slightly since then – the rate is now between 0.3% and 0.6% a year – thanks to stronger recognition of the ecological benefits of mangroves and expanded management and protection. But Associate Professor Daniel Friess from the National University of Singapore cautions against being overly optimistic, telling Science Daily that “conservation gains are not evenly spread, nor guaranteed in the future”.
This caution should apply to Hong Kong as well. With space limited, urban development looms ever on the horizon. This is clearly in evidence in Starfish Bay in the New Territories, where a new housing development towers over the beach. Long-time resident Mr So is only too aware of the issues: “There’s too much pollution, and look at all these new buildings that have been built nearby. It’s changed so much.”

Across the Mai Po mudflats, the high-rise buildings of Shenzhen are a hard-to-miss reminder of urban spread along this coastline. A less visible threat to the Mai Po mangroves are invasive species. The fast-growing Mangrove Apple (Sonneratia caseolaris) is a non-local species that was originally planted in the Futian Nature Reserve on the Shenzhen side of Deep Bay as a quick way to restore the mangrove forests there. The species has since invaded the Mai Po reserve, where regular clearance efforts are needed to prevent it overtaking native mangroves. To address the issue, Futian and Mai Po have also formed a collaboration, holding regular workshops and exchanging data. (Image: Katherine Cheng / China Dialogue)

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WWF staff member Cindy Chau identifies a bird spotted in the distance on the Mai Po reserve, home to Hong Kong’s largest remaining patch of mangroves. This expanse of marshes and mudflats at the head of Deep Bay supports a wide variety of birds, as well as other animals. The winter months are especially busy – an estimated 60,000 migratory birds spend the season here. (Image: Katherine Cheng / China Dialogue)

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The protected mangroves of the Mai Po reserve in the north of Hong Kong are a richly biodiverse habitat supporting numerous species. With its many hides, the reserve is also popular with nature lovers, many on the lookout for birds like this black-faced spoonbill (Platalea minor). (Image: Katherine Cheng / China Dialogue)

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The house of a modern-day fish pond can be seen near the Mai Po reserve. Aquaculture is a major driver of mangrove loss, not only through the removal of the trees to make way for ponds, but also due to pollution. If managed well, however, aquaculture and mangroves can be mutually beneficial to each other. Here, the fish ponds provide a buffer for the Mai Po reserve, protecting against creeping urban development. In their turn, the mangroves help clean the water and generally support the health of the local environment. (Image: Katherine Cheng / China Dialogue)

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The loss of mangroves means more than just the loss of another species of plant. Walking through the Mai Po Nature Reserve at low tide, it’s clear how important this ecosystem is. The tangled, gnarly roots are teeming with life, a haven between fish ponds and exposed mudflats.

But the benefits of mangrove forests go much wider. As Eddie Leung, assistant manager of WWF’s Mai Po Habitat and Infrastructure programme, explains, when seen as a nature-based solution, the ecosystem “addresses coastal erosion, prevents typhoon damage, provides a rich nursery for biodiversity, and more recently, is known for storing carbon”. For people who live in low-lying areas, this means the trees not only help protect their homes from flooding, but also provide a source of income through the fish stocks they nurture.


A barrel float used by local fishers is caught in a tangle of mangrove branches in Three Fathoms Cove on the eastern side of the Hong Kong SAR. Protected as a site of special scientific interest, the mangroves here remain vulnerable to overfishing and pollution from aquaculture due to limited capacity for monitoring and enforcement. (Image: Katherine Cheng / China Dialogue)

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A floating fish farm in Three Fathoms Cove, home to one of Hong Kong’s few remaining areas of mangroves. If practised sustainably, aquaculture can benefit from the ecosystem services mangroves provide. A richly biodiverse habitat, these intertidal forests offer a safe haven for fish to breed and raise their young, and also help clean the water of pollutants. (Image: Katherine Cheng / China Dialogue)

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This elderly resident of Tai O, a fishing village on the western side of Hong Kong on Lantau island, sells traditional salted fish for a living. A decline in the number of tourists over the past year due to Covid-19 means she’s only able to sell about a third of what she used to on a good day. “But I have enough to eat,” she says. Fishing around Tai O benefitted greatly when a new patch of mangroves was planted on abandoned salt pans next to the village between 2005 and 2007. (Image: Katherine Cheng / China Dialogue)

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To conserve mangroves, protected areas and restoration projects are vital. But for Hong Kong’s mangroves to continue to recover, and for these coastal forests to start making a come-back elsewhere – including on the Chinese mainland – best practice needs to be followed. Reserves need to be properly monitored and policed. And planting should only occur in areas suitable for mangroves, using only suitable local species. To this end, the exchange of scientific knowledge is key – a cooperation established in 2012 between the Mai Po Nature Reserve and the Shenzhen Futian Mangrove Ecological Park is a good example of this.

What’s happening in Hong Kong shows there is hope for mangroves. But much more needs to be done if we want to prevent this vital coastal ecosystem from disappearing by the end of the century.

A researcher from the Chinese University of Hong Kong collects samples of mangroves and mud in the Mai Po reserve. She is studying the diet of mangrove crabs, an important part of the mangrove ecosystem. The scientific research supported by the reserve plays a key role in efforts to conserve mangroves, both in Hong Kong and on the Chinese mainland. (Image: Katherine Cheng / China Dialogue)

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A ghost net can be seen near Three Fathoms Cove. Discarded fishing nets are often found near the coasts of Hong Kong, posing a danger to local biodiversity. (Image: Katherine Cheng / China Dialogue) 

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Across the Mai Po mudflats, the high-rise buildings of Shenzhen are a hard-to-miss reminder of urban spread along this coastline. A less visible threat to the Mai Po mangroves are invasive species. The fast-growing Mangrove Apple (Sonneratia caseolaris) is a non-local species that was originally planted in the Futian Nature Reserve on the Shenzhen side of Deep Bay as a quick way to restore the mangrove forests there. The species has since invaded the Mai Po reserve, where regular clearance efforts are needed to prevent it overtaking native mangroves. To address the issue, Futian and Mai Po have also formed a collaboration, holding regular workshops and exchanging data. (Image: Katherine Cheng / China Dialogue)

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Bartosz Majcher, a tropical ecologist at the University of Hong Kong, points to a mangrove sapling growing in Three Fathoms Cove. With its mangroves, mudflats and sandy shores, this is one of Hong Kong’s most ecologically diverse locations. Hong Kong has lost almost all of its mangroves due to coastal development. (Image: Katherine Cheng / China Dialogue)

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