The Science and Sustainability Behind Hong Kong’s Oyster Restoration

An Expedition to SAR, Hong Kong 

Oysters are often celebrated as culinary delights, but their true importance extends far beyond the plate. These bivalves are nature’s multitaskers, serving as powerful water filters that remove pollutants and excess nutrients, improving water clarity and quality. They build reef systems that act as essential habitats for marine life, boost biodiversity, and protect coastlines from erosion and storm surges. In a world grappling with climate challenges, oysters emerge as unsung heroes—key players in restoration and resilience. This realization captivated our expedition team, so much so that it brought us back to Hong Kong for the second time in just 24 months.

In one of the world’s most dense cities—better known for its dim sum than its underwater ecosystems—we set out to explore a lesser-known story: the role of oyster reefs in rebuilding marine habitats and protecting urban shorelines from the impacts of climate change. Hong Kong, quite literally, was built on oysters. In its early days, the lime for cement used in construction was derived from harvested oysters, fueling the city’s rapid development but decimating its once-thriving reef ecosystems in the process. 

(Studying the oyster reefs surrounding Hong Kong. Photo Credit: Adam Moore)

For this mission, we reunited with Marine Thomas, Associate Director of Conservation at The Nature Conservancy (TNC), and Sam Klein, a research assistant and incoming PhD student at the Swire Institute of Marine Science (SWIMS) at the University of Hong Kong. These two women are at the forefront of an ambitious effort to restore Hong Kong’s long-lost oyster reefs—an initiative that’s breaking ground as the first of its kind in the region.

Our previous visit in September 2023 had given us a glimpse of their work as the team embarked on initial oyster assessment surveys, determining how much of Hong Kong’s historic reef system was still intact and resilient. Now, several months later, we returned to see their progress firsthand.

(Marine Thomas with The Nature Conservancy. Photo Credit: Marla Tomorug)

A Vision for Restored Shellfish Reefs

Andi: Tell us what the TNC team has been up to when it comes to oyster restoration!

Marine: We started exploring the potential for oyster reef restoration in Hong Kong back in 2017. TNC has actually been working on shellfish reef restoration more broadly for more than 20 years now! Restoration always starts with understanding the history: where these ecosystems were, what happened to them, and whether they can be restored. There’s a lot of science involved—exploration, expeditions to assess what’s left, and historical ecology research to piece together the past. That’s exactly what we did when we started, and here we are, seven years later. 

Through this journey, we’ve gained a much clearer understanding of what happened to these systems in this region, the cultural importance of oysters here, where they used to thrive, and how they were lost. There are still some gaps we’re working to fill, but we’ve reached a point where we know enough to begin proper restoration. We’ve figured out the where and how, and today, we’re starting to make projects happen on the ground. Right now, we have pilot projects at four locations across Hong Kong. Our biggest project to date is at the airport runway, where we’ve deployed a little over 4,000 square meters of oyster reef that once was there but has been lost. 

(Back in September 2023, the Edges of Earth team shadowed TNC and SWIMS underwater. Photo Credit: Jeff Wilson)

Andi: That’s an incredible effort. Can you tell us more about why shellfish reefs like these are important?

Marine: Shellfish reefs are what we call the ecosystem engineers of estuaries and bays. They’re the foundation of life in these ecosystems. A lot of people don’t realize this, but in estuaries and bays—especially in temperate regions—you don’t have coral reefs. Instead, you have shellfish reefs.

These reefs perform many of the same critical functions as coral reefs do in tropical areas. They’re essential for biodiversity, providing habitat and shelter for a wide range of marine species. They support fish populations, stabilize shorelines, and help reduce erosion, which is especially important in urbanized or vulnerable coastal areas. What sets shellfish reefs apart is their ability to filter water. Oysters can filter gallons of water daily, removing excess nutrients and pollutants. This not only improves water clarity but also helps regulate the overall water quality in coastal areas. 

(Oysters are critical ecosystem engineers, not just food. Photo Credit: Adam Moore)

Andi: You mentioned expeditions and going out into the field. What does that entail when you're working with oysters?

Marine: The fieldwork is really where everything starts. From the historical research we’ve done, we have a good idea of where these ecosystems used to exist, but it’s not precise. Hong Kong’s history with shellfish reefs goes back centuries, as oysters were not only a food source but also a crucial component of the local economy. Oyster shells were extensively harvested for the lime industry, where they were burned to produce lime for construction materials. This practice, which peaked roughly 200 years ago, caused significant damage to the reefs, depleting the ecosystems to the point where many are now functionally extinct. The reefs that once thrived in Hong Kong’s waters supported diverse marine life and stabilized shorelines, but today, their remnants are largely invisible, hidden beneath layers of sediment or destroyed entirely.

We also don’t have a full picture of what these ecosystems were composed of—for example, the specific oyster or shellfish species that formed them. Much of this loss happened so long ago that it’s beyond living memory. Without visual data or detailed historical records, we’re left with gaps in understanding what these systems once looked like and how much remains today. These expeditions are about bridging that gap. It’s literally getting into the water in areas where, based on historical and ecological data, we think there might still be remnants of these ecosystems. We scuba dive in to see what we can find and collect samples.

(Marine and the dive team preparing to assess their underwater work. Photo Credit: Adam Moore)

Oysters, as it turns out, can be quite tricky to identify by species just by looking at them. That’s where our academic partners, particularly the SWIMS team, come in. They help us with DNA analysis to determine exactly what species we’re dealing with, their density, and their health. 

Andi: So, it’s really about piecing together the past to guide restoration in the future?

Marine: Exactly. By understanding what’s left and the ecosystem’s current health, we can make informed decisions about what would thrive if we tried to restore it. For example, the South Lantau site is one of the most significant areas for us historically, as it was heavily impacted by the lime industry. Our expeditions here have already given us insight into what remains, which, unfortunately, isn’t much. Now, we’re trialling restoration methods. In cases where ecosystems have been degraded to the point of what we call "functional extinction," natural recovery isn’t possible without intervention. That means reintroducing oysters in sufficient quantities to create self-sustaining reefs.

(Observing TNC and SWIMS’ more recent work in 2024. Photo Credit: Marla Tomorug)

Andi: So, how do you go about reintroducing oysters in an environment like this?

Marine: Over the summer, we deployed a restoration technique called spat-on-shell that’s a first here in Hong Kong, but a method used for restoration at scale. We start by using recycled oyster shells, which provide the ideal substrate for new growth. These shells are placed in tanks where we introduce larvae from different native oyster species. The larvae settle on the shells, growing into spat—essentially baby oysters. Once the spat reaches about a week old, we transfer them to the restoration site, where we track their survival and growth over the course of a year. This process helps us determine which species thrive in the local conditions and are viable candidates for larger-scale restoration. Right now, we’re trialing multiple native species at sites like South Lantau to see which ones are best suited for the environment.

This is really a trial-and-error process. If one or both species we’ve chosen don’t survive, it means they might not be the right fit for this site. At that point, we reassess and try other species. During our expeditions, we found traces of the Magallana HongKongensis, a large reef-building oyster. It’s a species we’re particularly excited about because it creates extensive habitat and is already present, albeit in very small quantities, on some of the remaining reefs. There’s a good chance it can thrive if reintroduced. However, if it doesn’t survive, we’ll need to figure out why. Maybe the batch we propagated wasn’t strong enough, or there could be something in the local environment that isn’t conducive. This is the nature of restoration ecology—constantly piecing together puzzles and asking questions. 

(Collecting oyster samples. Photo Credit: Marla Tomorug)

Andi: What kind of gaps are we talking about?

Marine: In places like Australia or the West, they have detailed fisheries data from the 19th century that show precisely where oyster beds were located and how much was harvested. But here, we didn’t have that luxury of a eureka moment or a detailed map. Instead, we had to piece things together from fragments of archaeological data, cultural records like poetry, and socioeconomic data spanning centuries. For example, we delved into writings from the Tang Dynasty—over a thousand years ago. We scoured newspaper clippings, old maps, anything and everything that could give us clues. It’s taken seven years to get to a point where we have a scientifically robust understanding of the historical extent of these reefs, and even now, it’s not complete. But we’ve estimated that at least 80% of Hong Kong’s shellfish reefs have been lost.

Fighting Climate Change with Oysters

(Sam Klein with SWIMS. Photo Credit: Marla Tomorug)

Andi: Can you tell us more about your role in the project and how you got involved?

Sam: Absolutely. Our lab focuses on assessing marine ecosystem changes in response to climate change and other human pressures like coastal development, pollution, and ocean acidification. We were brought into this project by TNC to serve as scientific advisors. Our role is to evaluate the best methods for restoring oyster reefs in Hong Kong and to support monitoring efforts. It’s a unique opportunity to address local challenges while contributing to the broader understanding of how restoration can combat the impacts of climate change.

One major issue we’ve noticed is that temperatures are staying warmer for longer. In Hong Kong, we typically have a warm, rainy season during the summer, and cooler winters where temperatures can drop below 20°C. But now, the warm season is stretching out, and the cool season isn’t getting as cold. This prolonged warmth causes significant stress on marine organisms. Researchers in our lab are studying how marine heatwaves are affecting species like sea urchins, impairing their resilience and reproduction. For oysters, restoration programs like ours aim to add beneficial ecosystems into areas facing these pressures. 

(Extreme weather is plaguing Hong Kong. Photo Credit: Jeff Wilson)

Andi: Beyond climate change, what human impacts have contributed to the decline of shellfish reefs here?

Sam: Hong Kong and the Pearl River Estuary are fascinating case studies for understanding how marine habitats respond to human activity. Land reclamation, for instance, has been happening here for over a thousand years. Historical records and archaeological evidence show that shellfish beds used to exist miles inland from where they are now. Ancient techniques like dyke systems—used in fish-pond agriculture—increased sedimentation rates and pushed these habitats further out to sea. This process has only accelerated with modern urban development.

The result? Shellfish reefs were exploited to exhaustion for lime while the shorelines where they live were being reclaimed. Shells were harvested and burned to produce lime for construction, while the oysters themselves were collected for food—Hong Kong was renowned for having some of the tastiest oysters in China! This constant cycle of overexploitation left these habitats with little chance to recover. Even today, the remnants of these reefs struggle to survive because the hard substrates—like shells or rocks—that oysters need to attach to have been removed from the environment.

(Oyster samples collected by the research and in-field teams. Photo Credit: Adam Moore)

Andi: How do oysters play a role in addressing pollution in Hong Kong’s waters?

Sam: The waters around Hong Kong are incredibly nutrient-enriched, largely due to nitrogen and phosphorus pollution. With seven million people in Hong Kong and around 60 million across the Pearl River Estuary, the runoff from agricultural fertilizers and urban waste flows directly into these marine environments. This excess nitrogen can lead to harmful algal blooms, which deplete oxygen levels, kill fish, and degrade water quality.

Oyster reefs can help mitigate this. Through a process called denitrification, bacteria that live on and inside oysters remove nitrogen from the water, improving its quality. My research focuses on assessing how different oyster species can maximize this effect. If we can demonstrate the effectiveness of this natural solution, it could encourage the government to invest more heavily in restoration efforts, framing oysters not just as ecological assets but also as tools for pollution management.

(Sam at the kiln working to better understand how oysters were used for lime. Photo Credit: Adam Moore)

Currently, shellfish reefs aren’t protected in Hong Kong—or in many places around the world, for that matter. They’re not recognized as critical ecosystems, and that’s a mindset we’re trying to change. Our research is being used to push for an IUCN assessment for endangered ecosystems, which evaluates ecosystems in the same way the Red List assesses species. By looking at factors like spatial coverage, health, and ecological function, we can determine whether Hong Kong’s oyster reefs are endangered, critically endangered, or even collapsed. That kind of designation raises their profile in conservation policy, making it much harder to ignore their importance. It forces us to protect what’s left and to seriously invest in restoration efforts. Recognizing their value is the first step toward restoring them at a scale that truly matters.

Andi: Is that the team’s vision for the future of oyster restoration in Hong Kong?

Sam: Yes, our collective goal is restoration at scale. That’s what will truly make a difference here. In places like Chesapeake Bay in the U.S., they’ve restored hundreds of hectares of oyster reefs, leading to significant improvements in water quality and marine biodiversity. I believe Hong Kong has the potential to aim for something similar, provided we establish dedicated hatcheries and secure the necessary institutional and community support.

(The women at the kiln pioneering oyster reef restoration in Hong Kong. Photo Credit: Marla Tomorug)

Right now, we’re still in the early stages, but the more healthy oysters we can reintroduce into the water, the better off the entire marine ecosystem will be. Oyster restoration is still a relatively young field, but the potential is enormous. With the diversity of reef-building oyster species we have and the unique resilience of these ecosystems, there’s every reason to be optimistic.

Ultimately, the goal is to restore these reefs to a point where they’re fully functional again—actively contributing to the health of the oceans and the coastal communities that rely on them. It’s a story you see repeated across restoration efforts worldwide, whether it’s mangroves, seagrass, or coral reefs. For us, it’s about ensuring that shellfish reefs are recognized as vital, protected where they still exist, and restored where they’ve been lost. I’m confident we can bring them back to life.

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As Told to: Andi Cross | Photography By: Adam Moore & Marla Tomorug | Support From: The Nature Conservancy Hong Kong & SWIMS

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SHE Changes Climate collaborates with the Edges of Earth Expedition, a woman-led team dedicated to highlighting impactful stories from the environmental frontlines. This partnership focuses on amplifying the voices of women who are pioneering positive change in some of the world’s most vulnerable coastal and marine environments, many of whose stories have gone untold.