Bayden Russell, associate director of a marine institute at Hong Kong University, dives into the waters near the Chinese territory's main aiport, where they are checking on a seeded oyster reef.Photography by James Griffiths/The Globe and Mail
One hand treading water, Bayden Russell heaves a green mesh bag bulging with oyster shells above his head, as his colleagues stretch down from a small boat to retrieve it. On deck, the team works quickly, emptying the bag into a white cooler, filling it with shells, debris and a strong smell of the sea. They hose the catch down and begin picking through it, throwing most back across the side, separating a few shells and other marine wildlife into clear plastic bags.
The boat’s occupants, from Hong Kong University and the global non-profit The Nature Conservancy, haven’t come out on this drizzly November day to fish. They’re inspecting an artificial oyster reef, built up over years of work in the unlikeliest place: between two runways of Hong Kong International Airport.
Practically unseen today outside of restaurant menus, oysters were once pervasive in Hong Kong and southern China, where they have been cultivated for hundreds of years. A ninth-century text, written by Tang Dynasty official Liu Xun, refers to a population of fish-like barbarians occupying the area that is now Hong Kong, who “live in the wild, eat only oysters, and make their houses from piles of shells.”
The team sorts the shells they have recovered. Among the divers is conservationist Marine Thomas, who says oyster meat and shells were once critical to local people's livelihoods.
Marine Thomas, associate director of conservation at The Nature Conservancy Hong Kong, said oyster habitats “were absolutely everywhere and fundamental in the building of Hong Kong.”
But while earlier Hong Kong inhabitants may have used whole oyster shells in construction, they are a far more effective material when they are burned to create lime, which can be mixed with sand and water to form a rudimentary concrete. It was this industry, which exploded in the 19th century, that devastated the local oyster population, Ms. Thomas said.
“There were lime kilns all over. It was a very active industry in Hong Kong from before the British came until the 1950s,” she added.
Oyster reefs are the most endangered marine habitat on the planet, with an estimated 85 per cent already lost, according to the conservancy. In Hong Kong, a decline that began with dredging for lime has been exacerbated by pollution, land reclamation and shipping – the city’s container port is the 10th-busiest in the world – leaving few wild oysters.
Recent decades have also seen a sharp drop in the number of oyster farms operating in Hong Kong, with only a handful surviving in Deep Bay, across the water from the mainland Chinese city of Shenzhen.
This isn’t just bad for the oysters. Reefs are a key ecosystem for dozens of other species, providing nooks and crannies in which tiny crabs, other shellfish and fish can find protection from predators and the weather. Hong Kong is home to almost 6,000 marine species, more than a quarter of the total recorded across all of China, including bivalves such as oysters and mollusks, fish and a tiny and vulnerable population of pink dolphins.
Beyond providing a potential habitat, oyster reefs can help protect against storm surges and filter the water, cleaning it of pollutants that can deprive other species of space, light and oxygen. The local oyster subspecies – hongkongensis – is particularly good at this, with among the highest filtration rates in summer of any bivalve, according to a study co-authored by Ms. Thomas and Prof. Russell, who helps lead the Swire Institute of Marine Science at Hong Kong University.
The artificial oyster reef is between two runways of Hong Kong International Airport, which was built on a pre-existing island but mostly consists of reclaimed land.
While there are many reasons to restore Hong Kong’s oyster reefs, it wouldn’t do much good to seed new oysters in areas where they would face the same problems that devastated the original population – which is where the airport comes in.
Hong Kong International Airport is almost entirely built on reclaimed land, in an area that was once a thriving oyster habitat, Ms. Thomas said. Lime kilns that date back to the Tang Dynasty have been found in the vicinity.
“Because it’s a restricted area, if we can grow a reef there, it will be the most protected oyster reef in Hong Kong, if not the only protected reef,” she added.
Oysters will build on almost any hard surface, including other oysters. In 2021, The Nature Conservancy and Hong Kong University team began work on a reef along a seawall protecting the newly built third runway, using concrete rubble and oyster shells collected from restaurants across Hong Kong.
“There’s a cyclical aspect to it that I love,” Ms. Thomas said.
“Historically, we burned oyster shells to create lime and destroyed the habitat in the process. And now here we are using recycled concrete, smashing it up and putting it back in the water and growing oyster reefs on it.”
The fate of previously seeded oysters will give researchers an idea about whether the reef can sustain itself.
The Globe and Mail joined the team on its first inspection of the new reef, one of four such trips it will take over the next year, to see if the plan to seed oysters along the runway is working. “If it’s not self-supporting, there’s no point,” Prof. Russell said.
For hours, team members dived down into murky water, feeling with their hands for shells to bring up for study as planes landed overhead every few minutes. They had to find the reefs by referring to signs on land and then swimming down: Being this close to the airport means GPS or other trackers are forbidden, as is getting too close to the seawall itself, which could set off an alarm and risk shutting down one of the busiest airports in Asia.
Exhausted, sitting on the deck of the ship as it puttered back to port, Ms. Thomas said that, except for one patch of reef that had become inundated with sediment for reasons unclear, everything looked positive. There were signs of juvenile oysters growing on the old shells and a good number of other sea creatures that had made a home on the reef.
Prof. Russell’s team, which had spent the day on board bagging oysters and other sea life for study back in the lab, was only just getting started. He estimated it would take another one to two months to truly understand the state of the reef and how much progress had been made.
“This was the quick bit. There’s a lot of work to go,” Prof. Russell said. “Really, we won’t know if this has worked for five years or so; that’s when we need to be checking.”
As Ms. Thomas noted: “Conservation isn’t easy. It’s hard, hard work.”
Ms. Thomas and colleague Rhian Evans emerge from the water with their specimen bags. She and Mr. Russell say it will take many more such excursions to test the health of the reef in coming years.