Oysters, obviously: Restoring the hidden heroes of ocean health

Oysters and shellfish are living filtration systems. Now there is a growing movement to restore the shellfish reefs of centuries ago, and the work is coming to Hong Kong

author icon By Ryan Swift | 3 March 2021

The Nature Conservancy works on reef restoration in Hong Kong | Kyle Obermann Photography

You’ve likely only encountered oysters on a platter, perhaps with a glass of champagne. But don’t be discouraged, as this story is not about how bad oyster consumption is for the marine environment – quite the opposite.

The chilled oysters on your platter were likely grown in an oyster farm. In Hong Kong, oyster farms have helped oyster populations stay alive even if their once-abundant reef systems have been devasted by the construction industry, harvesting and land reclamation.

In oyster farms, oysters grow on strings suspended from floating rafts. In the wild, however, oysters build enormous reef structures that have much in common with the more famous coral reefs found in tropical waters.

Farmed oysters are helping re-establish the wild oyster reef systems lost centuries ago

Oyster larvae, ejected from oysters, will drift in the water above other oysters before attaching themselves to another oyster shell or a suitable hard structure. Repeated over centuries, this process results in enormous oyster reef systems. New York City harbour was once home to an oyster reef covering more than 800 square kilometres.

Like coral reefs, shellfish reefs encourage sea life, serving as nurseries for fish species, in turn creating a habitat for other species. By keeping water cleaner and clearer, oyster reefs encourage seagrasses to grow.

Professor Bayden Russell, who studies oyster reefs for the Swire Institute of Marine Science, Hong Kong University, says the interesting thing about oyster reefs for him is the diversity of species associated with them.

They are also filter feeders, which means they purify and clean seawater as they grow. Shellfish reefs also serve as wave barriers, keeping shorelines intact from rising seas.

Separate studies conducted by the University of Hong Kong found that local oysters could filter and clean up to 30 litres of water per hour each – the equivalent of two large office water cooler bottles. Oyster reefs covering enormous areas in Hong Kong would have had a tremendous impact on water clarity.

Oyster restoration project in Pak Nai

Russell says that the loss of oyster and shellfish reefs happened so thoroughly and over so many generations that people have simply forgotten what the sea was like when shellfish reefs were plentiful.

Shifting Baseline Syndrome – where people simply accept a degraded environment as the new normal – may be an explanation as to why there isn’t more concern over oyster reefs. People simply don’t know what they’re missing.

The rafts of modern oyster aquaculture

Because the damage to oyster reefs has occurred over centuries, scientists have yet to fully understand their function in marine ecosystems. A 2011 study published in BioScience estimated that approximately 85% of the world’s shellfish reefs had been destroyed through over-harvesting, habitat destruction caused by reclamation or for the production of lime.

Because the damage to oyster reefs has occurred over centuries, scientists have yet to fully understand their function in marine eco-systems

“That report is old but had good insights. At the time, there was no real data on losses in the Asia Pacific; the report probably underestimated the losses in Asia,” said Russell.

For hundreds of years, builders created lime, the forerunner of cement, by burning oyster shells, which are made of calcium carbonate. Burnt shells were also used to make early plasters and paints.

Farms ensure abundant larvae in Hong Kong waters

And contrary to today’s status as a delicacy, the oyster 150 years ago was known as poor person’s fare, and its exploitation was intense. The study noted that ancient piles of shucked oyster shells found in southern France contained more than 1 trillion shells each, while Londoners in 1864 consumed an estimated 700 million oysters. In the United Kingdom, oyster dredging employed 120,000 workers.

Marine Thomas, a project manager with The Nature Conservancy in Hong Kong, says that once a shellfish reef structure is destroyed, it is very difficult to get it back without intervention.

An oyster aquaculture industry that dates back 700 years to the Song Dynasty is credited with keeping oysters alive in the Pearl River Delta

Thomas estimates that there are 10,000 oyster rafts in Deepwater Bay, between Hong Kong and Shenzhen, meaning that oyster larvae are still present in Hong Kong waters.

“It makes life easy – all I need to do is part a substructure in the water and oysters will start to grow,” she says.

Thomas and Russell both credit the study with starting the “oyster movement” of conservationists focusing on restoring oysters to their once-glorious position. In the United States, shellfish reef conservation and restoration projects began at least 20 years ago. The knowledge gained from US work is now filtering around the world.

Early studies on the beneficial impacts of oyster reefs were done in the 1990s, while conservation and reef restoration programmes got underway in the 2000s with help from the National Oceanic and Atmospheric Administration (NOAA).

Contrary to today’s status as an expensive delicacy, the oyster 150 years ago was renowned as poor person’s fare and its exploitation was intense

In 2006, NOAA and TNC published a practitioner’s guide to the restoration of oyster reefs, saying that the guide had become necessary due to the number of oyster reef restoration projects underway in the US.

“It’s now well established what to do in order to restore oyster reefs,” says Russell. “The problem is that all the hard substrate that oysters need is now gone. All that’s left is muddy shoreline.”

In Hong Kong, an oyster aquaculture industry that dates back 700 years to the Song Dynasty is being credited with keeping oysters alive in the Pearl River Delta.

Conservationists and NGOs such as The Nature Conservancy in Hong Kong are now starting to look carefully at the oyster and how its reefs can be restored to their former glory.

“We understand the loss (of shellfish reefs), but what does it mean for ecosystem function? Restoration is expensive and really hard – when you push ecosystems so much, it might not be possible (to restore them),” says Thomas.

 

The Nature Conservancy embarked on a trial restoration project in Tolo Harbour, in which nine tonnes of oyster shells in 700 biodegradable mesh bags, retrieved from hotel dining rooms, were piled up on the seafloor underneath a local fish farm.

Thomas says the project succeeded in bringing new sea life to the area, including a sea turtle – something that surprised and delighted local fish farmers.

The reef, just four-square metres, collapsed after seven months because it lacked a concrete structure underneath. In Hong Kong, difficulty in getting permits to put down a solid structure suitable for oyster growth has held back the effort to create new oyster reefs.

“There are overlapping jurisdictions (in Hong Kong), so it’s not always clear who gives permissions for such things,” says Russell, who is originally from Australia.

He notes that oyster restoration work proceeded quickly when Australia’s many sport fishermen, who have a powerful lobby, pressed local governments to establish oyster reef restoration projects.

So keep dining on oysters. The farming that has kept them alive in Hong Kong may be the reason that oyster reefs one day make a return to Hong Kong waters.