Oysters: A slimy delicacy with a vital role to play in the sea

The boat glides through the tranquil waters of the Hawkesbury River near the village of Mooney Mooney, about 45 minutes north of Sydney, where only a small sign on a pier reveals that there is an oyster farm beneath the surface.

The farm belongs to Sheridan Beaumont and her family, who offer tours for visitors here. "Who wants to hold some baby oysters?" Beaumont asks, pulling two long containers out of the water containing several small oysters.

The visitors, all wearing waterproof fishing overalls with sewn-on wellies, cautiously take a bite.

Beaumont explains that the sea creatures are raised in oyster bags and rafts that float in the water until they are big enough to be sold.

The native Sydney rock oysters (Crassostrea commercialis) are cultivated, as well as Pacific oysters (Crassostrea gigas), the most commonly cultivated oyster species in the world.

Originally from the coastal waters of the western Pacific, the Pacific oyster has since been farmed in many parts of the world.

It has also been used in aquaculture in Europe for decades and has since spread rapidly along the coasts, competing with native species for space and food.

Among gourmets, the Sydney Rock Oyster is considered a special delicacy due to its tender meat and strong flavour. It is the most popular species in the state of New South Wales, where it accounts for around 80% of oyster farming.

However, Pacific oysters are also very popular throughout Australia. In contrast to the Sydney Rock Oyster, which takes three to four years before it can be sold, this species is said to reach the appropriate size after just one to two years.

Aboriginal Australians farmed oysters centuries ago

In New South Wales, oyster farming is the most important aquaculture. About 106 million oysters worth more than A$35 million (about $23 million) are produced every year.

For over 100 years, the sea creatures have been deliberately cultivated in bays and waterways.

Aboriginal Australians are also said to have collected and eaten the protein-rich delicacy from rocks since time immemorial - at least that's what Captain James Cook reported when he landed with the First Fleet in Botany Bay, what is now Sydney, in 1788.

Eating oysters is also the highlight of Beaumont's tour: the guests crack fresh oysters and eat them with a little lemon juice, accompanied by a glass of sparkling wine.

"It's gone to the point where we have like 400 people a weekend and it's been incredible," says the oyster farmer.

Tourism has become the 37-year-old's second mainstay as selling oysters has proved difficult in recent years. This is mainly due to climate change and the associated natural disasters.

Deadly floods and bushfires

"I just had massive bulka bags of just dead shell," recalls Beaumont of a flood in March 2022, when around 800,000 oysters died.

The floods not only contaminated the water with dirt, but also changed the salt concentration in the river, which led to the death of the marine animals.

Bushfires, which are always a major threat in Australia, can also jeopardize shellfish.

When the country battled devastating flames during the 2019-2020 bushfire season, known as the Black Summer, the ash reached the Hawkesbury River and clogged the oysters' gills.

Oysters filter water and provide more light

The climate crisis not only has economic consequences. Oysters are hugely important for the quality of coastal waters.

"[Shellfish] are all filter feeders, so they are really really good for water quality," says Marina Richardson, a marine biologist specializing in oyster reefs at Griffith University in Brisbane.

They filter out excess nutrients and pollutants from the water, she adds.

Filter feeders are marine animals that filter their food out of the water - including various types of fish, corals and tube worms. Oysters filter dozens of litres of water every day.

In the process, they also absorb pollutants and other particles, which are excreted together with food residues as so-called mock faeces. "They are the kidneys of the waterways if you will," says Richardson.

"[Oysters] will also increase the available light," the biologist continues. The water filtered by the little creatures allows more sunlight to reach the seabed, which supports the growth of seagrass and other plants.

In turn, seagrass helps in the fight against the climate crisis.

According to studies, the plant species can bind large amounts of carbon and thus reduce the carbon dioxide (CO2) content in the atmosphere.

Habitat for other species

Australian marine biologists are endeavouring to protect original oyster reefs and restore destroyed reefs.

This is also about preserving the region's biodiversity as oyster reefs provide a habitat for numerous animal and plant species.

For example, barnacles and sea anemones colonize them alongside other shellfish species, which in turn are a source of food for fish.

The health of oysters also provides a general indication of the condition of the water. Beaumont, who is the third generation to run the mussel farm, says they are a kind of a signalling device - "the canary of the river."

"When an oyster starts to die, you know that there's something is wrong."