Today, natural oyster beds have become rare in many places – this also applies to the once oyster-rich waters around Hong Kong. There, biologists have now uncovered the great importance these mussel beds have for cleaning the sea. A Hong Kong oyster can filter up to 30 liters of water per hour. The good news: Hong Kong’s oyster reefs could regenerate quickly if they were placed under protection.
There were once extensive natural mussel beds off many coasts of the temperate and subtropical latitudes, including colonies formed by oysters. But now there are only a few remains and commercial breeding facilities left. “Globally, we have lost 85 percent of all mussel beds – making them one of the most endangered marine habitats on earth,” explains Marine Thomas from The Nature Conservancy in Hong Kong.
Only remnants remain
The disappearance of the natural oyster beds is a major ecological loss in several respects, as the researcher explains: “Most people associate oysters with an edible delicacy, but it is less known that oysters are ecologically valuable reef habitats.” Because mussel beds are home to many young fish as well as other marine life a protected habitat, at the same time they form structures that break waves and create calm water zones. “Only by restoring these lost habitats can we regain some of their positive effects on the marine environment,” says Thomas.
She and her colleagues have now investigated to what extent this is possible in the waters around Hong Kong, what is the situation there with the once extensive oyster beds and what filtering performance the Hong Kong oyster (Crassostrea hongkongensis) performs. It has been shown that the remains of natural oyster beds still exist in many areas of Hong Kong’s lower intertidal zone. “However, these areas are severely degraded and mostly only have low densities of oysters, and most of them are less than two years old,” the researchers report. The reason for this: “The mussel habitats are hardly protected in Hong Kong. Therefore, wild harvesting is a big problem: as soon as the mussels are big enough to eat, someone takes them away. “
Enough oyster larvae for natural regeneration
But there is also several good news. Perhaps most importantly, there is great potential for the natural regeneration of Hong Kong’s oyster beds. In some areas, up to 500,000 young mussels per square meter settled, as the biologists determined. “Although the number of animals that survived into adulthood fluctuated, there were enough new larvae arriving and enough survivors to ensure the oyster beds even without targeted breeding and releasing larvae,” said Thomas and her team. They suspect that most of the oyster larvae come from the many traditional oyster farms in the nearby Pearl River Delta. Some of the larvae released by the mussels then also settle off the coast of Hong Kong.
More detailed studies also showed that the local Hong Kong oysters in particular make a particularly large contribution to sea cleansing. Even an annual oyster can filter up to 30 liters of water per hour in summer temperatures. This is one of the highest filtration rates known from oysters, say the researchers. Seven square meters of such an oyster bank is enough to clean an entire Olympic swimming pool full of seawater every day. Even the thinned-out remains of the oyster beds around Hong Kong that still exist today contribute to making the sea there cleaner. This is one of the reasons why the oyster beds in this region and on other coasts of Asia are particularly worthy of protection. “We are already working with the government to better protect these important ecosystems in the future,” says Thomas. She and her team suspect that the mussel beds in other areas of Asia would also regenerate themselves if they were better protected from destruction and premature harvest by humans.
Source: The University of Hong Kong; Professional article: Restoration Ecology, doi: 10.1111 / rec.13267