Restoring degraded oyster beds and reefs

European oysters © SNH

When we think of mass habitat extinction, colourful, diverse and highly visible ecosystems such as tropical rain forests and coral reefs come to mind. Approximately half of global shallow water coral reefs and forests have been lost over the last few hundred years, but there are glimmers of hope. Deforestation rates are declining and some corals have shown resilience to stress from climate change.

A far less visible ecosystem crisis has occurred relatively recently beneath the ocean’s surface. A study revealed that 85% of global oyster reefs have been lost over the last 150 years. Of those remaining, over one third are so depleted that they no longer function as ecosystems, particularly those in Europe, North America and Australia. Only a few healthy oyster reefs remain in South America. Even these are 50% of their prior abundance, making oyster reefs one of the most threatened habitats on Earth.

Ecosystem engineers

Oysters are “ecosystem engineers” like corals – they create three-dimensional structures as they settle and grow on each other. Left undisturbed, these oyster reefs provide a habitat for an incredible biodiversity of organisms. They serve as a food source, nursery ground and refuge for many species, boosting fish stocks.

Oysters also have an incredible ability to clean seawater. A single oyster can filter almost 200 litres of seawater daily, eating the phytoplankton and organic matter suspended within it. Oysters improve water quality and clarity, preventing large scale algal blooms and the potential consequences of mass fish mortality and deadzones due to depleted oxygen.

Removing oyster reefs increases wave energy and erosion of saltmarshes and the corresponding coastline. Just as tropical coral reefs protect mangrove forests, oyster reefs provide coastal protection for important temperate ecosystems such as seagrass and saltmarshes.

As climate change and pollution destroy marine life, the need for vast oyster reefs has never been greater. Sadly, this vital habitat is at its nadir.

The oyster reefs of yesteryear

Humans have been hand harvesting oysters since the Stone Age and cultivating them since Roman times. However, oyster extraction reached fantastic proportions from the mid-1800s as mechanised fishing boats replaced older fleets.

Chesapeake Bay – meaning “great shellfish” – in the US was named by the Algonquin Indians, because of the miles of incredibly dense oyster reefs that “lay thick as stones” in 1608. Between 1860 and 1920, three quarters of these reefs had been destroyed. By 1940 they had been almost entirely wiped out.

Across the Atlantic in Europe, oyster reefs were once plentiful and thought to be inexhaustible. The Olsen piscatorial atlas of 1883 depicts large oyster grounds at a depth of 34 metres in the North Sea, stretching from Helgoland, off the coast of Germany, to the Dogger Bank, between UK and Denmark, and through the English Channel. Though recently extirpated, oysters were once abundant in the Firth of Forth on Scotland’s east coast.

Roads and towns were built on shucked oyster shells in the US and Britain, while shell piles towered one trillion tall in France. In 1864, 700m European oysters were consumed in London alone.

The European native oyster (Ostrea edulis) population has declined by 95% since the 1950s. Oysters are now extinct in some regions of Europe, such as the Wadden Sea along the coast of Germany.

Removing these vast ecosystems has deprived the sea of essential services and reduced water quality. There’s even speculation that the North Sea is especially murky today, because oysters are no longer around.

Destructive fishing that exceeds reproduction rates and removes habitat is the main cause of this ecosystem’s demise. Pollution, climate change, invasive species and shellfish diseases have further decimated remnant oyster stocks. It’s clear that active intervention is needed to turn the tide for oysters.

Restoring oyster reefs

Efforts to restore oyster reefs have already had success in the US, and there are projects emerging in the UK and Europe.

In southern England, the Solent Oyster Restoration Project was established in 2017 to reseed the Solent strait with 5m oysters. Since then, more than 20,000 oysters have been placed in novel restoration aquaculture cages, suspended underneath marina pontoons across the Solent. These cages are designed to pump out larvae which settle on the surrounding seabed, where 30,000 juvenile oysters reared in hatcheries have already been reintroduced.

The Native Oyster Network in the UK and Ireland and the Native Oyster Restoration Alliance across Europe aim to restore native oysters around the north-west Atlantic. Both established in 2017, these networks are bringing together scientists, fishers, conservationists and governments to bring back a forgotten ecosystem.

Restoring European oyster beds and reefs is possible, but there is no quick fix. Work will take years and decades and depends on multinational cooperation and effective policy to protect habitats and regulate fishing. Nevertheless, recent efforts give hope we are at the beginning of a journey to restore a forgotten ecosystem to its once magnificent state.

This article was originally published as The World’s Most Degraded Marine Ecosystem Could Be About to Make a Comeback at The Conversation on 2/05/2019.

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