Europe’s exhausted oyster reefs ‘once covered area size of Northern Ireland’

Only a handful of natural oyster reefs measuring at most a few square metres cling on precariously along European coasts after being wiped out by overfishing, dredging and pollution.

A study led by British scientists has discovered how extensive they once were, with reefs as high as a house covering at least 1.7m hectares (4.2m acres) from Norway to the Mediterranean, an area larger than Northern Ireland.

The study involved dozens of researchers poring over government records, nautical charts, fishery reports, customs documents, naturalists’ accounts, scientific journals and newspapers from the 18th, 19th and early 20th centuries to piece together the spread of the European flat oyster (Ostrea edulis).

They found vivid – and poignant – accounts of often sprawling reefs at 1,196 locations off countries including the UK, France, Ireland, Denmark, Spain, Germany and the Netherlands. One report from a scientific article mentioned oyster reefs reaching 7 metres in height in the Black Sea.

A depiction of vessels dredging for oysters off the UK coast. Photograph: Handout

Ruth Thurstan from the University of Exeter, the joint lead author of the report, said she was “blown away” by the extent of the reefs. “I knew that oysters used to be caught in huge quantities, so we suspected that these reefs could be large, but to find information that evidenced such coverage of reefs, amazed me.

“Few people in the UK today will have seen a flat oyster, which is our native species. Oysters still exist in these waters but they’re scattered, and the reefs they built are gone. We tend to think of our seafloor as a flat, muddy expanse, but in the past many locations were a three-dimensional landscape of complex living reefs.”

The reefs created rich ecosystems, providing a habitat for almost 200 fish and crustacean species including the common stingray, the short-snouted seahorse and the European sturgeon. They also played a vital role in stabilising shorelines, nutrient cycling and water filtration.

Thurstan said: “There are a handful of remnant reefs in a few parts of Europe, including the coast of Brittany and the west coasts of Ireland and Scotland. But these are at most a few square metres in extent, as opposed to square kilometres in the past. The significant ecological functions these reefs used to provide no longer exist, which is what we mean by functionally extinct.”

Reefs have largely disappeared but clumps of oysters can still be found. Photograph: Stephane Pouvreau/Ifremer

Some of the accounts the researchers found make for sad reading as people realised what was being lost. One writer in 1852 reported: “In the Wash, about 50 years ago, were enormous oyster beds; one extending nearly the whole length of the Wash and continuing outside about 50 miles.”

A description of a “great” reef three miles off the Isle of Man read: “It took 20 boats seven years to dredge away these oysters. The oysters were thick on that bed … One boat has got 30,000 oysters in a week.”

An account from France describes oyster fishing in 1909: “From 10 April to 24 April, fishing took place. The number of oysters caught was 16 million.”

Oyster restoration projects are under way but the researchers say they need to be scaled up.

Philine zu Ermgassen, an honorary researcher at the University of Edinburgh, said the destruction of the slow-forming reefs had been rapid. “These were huge areas that were thickly crusted with oysters and crawling with other marine life. There has been a fundamental restructuring and flattening of our sea floors.”

The report, The world was our oyster: Records reveal the vast historical extent of European oyster reef ecosystems, is available online.