Oysters once formed extensive reefs along much of Europe’s coastline – but these complex ecosystems were destroyed over a century ago, with the advent of steam ships with powerful dredges accelerating their disappearance through overharvesting, new research shows.
Researchers found past evidence of reefs almost everywhere, from Norway to the Mediterranean, covering at least 1.7 million hectares, an area larger than Northern Ireland. 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.
Photo: Stephane Pouvreau
Native oyster reefs created their own ecosystems, full of a diverse range of underwater life – supporting a greater number of species than surrounding areas. Over decades and centuries, oysters grow on top of the shells of their ancestors, building up reefs that serve as habitat for spawning fish. The reefs of some species can become robust enough to protect shores from storms. The oysters also played a vital role in nutrient cycling and water filtration – with a single adult oyster filtering up to 200 litres of water a day.
“Oyster reefs are slow to develop, with layers of new oysters building up on the dead shells of their predecessors, but their destruction through overfishing was relatively rapid,” said Dr Philine zu Ermgassen, honorary researcher at the University of Edinburgh and joint lead-author.
“This has caused a fundamental restructuring and ‘flattening’ of our seafloors – removing thriving ecosystems and leaving an expanse of soft sediment behind. Thanks to this historical ecology research, we are now able to quantitatively describe what oyster reefs looked like before they were impacted, and the spatial extent of the ecosystems they formed.”
Historical research
One of the oldest documents uncovered in the research, published in 1715 by Luigi Ferdinando Marsili, an Italian naturalist, described the abundance of oyster reefs in the Adriatic Sea: “The sea floor is filled with oysters, almost placed one on top of the other like stones, forming a wall.” In 1885, The Fisherman’s Practical Navigator surveyed oyster beds of “enormous dimensions” in the North Sea, extending 320 kilometers by 110 kilometers. Other documents hinted at the overharvesting: An 1879 fisheries report from the Isle of Man describes a single boat taking 30,000 oysters from the Irish Sea in one week.
By combining all these records, the team created a map showing dense oyster beds extended along the coastlines of the United Kingdom, Ireland, and continental Europe.
The first day of oysters: a London street scene. Published in The Illustrated London News of 1861. Showing people gathered around the stall of an oyster seller, some opening oysters, others eating them. Taken from Old Book Illustrations under a Creative Commons 4.0 License.
Restoration efforts
“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”, said Thurston.
About 30 small restoration projects are underway in Europe. They drop shells or other substrate into coastal waters to provide habitat for transplanted oysters. A few are beginning to scale up to add oysters across a hectare or so. “It’s at a very hopeful stage,” zu Ermgassen said.
Rachel Smith, a restoration ecologist at the University of California, Santa Barbara, notes reestablishing oyster reefs is a “daunting challenge.” Introduced diseases, for example, can set back recovery efforts. Smith participated in successful oyster restoration projects in Virginia, but she says the oyster species in Europe (Ostrea edulis) may be slower to rebound because it reproduces by brooding larvae rather than spawning offspring that drift and settle.
Report co-author Professor Joanne Preston, University of Portsmouth Institute of Marine Sciences, said: “The paper acts like a time portal; hopefully it will create a radical shift in our perception of what European coastal waters used to look like, and therefore provide evidence to set more ambitious goals for future restoration of habitats and biodiversity.”
The research team was made up of more than 30 European researchers from the Native Oyster Restoration Alliance. The paper, “Records reveal the vast historical extent of European oyster reef ecosystems.”, is available online.