Inventory of the last flat oyster reefs

Europe has lost thousands of hectares of forests. The disappearance was so radical that the men lost their memory. The only species of oyster native to Europe, Ostrea edulis it is now present only in a few very limited places in Brittany. It is no longer found in Germany and is very rare in the rest of Europe. “We have identified a dozen stations where flat oysters remain in the wild, and above all we have identified ecological conditions that would allow the restoration of these mounds very similar to corals as they existed until the 19th century.“, rejoices Stéphane Pouvreau, marine biologist at Ifremer Brest.

Forest: This term may seem strange to shellfish living in a marine environment. The analogy, however, is correct: flat oysters create a homogeneous environment like trees and produce ecosystems that benefit other animals. “If they form clumps of lime shells, it’s simply because the juveniles attach to the shells of the adults,” explains Stéphane Pouvreau. Unlike Japanese cup-shaped oysters Crassostrea gigas which we grow and consume today mainly in mainland France, Ostrea edulis forms benches almost always submerged. If its wild form has become extremely rare, cultivation continues with about 1,500 tons produced annually, mainly in the bay of Quiberon in southern Brittany, in the bay of Mont-Saint Michel and a little in the bay of Brest called “belon”.

It was considered inexhaustible, but the source eventually disappeared

The flat oyster has disappeared mainly due to overexploitation over several centuries. Consumed since the Paleolithic, it was a delicacy among the Romans and throughout the Middle Ages. Then the resource is considered inexhaustible. But from 17e century, the improvement of dredging techniques and the increase in demand caused a real “deforestation” along the entire European and British coast. Consumption is such that the remains appear at the village gates, “grenade embankments”. The weakening of production was compensated for in the early 1870s by the invention of oyster farming by Victor Costa and Ferdinand de Bon. Cultivation allows to maintain an annual production of 30,000 tons of flat oysters during the 20se century. The line is then pushed by a glass oyster Crassostrea angulata known as the Portuguese oyster which naturally takes root from individuals released into the Gironde estuary by sinking a ship that brought this species of Asian origin from Portugal and then introducing its cousin Crassostrea gigas sixties. 1968. arrival of parasites Marteilia then in 1980 Bonamia to decimate both hollow and flat oysters. If the Japanese oyster, the apartment, was saved by the production of hollow, the apartment did not recover. Today, out of 90,000 tons produced in France, 1,500 are flat.

This historical reminder partly explains the unusual amnesia. There are few written, drawn or photographed testimonies left about the immense reefs of the past, so much so that we do not know what they looked like. “Our research work on the last natural sites was based on an atlas made in 1908, which is the only memory of the areas that at that time could be occupied by oyster reefs that were called” oysters “at that time.”, Continues Stéphane Pouvreau. Finding them was no easy task. Acoustic techniques proved ineffective due to the low density of the shells, the researchers had to collect testimonies from oyster farmers and put on boots and fins for diving in bottles. This census, conducted as part of the European research program FOREVER (recovery of flat oysters), takes a long time. Until 2010, research was focused on improving the quality of oysters and their resistance to parasites. “But interest returns to understanding natural heritage and preserving it,” notes the biologist.

Oyster ridges can be found for several centuries

Studying the species also allows us to better understand the insidious changes at work in coastal areas. If there are so few reefs, it is because their environment is increasingly unfavorable. The flat oyster sees its favorable environment diminish. This one does not like mud, and as for coral, its accumulation between relatives allows it to be extracted from the sediment and grow in the water column to optimize its filtration. However, due to soil erosion caused by intensive agriculture, the estuaries are constantly blurred. Oysters therefore need to move more along the sea. “There is another obstacle they face, sorry Stéphane Pouvreau. In summer, they need water at 18 ° C to be able to reproduce, temperatures that cannot be found in the sea. “

Reef test to attract flat oyster larvae. © Stéphane Pouvreau / Ifremer

A better understanding of the ecosystem allows thinking about promoting species return. FOREVER program, whose results have just been published, did so by finding a way to encourage oyster larvae to prepare in schools. “We tested various techniques and showed that coarse limestone carriers near the natural stripes of adult oyster shells allow the saliva to be encouraged to settle in these places if possible,” researcher details. This is the second phase of the operation. With the support and technical assistance of an oyster breeders ’union in southern and northern Brittany, biologists will identify the most favorable sites for the restoration of that species. If we can reasonably think that this stimulus should bring its “fruits” to nature, it will take a lot of patience to find these amazing limestone formations that were supposed to be built Ostrea edulis three centuries ago. It will take decades to recover.

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