Brest (AFP)

Owing to high mortality for ten years, oysters could benefit from cohabitation with other marine species according to the model of permaculture, revealed this week researchers gathered around a European program dedicated to the health of molluscs.

As part of this research program, which brings together ten countries since 2016, scientists from the French Institute for Research on the Exploitation of the Sea (Ifremer) have been interested in the role of biodiversity on the sensitivity of oysters to diseases .

Over the past ten years, the sector, which employs more than 40,000 people according to the European Union, including 10,000 in France according to the National Committee for Shellfish Culture (CNC), is facing a high mortality of shellfish.

Among the causes of these recurring episodes of mortality, the OsHV-1 virus that affects young oysters in several European countries or Vibrio aestuarianus bacteria that affects adult oysters, especially in France and Ireland.

Researchers from Ifremer's Laboratory of Invertebrate Physiology have looked in particular at the question of whether a diverse, healthy and rich ecosystem could be able to protect oysters from disease.

Oysters from Brest harbor have been raised in the laboratory, in a semi-natural environment, with other marine species, including mussels and sea-squirts, marine sponges.

- Filtering animals -

"Our work has shown that when we associate the oyster with other filter feeders, it is more resistant to disease," says in an interview with AFP Fabrice Pernet, a researcher at Ifremer.

Not that the viruses and bacteria were diluted between several organisms, but with a less abundant food because shared, the oyster metabolism was slowed down, leading to a lower capacity for spreading diseases.

"By eating less, oysters grew a little slower which made the diseases spread less," says Fabrice Pernet. "This is the phenomenon of marine version permaculture," he says.

In the oyster population reared with other marine animals the survival rate was 97% compared to 75% in the high control population alone. For comparison, in Brest harbor the survival rate of oysters is 50% on average.

Research carried out in the harbor of Brest, on the other hand, showed an increase in mortality in the case of an association of oysters with green algae.

The survival rate of oysters raised in the presence of green algae was 50% compared to 75% for the control population. Green algae have indeed helped to destabilize the microbial flora of oysters, making them more susceptible to diseases.

"The professionals could put small periwinkles on their oyster pockets so that they graze the green algae", recommends the researcher, borrowing there also to the technique of the permaculture of which one of the keys is the integration rather than the exclusion .

Such recommendations, the Vivaldi program, which ends this week in Brest with a report in the presence of some 200 researchers, abounds.

"We got a lot of results," says Isabelle Arzul, coordinator of the program that brings together French, Spanish, Italian, Irish, Norwegian, British, German and Danish scientists.

A manual gathering these good practices is being prepared in consultation with the producers and the competent authorities in the countries participating in the program.

Some of these recommendations, including the non-displacement of shellfish in case of mortality or the choice of specific dates and ideal temperatures to immerse spat, have already been put into practice in some countries, such as Spain.

"Oyster producers in the Ebro Delta have gone from a 80% mortality rate to a 4% rate after following our recommendations," says Dolors Furones, a researcher at the Institute of Agrifood Research and Technology. (Irta) of Barcelona.

© 2019 AFP