For nearly 40 years, the Bay of Morlaix has experienced an annual cycle of green algae growth, particularly in the spring. This phenomenon is linked to the eutrophication of the environment, which is in fact the excess of nutrients in the environment. In this case, it is the excessive supply of nitrates and/or phosphates that unbalances the ecosystem.

In themselves, green algae are natural and naturally regulated species. It is the overabundance of green algae species that can cause problems.

A change in oyster farming practices

Fifty years ago, oyster dredging did not require any special preparation of the parks because no green algae were present. Today, prior harrowing ("raking") is necessary to remove the algae from the parks before recovering the oysters.

For 5 years, the proliferation of green algae has been noted by oyster farmers in the Bay, both by the period of presence which extends into the summer and the surface area affected, particularly in the deepest parks. It particularly impacts oyster farmers who continue to practice land farming (see article "L'Huître de sol - Un savoir-faire traditionnelle et respectant de l'environnement"). These green algae will cover the oysters on the ground and constitute a layer of material that sea movements (wind or current) will no longer be able to move.

The overabundance of green algae becomes problematic in the phase of decomposition of the algae. The process of putrefaction will indeed raise the temperature and kill all life under it, in particular oysters but also the entire ecosystem (crustaceans, gastropods, other molluscs and other algae).

What solutions are there to address oyster mortality?

To prevent this phenomenon, which could cause up to 100% mortality of oysters, particularly in the smallest oysters (spat), we must harrow ("rake") the algae to resuspend them in the water so that their decomposition can be better managed by the sea.

This harrowing activity, normally limited to periods of oyster growth to ensure the shape and spacing of the oysters, becomes very frequent during the period of algae proliferation.

Beyond the work overload and the economic impact on oyster farming businesses that this phenomenon generates, it is above all indicative of an imbalance in the ecosystem and a degradation of its biodiversity.

Oyster farming is one of the last professions where man contributes nothing to nature (neither food, nor medicine, nor other input) except his work.

For the moment, it is therefore not possible for oyster farmers to seek solutions other than manual ones to deal with the phenomenon. Consequently, the only good solution is to understand this phenomenon well by mobilizing the scientific skills necessary to effectively treat its causes.