Integration of shellfish farming activies in three French coastal environments : Mont Saint-Michel Bay, Gulf of Morbihan, and Marennes-Oleron Bay

Integration of shellfish farming along the French shoreline (around 5500 km) has been progressive, as marine bivalve culture is a traditional activity in our country. The history of mussel culture relates that the "bouchot" technique (intertidal culture on wooden poles, producing annually around 60,000 tons) was developed as early as the XIIIth century, by a refugee attempting to capture birds by sticking branches of brushwood along the shore. The European flat oyster, Ostrea edulis, already appreciated at the Roman period, was first cultivated in France after Victor Coste introduced the technique of spat collection at the end of the XIXth century. The Pacific cupped oyster, genus Crassostrea, now the most-cultivated in France (around 120,000 tons marketed annually), has a curious history of introduction; C. angulata (called the Portuguese oyster) is supposed to have reached Europe, with the overseas expeditions in the XVIth century; then it was introduced in Southern France, at the end of the XIXth century, after a shipwreck in the Gironde estuary. The Pacific oyster, C.gigas, was voluntarily introduced around 1970, to replace the former Portuguese oyster that had been almost completely eradicated by a viral epizootic and to sustain the oyster industry. Integration of shellfish farming along the French coasts may be described from three representative mollus cculture sites : la Baie du Mont Saint-Michel (North Brittany, West of France), producing annually 15,000 tons of Pacific cupped oysters and blue mussels Mytilus edulis in intertidal areas; le Golfe du Morbihan (South Brittany, West of France), where 5,000 tons of Pacific oysters are harvested annually; and le Bassin de Marennes-Oléron (mid-West France), a famous place for oysters in France (stocking biomass>120,000 metric tons), with activities ranging from spat collection to growth (35,000 tons harvested annually), "affinage", and then trade (60,000 tons marketed each year). From a production standpoint (growth rate, mortality, yield per ha, meat content), oyster figures from the three sites are not very different, in spite of significantly different seawater parameters (temperature, salinity, turbidity, phytoplankton, terrestrial contaminants...). This consistency may be attributed to the high adaptability of the shellfish species. However, great differences among the three sites can be described regarding the constraints: - Space for mollusc cultivation seems widely available in Mont-Saint-Michel bay (15 km² cultivated intertidally, out of 250 km² available), although the most favourable places have been occupied. In Morbihan Gulf, a larger proportion of intertidal area, as high as 25%, is occupied by oyster leasing grounds (16 out of 65 km²) ; in the Marennes-Oléron Bay, the extent of occupation is still higher (20 km² of leases, out of 96 km² of strand). Food availability also differs between the three sites: in Mont-Saint-Michel bay, widely open to oceanic influence, phytoplankton is rather diluted (around 2 µg .l(-1) chlorophyll a, as an average), but extremely well renewed; although more enclosed, Morbihan Gulf shares the same characteristics; Marennes-Oléron basin, for its part, has already exploited a great part of its trophic capacity, leading to suboptimal, food-limited growth. - The increasing public interest for nature conservation exerts more and more influence on shellfish farming: in Mont-Saint-Michel bay, mollusc culture has already modified the scenery of the western part of the bay, but natural forces (tides in particular) are so strong that this imprint is likely superficial. However, public support is progressively shifting from production enhancement to environmental protection: oyster culture in Mont-Saint-Michel bay is questionned for invading Sabellaria reefs; there is also a dilemma between preservation of common scoters Melanitta nigra and their frightening away as mussel predators. Oyster culture in Morbihan Gulf has been questionned for its impact on biodiversity, including siltation on seagrasses (Zostera), but only a marginal impact was demonstrated. Conflicting uses of the same public resource, between shellfish farmers and other stakeholders, are the main concern: regarding the landscape (acrimony against mollusc growout facilities, when they are judged unaesthetic or too dispersed as is the case in Morbihan Gulf); in neighbouring salt marsh areas (oyster "claires" from Marennes-Oléron basin, looked upon as natural ecosystems by environmentalists, in spite the fact that the hydraulic system is entirely human mediated ); in intertidal, coastal zones (by way of example oyster trestles are considered to interfere with the free access to beaches in Morbihan Gulf); deep conflicts also exist with fishermen, whose trawls may capture cultured oysters or accidentally damage the bordering bouchots (Mont-Saint-Michel bay). Less directly, but often more contentiously, water quality may be a disputed issue: an agreement made in Morbihan Gulf for better management of seawage plants is an example of successful cooperation for improved water quality. Such an agreement remains to be found between corn farmers from the watershed of Marennes-Oléron, and oyster farmers, for freshwater availability in summer, when it is scarse and much needed by both sectors. A recommendable way to cope efficiently with these potentials and constraints would consist in marrying a scientific approach (numeric models coupling physical and biological processes), and a participatory one. Integrated Coastal Zone Management has been identified as a framework to analyse interactions among human activities. A project on Science and Policy Integration for Coastal System Assessment (SPICOSA, http://www.spicosa.eu/) supported by the European Commission has started in 2007. Within the Marennes Oléron bay study site, the use of water resources will be adressed with respect to land uses and needs for shellfish aquaculture.