Le risque sédimentaire

The "sedimentary risk" has been identified since the mid-nineties in the Marennes Oléron Bay. The proximity of the sediment (from 0 to 15cms) increases the mortality rate. Depending on the year, mortality can be 10 to 20 % higher than in "high" conditions (50 to 70cms). These zootechnic results, gathered since 1996 on the working site of Ronces – Perquis, lead to the research of a pathogenic agent or a toxic substance released by the sediment. Studies conducted between 2002 and 2003 [12, 18] did not allow the identification of any pathogenic agents and ruled out the infectious risk. In 2004, the chemical risk (sulphides, ammoniacal nitrogen) was studied on 4 sites (3 working sites + Quiberon). In 2004, the "on-bottom/off bottom" mortality model of the Marennes Oléron Bay was transferred to Auray, Quiberon and Bay of Veys. The sedimentary risk has been confirmed for the Bay of Veys and for Quiberon (?) but not for Auray where the lack of significant differences between 15 and 70cms could only be the consequence of the homogeneity of the water column on a more hierologically unstable site!