Mussels and oysters growth in Brest bay, indicators of water quality and molluscs farming potential

Abstract

The growth of bivalve molluscs has been selected as a biological indicator of water quality in the bay of Brest (Brittany, western France) thanks to the biology of this group (sedentary living, filter-feeding...) and its importance in national aquaculture. From 1992 to 1995, oysters and mussels have been laid at intertidal and subtidal defined locations, and sampled every third month to assess meat and shell growth. References from other exploited sites in Brittany were added to the comparison. The pacific oysters Crassostrea gigas show two shell affections in Brest bay: an over-thickness, characteristic of tributylstain effect, and a Polydora sp. infestation generating mud blisters. Both symptoms decrease from north to south of Brest bay. The oysters in Roscanvel (nearest site from Brest bay mouth), which site is actually cultivated, can be considered almost normal. The mussel Mytilus edulis productivity of the two sites at the south of Brest bay is moderate (inferior to the best Breton references), while it is exceptionally high in the Elorn north site, the only site where mussels are farmed in Brest bay. This species, less sensible to Tributylstain, seems to take advantage of nutrients from estuaries. As clams and flat oysters suffer from endemic diseases, mussels may be considered as a favourite candidate for mollusc aquaculture in the bay of Brest.