| Summary: | National audience 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 epidemies, mussels may be considered as a favourite candidate for mollusc aquaculture in the bay of Brest. La croissance des mollusques bivalves a été choisie comme indicateur biologique de qualité d'eau de la rade de Brest, en raison de certaines caractéristiques biologiques de ce groupe (sédentarité, filtration.), et de son importance dans l'aquaculture nationale. Entre 1992 et 1995, des échantillons d'huîtres et de moules ont été mis en place à quelques stations intertidales et subtidales, et leur croissance en chair et coquille évaluée chaque trimestre. Des références ont été obtenues, selon le même protocole, dans quelques sites conchylicoles extérieurs à la rade de Brest. Les huîtres creuses Crassostrea gigas manifestent en rade de Brest deux ...
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