The assessing of Pacific oyster (Crassostrea gigas) rearing performances by the IFREMER/REMORA network: method and first results (1993–98) in Brittany (France)

Abstract

Every year since 1993, from February to December, the French network IFREMER/REMORA has monitored mortality, growth and quality criteria of two oyster year-classes (‘juveniles’ of the first-year class and ’adults’ of the second-year class), distributed among various stations of French oyster farming areas. This has provided a standard and simple annual assessment of the rearing results, generally tallied with professional observations. Each year, data series have been obtained, from which mean values (references), chronological trends or spatial differences can be analysed. Results in Brittany, from 1993 to 1998, show that 80% of oysters of 30 g in February grow to a range of 55–85 g at the end of the year, with a mortality rate lower than 20%. Mortality occurs mainly in spring for adults and in summer for juveniles. Although annual fluctuations exist, connected to climatic and hydrological influences, differences remain high between the monitored stations. This allows characterization of sites. Some chronological trends appeared significant, but the period of time remained too short for any prediction. On the other hand, REMORA data, especially when compared to climatological or hydrological data series, may support various types of studies: description of oyster quality, biological indicators in coastal water monitoring, explanatory models of the oyster-farming ecosystems. Lastly, unusual mortalities (in 1995), lack of growth (1998) and problems with quality (infestation by the worm Polydora) could be pointed out and quantified. Moreover, unusual results could be observed, which might not be explained only by annual variations or geographical specificities. This gives to REMORA network the role of watchdog and the possibility of giving advice for collective oyster management.