Etude de la reproduction de l'huître creuse dans le Bassin d'Arcachon

The Arcachon Basin is an environment particularly favorable to the reproduction of the Japanese oyster Crassostrea gigas, introduced in France in 1970. An estimated 60 to 70% of oysters sold in France are native to the Bay of Arcachon.This is why following the strong deficit of spatfall of 1998, a study was undertaken to determine whether this event was part of or not fluctuations in 'natural' catchment. To do this, we had to identify the factors responsible for the variability of larval recruitment and study their impact on the various stages of reproduction.The results of this study are based on analysis of historical data (1985-2003) and the acquisition of new data during five breeding seasons, with very different intensities of recruitment (1999 to 2003). This work led to the following conclusions. - The intensity of spatfall depends, first of all, on survival of larvae during their development and, secondly, on the abundance of veligers formed after the main spawn. The years of abundant collection are characterized by high values of the number of veligers and their survival, while these two parameters are particularly low for years of very poor spatfall (1998 and 2002) . - The number of veligers depend on reproductive effort of parents, which is itself conditioned by the phytoplankton richness of the medium between the end of winter and spring before egg laying. - The cohort survival depends on the temperature of the water in which they develop, both directly (by accelerating their growth) and indirectly (by influencing the amount of food available for the larvae): the abundance of phytoplankton cells 4 to 20 microns (nanoplankton), which appear to be the major source of larval food, is indeed positively related to temperature - It was not possible to demonstrate an influence of other factors studied (dispersion conditions of the larvae, salinity, zooplankton predator or competitor of oyster larvae and disease) on survival of cohorts. - The concentration levels of major organic and inorganic molecules that may affect the settlement (coming from the watershed and anti-fouling paints) was established, the summer levels being quite variable depending on the year studied. Under the conditions observed between 1999 and 2003 (and from 1982 to 2003 for copper), this contamination did not appear sufficient to directly affect the survival of oyster larvae. Nevertheless, during the summer of 2000, the levels of herbicides slightly higher than in other years could adversely affect the abundance of available food (nanoplankton). The results of the low sparfall in 1998 and 2002, as well as those of the very good one in 2003, are well explained by the parameters "spring chlorophyll" and "temperature during the development of cohorts" during these years. However, the relatively low survival of the unique cohort of 2000 summer which is not explained by the low temperatures, appears to be due to a lack of food for the larvae may be related to a little higher concentrations herbicides in the waters than other summers.