Testage de souches d'huîtres plates Ostrea edulis tolérantes à Bonamia ostreae: élevage contrôlé, écophysiologie et modélisation de la croissance

Since the 1960's, breeding of the flat oyster Ostrea edulis has been under threat from two protozoan parasites, one of which is Bonamia ostrea. Several attempts have been made to introduce flat oysters of the Ostrea genus on the French coast, without much success. The IFREMER genetics laboratory. at Ronce les Bains has used selective pressure with Bonamia ostrea to obtain bonamiosis-resistant strains of the flat oyster, Ostrea edulis. The growth and physiology of these animaIs was studied in an intensive breeding system in which the algae Skeletonema costatum was used as fodder. The growth performance of the tolerant animals was compared with that of non-selected controls over a period of three years (1994,1995,1996). An analysis of the results showed the following: - The populations of tolerant flat oysters grew better than the non-selected controls. - The selected animals were less sensitive to environmental stress (increase in temperature, fall in oxygen concentration...) than the controls. - In the absence of stress, both populations had identical survival rates. Mortality was low and didn't rise above Il % throughout the duration of the experiment.- This species seems well -adapted to intensive breeding, on condition that a sufficient quantity of phytoplankton-enriched sea water be supplied (31/h/animal). A study of the physiology of the flat oyster was made in order to measure the change in clearance rate and oxygen consumption with increasing temperature (10 to 30°C) as a function of the animal 's dry weight. The allometry for the animal's body weight was 0.62 for clearance rate and 0.74 for oxygen consumption. Both these physiological functions increased with temperature. The ponderal growth performance was correlated with the physiological response of the flat oyster. The animal' s clearance rate was found to be the physiological parameter that correlated best with growth (r2=0.68). Finally. a deterministic model was made of the growth of the flat oyster Ostrea edulis in intensive breeding conditions. The results validate the clearance rate and oxygen consumption models and also show the sensitivity of the efficiency of absorption as a function of the dilution or organic matter by the mineral seston. However our knowledge of the mechanisms leading to the induction of reproduction are still too sparse, with as yet too many unknowns to provide an accurate model for flat oyster growth in intensive breeding conditions.