COMPARATIVE GROWTH AND MORTALITY OF BONAMIA-OSTREAE-RESISTANT AND WILD FLAT OYSTERS, OSTREA-EDULIS, IN AN INTENSIVE SYSTEM - I - FIRST YEAROF EXPERIMENT

Since the early nineteen-seventies, the parasitic protozoans Marteilia refringens and Bonamia ostreae have consistently deleteriously affect ed the French production of Ostrea edulis (Linne). Purification and in oculation of B. ostreae that were achieved in the late nineteen-eighti es allowed a selection program to be initiated. Two O. edulis strains, S85 and S89, were raised that displayed significant resistance to B. ostreae in the held. Growth was monitored to ensure that the resistant strains grew at a rate comparable to that of controls, using two cros ses between the first generation of S89 and the second generation of S 85 (G1G2), and between the first generation of S89 and wild oysters (G 0G1). Growth and mortality were monitored for seven consecutive months in an intensive open circulating system at the IFREMER station of Bou in (Vendee, France). A Chapman-Richards model of growth revealed that the two ''resistant'' crosses grew significantly better than two contr ols from Quiberon Bay (Brittany), an area in which B. ostreae is endem ic, and Palavas (Mediterranean Sea), where oysters are less infested b y this parasite. The asymptotic values for growth curves ranged betwee n 27.2 and 28.2 g for G1G2 and G0G1 vs 21.0 and 22.8 g for the control s. There was a similar trend in mortality, with G1G2 (11.8%) surviving better than the three other populations (from 28.7 to 57.5%). B. ostr eae was not detected during the experiments and mortalities were attri buted to feeding conditions that were not optimal, reinforced by gamet ogenesis and high temperatures during the summer. It is suggested that the better performance of G1G2 and G0G1 resulted from their increased resistance to stress.