Effects of food ration, water flow rate and bacteriological levels of broodstock on the reproductive conditioning of the European flat oyster (Ostrea edulis, Linnaeus 1758)

•Oysters conditioned with a food ration of 3 % produced the largest number of larvae.•Oysters conditioned at flow rate of 3 L h−1 oyster−1 had the highest mortality rate.•Highest levels of vibrio and heterotropic bacteria were found at 3 L h−1 oyster−1.•Best larval production obtained at 3% food ration and 2 L h−1 oyster−1 flow rate.•Influence of bacteriological charge of broodstock is studied for the first time. The natural production of the native flat oyster Ostrea edulis and the production of seed in hatcheries are insufficient to meet demand. Several projects have recently been developed to optimise flat oyster production and to establish a reliable protocol for broodstock conditioning in hatcheries. In this sense, the positive effect of a gradient of temperature and a gradient of daylight on the gonadal development, spawning and larval production of O. edulis during winter and autumn conditioning was previously confirmed. No other study has yet been optimised the effect of other factors on flat oyster conditioning. This work examines the combined effect of two important factors on broodstock conditioning, namely the food ration and the water flow rate, for the first time. Furthermore, the bacteriological load of oyster broodstock and its influence on O. edulis conditioning had not been studied before. The results showed that the broodstock conditioned with a food ration of 3% produced the highest number of viable larvae. Oysters conditioned at a flow rate of 3 L h−1 oyster−1 had the highest mortality rate, coinciding with the highest levels of vibrio and heterotrophic bacteria in both meat and intervalvar fluid. The best larval production results were obtained when the 3% food ration was combined with a water flow rate of 2 L h−1 oyster−1.