Hatchery production of great scallop, Pecten maximus, remains unpredictable, notably due to poor larval survival. Large‐scale flow‐through systems up to 3500 L have been developed to avoid the use of antibiotics in static systems. Alternatively, small‐scale flow‐through systems have been successfully applied for oysters but they proved to be unsuitable to rear scallop larvae. By focusing on physical factors presumed to limit P. maximus larval development, this study aimed to optimize great scallop larvae rearing parameters under controlled conditions. First, the influence of aeration on larval performances, energetic metabolism and antioxidant defences were studied both in static and flow‐through systems. Aeration depressed larval food intake, regardless of the intensities of flow tested (100 ml/min, 155 ml/min and 270 ml/min). On the other hand, antioxidant enzyme activities remained constant or decreased, suggesting that antioxidant defences were inactivated. The increase in citrate synthase activity suggested an increase in metabolic rate possibly due to a turbulent stressful environment. All larvae exposed to such turbulence died before reaching metamorphosis, whereas those reared without aeration survived well (≈ 95%). The effects of water renewal were thereafter studied in 50‐L flow‐through flat‐bottomed tanks. No differences in survival (20.4 ± 0.5%), growth (3.8 ± 0.2 μm/d), competence (5.6 ± 0.2%), energetic metabolism level and antioxidant enzyme activities were observed when comparing 12.5 and 25 L/hr water renewal. Whereas air bubbling leads to detrimental effects, flow‐through in small flat‐bottomed tanks appears to be a suitable technique for scallop larvae rearing.

Pathogenic bacteria are known to be one of the main factors affecting Pecten maximus larval survival in hatcheries. As a result, juvenile production often relies on the use of antibiotics during larval culture. However, limitations of the usage of chemicals such as chloramphenicol in aquaculture have been reinforced due to their negative environmental impact and alternatives are accordingly needed. Thus, the importance of bacterial transfer from oocytes to larvae has been studied here as well as procedures to limit larval mortality in P. maximus. In order to reduce bacterial contamination during larval development, we focused on two periods, broodstock conditioning and post-fertilization. The animals were conditioned for 2 months with two erythromycin treatments of 6 days, with (SA) or without sandybottom (NSA). The absence of sediment strongly reduced contamination by Vibrios of oocytes (NSA: 0.003 ± 0.002 CFU oocyte-1SA: 0.57 ± 0.17 CFU oocyte-1) and D-larvae (NSA: 0.14 ± 0.05 CFU oocyte-1SA: 0.51 ± 0.002 CFU oocyte-1). It also enhanced survival by 52% at 15 days post fertilization, whereas a two days antibiotic treatment of D-larvae did not improve subsequent survival, regardless of broodstock conditioning method. Furthermore, both treatments led to similar fatty acids profiles of oocytes and resulting larvae, suggesting that broodstock conditioning was physiologically similar with or without sediment. This work has shown that gametes contamination could be reduced when sandy-bottom was removed during broodstock conditioning under bacterial control with erythromycin. The present results contribute to a reduced utilization of antimicrobial agents for great scallop larval rearing in controlled condition. Highlights ►The absence of sediment during scallop broodstock conditioning strongly reduced contamination by Vibrios of oocytes. ►It also enhanced larval survival by 52% at 15 days post fertilization. ►Two days antibiotic treatment of D-larvae did not improve subsequent survival, regardless broodstock conditioning method. ►The use of selected antibiotic targeted on broodstock for a brief and early antibiotic treatment depressed sharply bacterial flora.