The physiological and behavioural responses of early life phases in American Atlantic sturgeon (Acipenser oxyrinchus) towards sand and gravel substrate were examined during the first 15 days post-hatch. The free embryos were reared in circular tanks with approximately 30% of the bottom surface covered with either coarse gravel or sand. A group reared in tanks without additional substrate served as a control. Diurnal differences in activity patterns were observed. Substrate use by the free embryos revealed significant differences during the first 5 days post-hatch, being higher in the gravel group than in the sand group. The results in size of the free embryos revealed significant differences, with the gravel group showing the lowest total length and wet mass until the onset of exogenous feeding -although dry mass and energy contents were highest. In contrast, length and wet mass during yolk sac absorption were highest in the control group, but energy content at onset of exogenous feeding was 14% lower compared to the gravel group. The onset of exogenous feeding in the gravel group had a 1-day delay when compared to the two other treatments. On day 14, following the successful establishment of exogenous feed uptake, the specific growth rate in wet mass (SGR) for the gravel group (0.250 ± 0.088) exceeded those of the two other treatments (sand 0.132 ± 0.038 and control 0.095 ± 0.020) significantly (DunnÕs n = 10 and n = 5, P < 0.05), indicating a compensational growth pattern. Cortisol concentrations in free embryos did not differ significantly among the three groups, providing no evidence for differences in stress levels among the three treatments until onset of exogenous feeding. The results strongly suggest that gravel is the most beneficial substrate for free embryo development of American Atlantic Sturgeon, impacting the physiological performance of their early life phases and reducing predation.