Diet and stable isotope signatures of Atlantic sturgeon (Acipenser oxyrinchus) caught in Minas Basin, Bay of Fundy, and the Saint John River, Canada

The feeding habits of Atlantic sturgeon Acipenser oxyrinchus (AS) juveniles and sub- adults from Minas Basin (MB), and adults taken during their spawning run in the Saint John River (SJR) were investigated by stomach content and stable isotope analysis. While the diet of AS from MB was mainly composed of Corophium volutator (% index of relative importance = 98.6 %), the stomachs collected from the specimens caught in the SJR were empty. In SJR AS, no correlation was found between δ¹⁵N and C:N ratios suggesting that lipid reserves satisfy energy costs related to the spawning run. Trophic positions (TP) were estimated using isotopic data of C. volutator as the food web baseline for the region. Blood isotopic data indicated that SJR AS occupied a higher TP than MB AS (3.9 ± 0.3 and 3.7 ± 0.1, respectively), whereas the opposite was indicated by muscle data (4.0 ± 0.2 for SJR AS, and 4.5 ± 0.1 for MB AS). Isotopic niche areas estimated by standard ellipse areas and kernel utilization density analyses indicated that MB AS had a more diverse diet than SJR AS. There was no isotopic niche overlap between MB AS and SJR AS indicating that the two groups had distinct feeding grounds. These findings are directly relevant to the conservation and management of the AS, providing critical empirical data to support ecosystem-based models, particularly in the Bay of Fundy and Saint John River regions.