Natural chemical tags reveal trophic resilience of demersal fish to hypoxia exposure

Seasonal hypoxia may affect benthic ecosystems by altering trophic relationships between benthic prey and mobile predators. Identifying the sublethal effects of hypoxia on mobile fishes is difficult without long-term chronological geochemical records of individual exposure histories. In this study, trace element profiles in otoliths of demersal Atlantic croaker Micropogonias undulatus collected from the “Dead Zone” region of the northern Gulf of Mexico were used to quantify environmental exposure histories over the 2-3 months prior to capture. Otoliths were analyzed to quantify profiles of manganese (Mn) as a proxy of redox conditions and barium (Ba) as a proxy of estuarine habitat use. Estuarine and hypoxia exposure indices based on these proxies clustered fish into four recent exposure groups that included: late estuarine migrants, early estuarine migrants, normoxic coastal residents, and hypoxic coastal resident fish. Muscle tissue stable isotope values of carbon and nitrogen that also reflected 2-3 months of recent dietary history were used to estimate isotope niche area using standard ellipses. Fish demonstrating more estuarine habitat use displayed larger niche areas, while normoxic and hypoxic coastal resident fish exhibited small and statistically similar niche areas. These results suggest trophic resilience of demersal croaker to seasonal hypoxia in the northern Gulf of Mexico. A combination of otolith chemistry and tissue stable isotopes further enhances our understanding of responses to sublethal hypoxia and the potential consequences for ecosystem functioning.