Redox-Sensitive Chemical Markers in Otoliths Reveal Evidence of Exposure, and Tolerance to Hypoxia in a Ubiquitous Demersal Fish

Knowledge of past exposure history to hypoxia in mobile fishes is essential to estimate population responses to this widespread environmental stressor in freshwater, estuarine and oceanic habitats. Otolith chemistry offers a unique opportunity to identify sublethal hypoxia exposure using redox-sensitive chemical markers, such as manganese (Mn), that record lifetime exposure histories for individual fish. We applied this approach to quantify the proportion of Atlantic Croaker (Micropognias undulatus) exposed to hypoxia in the northern Gulf of Mexico, which experiences widespread summertime hypoxia every year. We found the proportion of fish with sublethal hypoxia exposure during their first year of life varied between sampling years (34 - 48%), indicating that exposure was common enough to have important consequences for reproductive sustainability. However, we found limited evidence for exposure effects on somatic growth or survival for Atlantic Croaker across multiple cohorts, suggesting some life history parameters in this species are relatively resilient to hypoxia. Thus, otolith chemistry offers a novel way to elucidate the extent of exposure to this growing environmental stressor, while providing information needed to draw inference about potential population responses.