Geochemical indicators of sublethal hypoxia exposure in otoliths of a ubiquitous demersal fish species in the northern Gulf of Mexico

Hypoxia in both marine and freshwaters represents a serious and growing problem around the world. One of the largest seasonally occurring anthropogenic hypoxic zones in the world occurs in the northern Gulf of Mexico (nGoMex) where nutrient input and summer stratification facilitates seasonal hypoxia in the region. While acute behavioral responses of mobile fishes to hypoxia may include avoidance or reduced activity, long-term individual and population level consequences of hypoxia exposure are relatively unknown. However, the cycling of redox-sensitive elements such as manganese in hypoxic zones offers an opportunity to reconstruct lifetime hypoxia exposure in otoliths of mobile fish species. We sampled bottom water elemental concentrations in the nGoMex during hypoxic and non-hypoxic seasons and found a strong negative correlation between dissolved oxygen and dissolved manganese. Analyses of otoliths from our study species, the ubiquitous Atlantic croaker (Micropogonias undulatus), reveal distinct lifetime patterns of manganese fluctuations that indicate differential individual exposure to hypoxic waters, with hypoxia-exposed fish constituting a large proportion of fish obtained from sampling stations across the nGoMex “Dead Zone”. These results suggest that Atlantic croaker may be relatively resilient to hypoxia exposure, although the effects on growth performance may be cryptic. We suggest identification of an individual’s exposure history to hypoxia across multiple cohorts and between years could help define potential impacts on growth and differential survival. Subsequently, this information could be used to more accurately predict long-term population level consequences that until now have relied primarily on short-term indices of hypoxia exposure.