The Legacy of Hypoxia: Tracking Carryover Effects of Low Oxygen Exposure in a Demersal Fish Using Geochemical Tracers

The increasing prevalence of hypoxia in aquatic ecosystems emphasizes the need to understand sublethal effects on organisms to anticipate potential changes in population dynamics. Carryover effects of hypoxia exposure during early life stages on lifetime survival and growth performance are rarely known, which leaves major knowledge gaps about the long-term impacts of sublethal exposures. Geochemical signatures incorporated into fish hard parts can track lifetime hypoxia exposure, thereby allowing sublethal carryover effects across age-classes to be quantified. We examined the impact of hypoxia exposure on the Atlantic Croaker Micropogonias undulatus, a demersal fish inhabiting the northern Gulf of Mexico, which is one of the largest hypoxic systems in the world. Using an otolith-based, dual-proxy (Mn:Ca and Ba:Ca) geochemical method coupled with intracohort comparisons, we assessed changes in the traits of Atlantic Croakers that survived hypoxia exposure during their first year of life. We also investigated whether somatic growth was affected by exposure to sublethal hypoxia during the first year of life. Within three cohorts of Atlantic Croakers, there was no evidence that the otolith-based traits of survivors changed from one year to the next. Additionally, somatic growth in individuals that had previously experienced sublethal hypoxia during the first year of life and survived was not depressed relative to somatic growth in individuals that did not experience hypoxic conditions. These results indicate that Atlantic Croakers are tolerant to hypoxia experienced during early life and that sublethal carryover effects do not manifest as reductions in survival or growth. Geochemical indicators of hypoxia exposure in fish otoliths can thus provide vital insight into the range of potential sublethal responses by fishes with variable sensitivities to this growing environmental stressor across the world.