MEPS 560:207-221 (2016)  -  DOI: https://doi.org/10.3354/meps11934

ABSTRACT: Seasonal hypoxia may alter trophic relationships between benthic prey and mobile predators if consumers with low hypoxia tolerance are vertically displaced from bottom waters and switch to pelagic prey. Alternatively, consumers with greater hypoxia tolerance may continue to forage on stressed benthic prey. Identifying the trophic effects of hypoxia on mobile fishes requires long-term records of individual exposure histories. Elemental profiles in otoliths of demersal Atlantic croaker Micropogonias undulatus from the northern Gulf of Mexico (nGoM) were used to quantify hypoxia exposure and estuarine occupancy histories over the 2 to 3 mo prior to capture. Elemental patterns in croaker otoliths clustered fish into 4 groups: early or late estuarine migrants and normoxic or hypoxic coastal residents. Stable isotope values of δ¹³C and δ¹⁵N in croaker muscle were compared between clusters to determine trophic shifts associated with environmental histories, while isotope niche areas of clusters indicated whether trophic shifts were uniform or variable among individuals within each cluster. Estuarine migrants displayed lower δ¹³C and δ¹⁵N values, indicating greater contribution of terrestrially derived diets, although the relatively larger isotope niche areas for estuarine clusters was consistent with individually variable emigration timings. Coastal normoxic and hypoxic fish both had similar δ¹³C and δ¹⁵N values, suggesting limited vertical displacement to pelagic food webs in hypoxic fish. These results indicate trophic resilience of demersal croaker to seasonal hypoxia in the nGoM, with no detectable change in trophic dynamics over monthly time scales. This paired natural tag approach further enhances our understanding of sublethal trophic responses to hypoxia and consequences for ecosystem functioning.

KEY WORDS: Hypoxia · Trophic resilience · Otolith chemistry · Stable isotopes · Gulf of Mexico · Micropogonias undulatus