Tracking hypoxia induced trophic shifts of Atlantic Croaker (micropogonias undulatus) in the Gulf of Mexico using stable isotopes

Seasonal hypoxia in the northern Gulf of Mexico (nGoMex) occurring during the summer months can have sub-lethal effects on fishes by impairing reproductive capabilities, reducing growth rates, displacement to sub-optimal habitat, and altering trophic interactions. Atlantic Croaker, (Micropogonias undulatus) hereafter referred to as croaker, are demersal omnivorous fish found throughout the nGoMex including the area affected by the hypoxic zone. Bottom-water hypoxia may displace croaker from preferred benthic prey to pelagic alternatives. Pelagic shifts will not occur if resilient consumers can withstand hypoxia enough to continue foraging on stressed benthic prey. Stable isotopes can be used to resolve benthic to pelagic food web shifts given known differences in primary producers, known as isotopic endmembers. To identify recent hypoxia and estuarine residence, I used microchemical otolith markers for hypoxia (manganese) and salinity (barium), thereby clustering fish by exposure histories. Time periods of recent otolith exposure histories were matched to experimentally-validated turnover rates of δ13C and δ15N in croaker muscle allowing direct comparisons between exposure type and food web dynamics. Isotope niche widths revealed variable trophic shifts among individuals across two years using standard ellipse area (SEA). On average, hypoxia exposed fish had depleted δ13C values relative to normoxic fish indicating shifts to pelagic food webs, although the magnitude of displacement differed among individuals. SEAs for hypoxic individuals showed expanded isotopic niche widths of δ13C and δ15N indicating variable trophic shifts following benthic food web displacement. Greater magnitudes of hypoxia exposure index values in the otolith were correlated with pelagic δ13C values in muscle tissue, suggesting individual differences in benthic displacement may have been driven by variability in hypoxia exposure. Combining otolith microchemistry with stable isotopes enhances our understanding of sub-lethal hypoxia, trophic webs, and feeding ecology and will inform management of key demersal fish species in the northern Gulf of Mexico