Distinguishing West Florida gag (Mycteroperca microlepis) nurseries with otolith stable isotopes

Numerous reef fishes utilize estuarine nurseries following spawning in marine habitats. Nursery origin affects their life cycle space use, and otolith chemistry can be an effective tool for tracking movement over this lifetime scale. Gag Mycteroperca microlepis occupy West Florida (WF), USA, estuaries as juveniles and have a spatially complex life cycle with ontogenetic habitat shifts and offshore spawning. We tested whether age-0 gag otolith stable isotope values distinguished WF nurseries and assessed isotopic variability at multiple temporal and spatial scales. Northern and southern juveniles had distinct otolith carbon (δ 13 C) and oxygen (δ 18 O) stable isotope values despite evidence of interannual variability, and both year-specific and combined year models had high (>80%) classification accuracies. We investigated long-term stability of otolith δ 13 C and δ 18 O by comparing age-0 gag collected during 2009-2012 and 2020-2021. The δ 18 O values of southern age-0 otoliths decreased over time, thus decreasing inter-regional differences and the classification accuracy of the combined year model (79%). Concomitantly, age-0 otoliths became depleted in 13 C across regions, which may result from increased limestone dissolution (i.e. dekarstification) in the Floridan aquifer. We identified greater decadal variability in region-specific stable isotope values than has been documented in previous studies, indicating that a cohort-specific approach will likely be required to estimate gag lifetime movement.