Natural variability in δ18O values of otoliths of young Pacific sardine captured in Mexican waters indicates subpopulation mixing within the first year of life

Oxygen stable isotopes were measured in whole sagittae of young Pacific sardine (delta O-18(oto)) collected throughout their range in the Mexican Pacific to quantify natural variability, to reconstruct temperature histories, and to infer whether fish mix at a population or subpopulation level. Isotopic values and derived temperature estimates (T-oto) of sardine captured simultaneously showed high variability (up to 2.0 parts per thousand and 10 degrees C at a given location). Given limited variations in salinity, this implies differences in thermal history and the prevalence of subpopulation-level mixing processes. We tested the null hypothesis of local residence by comparing delta O-18(oto) values with predicted isotopic values on a location-specific basis, and age- and location-specific average sea surface temperatures (SSTs) with derived T., Some fish exhibited values outside the local range of predicted oxygen isotope values and SSTs, suggesting that they were not permanent residents. Using an otolith growth model, we show that otolith growth and age differences cannot fully account for the variability in delta O-18(oto) values. The absence of significant differences in delta O-18(oto) values between the Pacific and Gulf indicates that oxygen isotope ratios cannot be used to examine population structure or migration among these regions. However, they can be used to infer mixing within and among subpopulations.