Otolith‐Based Age Estimates of Alligator Gar Assessed Using Bomb Radiocarbon Dating to Greater than 60 Years

Abstract

AbstractConsiderable efforts have been undertaken to characterize population dynamics of the Alligator Gar Atractosteus spatula. Accomplishing this task required identification of suitable structures to estimate age. Buckmeier et al. (2012) validated sagittal otoliths to 31 years—the oldest fish in their study. However, they noted that interpreting annuli became increasingly difficult with age and cautioned that difficulties may result in underestimates of age for older fish. In recent years, sampling efforts have resulted in the collection of otoliths from a number of large (>1,800 mm TL) individuals. Age estimates of these fish ranged to 68 years; however, the accuracy of these ages was unknown. Therefore, we used bomb radiocarbon dating to assess accuracy. The technique measures the time‐specific increase in the radioactive carbon isotope (14C) in calcified aging structures resulting from the testing of thermonuclear devices in the 1950s and 1960s. An abrupt, worldwide increase in environmental radiocarbon occurred in approximately 1958 and acts as a time stamp, providing a reliable means of corroborating the age estimates of long‐lived fishes. Core material (i.e., hatch year), along with a subsample from early growth years, was extracted from the otoliths of 24 Alligator Gar (1,805 to 2,540 mm TL) and analyzed for Δ14C. Estimated age from growth zone counting ranged from 26 to 68 years, corresponding to hatch years spanning from 1946 to 1989. Otolith Δ14C was then plotted against the corresponding hatch years and compared to reference curves for environmental 14C levels across time. Despite evidence of regionally specific trends attributed to differences in groundwater and surface inflows, Δ14C levels in the otoliths were highly correlated with reference data, indicating that age estimates were accurate to within a few years. Thus, our results extend the utility of methods developed by Buckmeier et al. (2012) for Alligator Gar in excess of 60 years and provide greater insight on longevity.