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

Considerable 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. There- fore, 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.

Publication:Daugherty, D.J., Andrews, A.H., and N.G. Smith. 2019. Otolith-Based Age Estimates of Alligator Gar Assessed Using Bomb Radiocarbon Dating to Greater than 60 Years. North American Journal of Fisheries Management (dx.doi.org/10.1002/nafm.10390

Collaboration: Texas Parks and Wildlife

HISTORIC ALLIGATOR GAR PHOTO - The origin of this photo is in question (likely a 10 foot fish caught in 1910 at Moon Lake in Mississippi) and the actual size of this fish is not well known but this was a huge fish and likely very old. At this time, colleagues in Texas have an otolith from an Alligator Gar (Atractosteus spatula) that is estimated to have lived 95 years. We have since this paper applied LA-AMS to validate maximum age.

This collaboration with the Inland Fisheries Division of Texas Parks and Wildlife Department to determine the age, growth and longevity of Alligator Gar using otolith sections and bomb radiocarbon dating continues with potential applications to other freshwater fishes of North America ... stay tuned.

US distribution of Alligator Gar where this ecologically beneficial species has been extirpated from much of its original range because it is mistakenly viewed as a trash fish — alligator gar may function as a useful control mechanism for invasive Asian carp species.