Validity of daily and annual age estimation and back-calculation methods for early life stages of a subtropical-tropical species, the tarpon (Megalops atlanticus)

Abstract

Much emphasis has been placed on challenges related to estimating ages of old fish, yet accurately estimating age for young individuals has proved equally challenging for some species. Indices of recruitment are often based on assigning ages to young fish to estimate the strength of a given year class. Similarly, growth is most commonly estimated using age-based methods and has been proposed as a metric for evaluating the quality of nursery habitats. Unfortunately, there is a paucity of information on the validity of age estimates for many fishes, and thus the accuracy of age-based metrics used to guide management of these species is unknown. To address such needs, we conducted a validation study of age and growth estimation methods for juvenile tarpon (Megalops atlanticus), using oxytetracycline to chemically mark otoliths and scales. Oxytetracycline marks were observed on all the otoliths of recaptured tarpon (n = 23), prior to a newly formed annulus, validating true age and that one annulus is deposited yearly. Marginal increment analysis indicated that annuli form in tarpon otoliths and scales during March–April. Annuli in scales were more easily identified by readers leading to more accurate and precise estimates of age from scales (100 % accuracy of age estimates for age-1 fish) compared to otoliths (88 % accuracy). Although annual age estimates were accurate, we observed substantial error and low accuracy for daily age estimates based on otoliths (10 % accuracy). Additionally, there was a poor relationship between otolith radius and tarpon length (R2 = 0.46), which resulted in inaccurate estimates of back-calculated length-at-age and growth rates. Our results indicate that scales can be used to differentiate between age-0 and age-1 tarpon, which will aid in conservation efforts by allowing for more accurate evaluation of the age structure within nursery habitats and the quality of these habitats. Further research is needed to better understand factors influencing daily age and growth estimates from otoliths.