Bomb radiocarbon analyses validate and inform age determination of longnose skate (Raja rhina) and big skate (Beringraja binoculata) in the north Pacific Ocean

Age and growth estimates based on growth band counts of sectioned vertebrae have been produced for longnose skate (Raja rhina) and big skate (Beringraja binoculata [formerly Raja binoculata]) populations in the Gulf of Alaska, British Columbia and California. Previous growth studies involving estimates from different laboratories in the USA (Alaska Fisheries Science Center, AFSC; Pacific Shark Research Center, PSRC at Moss Landing Marine Laboratories) and Canada (Fisheries and Oceans Canada, DFO) have produced dissimilar results for either species, highlighting the need for development of a consistent age determination protocol and more importantly an age validation study. Archived large specimens of longnose skate and big skate collected in Monterey Bay, CA, in 1980 and 1981 had minimum preliminary age estimates from vertebral growth band counts old enough to suggest that radiocarbon (14C) signals from bomb testing conducted in the late-1960s could be used to establish dates of growth band formation. To this end, we micro-milled skate vertebral thin sections, measured Δ14C using mass spectrometry, and estimated year of growth band formation based on the estimated age from growth band counts using both unstained and stained preparation methods Non-linear random effects modeling, implemented in a Markov Chain Monte Carlo (MCMC) simulation, was used to compare the skate Δ14C data set to a marine teleost otolith reference chronology for the California Current System. Results showed Δ14C measurements for big skate were non-informative as none of the archived samples were old enough for comparison to the reference curve, hence validation of the age estimation approach was not possible. However, for longnose skate, Δ14C data were more informative to fit pulse function models and compare results to the reference chronology. Modeling results indicated longnose skate age estimates based on unstained vertebral thin sections were less biased (overestimated to a smaller degree) than estimates based on stained vertebral thin sections. The degree of bias depended on agency ageing criteria, with the least biased age estimates produced by age readers from the AFSC. The AFSC age estimates had about a 70% probability that age estimates of longnose skate was within +/− 2 years of the expected age based on the radiocarbon assays. We were able to validate the age estimation methodology for longnose skate and establish criteria for growth band counts, which should now be useful to generate region-specific accurate growth and life history parameters required for more reliable stock assessment approaches.