Age Validation of Pacific Cod (Gadus macorcephalus) Using High Resolution Stable Oxygen Isotope (d18O) Signatures in Otoliths

Pacific cod (Gadus macrocephalus) age determination is difficult. To address ageing inaccuracy, we conducted an age validation study using the stable isotope ¹⁸O (δ¹⁸O). This uses the principle that seasonal changes in temperature are reflected in otolith δ¹⁸O values. We sequentially microsampled Pacific cod otoliths, from core to margin, to measure δ¹⁸O. We developed individual life history signatures of δ¹⁸O from specimens (n=40) with estimated ages of 2 to 5 years. Our goals were to identify the annual cyclical variation and determine if the number of maxima and minima was consistent with the age estimated from growth zone counts. We also estimated the probability of age reading bias by treating the number of maxima and minima as the “true age.” The relationship between δ¹⁸O in otoliths and known water temperature was also verified (r² = 0.74). Overall, the probability of an estimated age being equal to the true age was 64%; the probabilities of assigning an age 1 year greater was 19% and 1 year less was 17%. At age 5, the probability was non-symmetric, and indicated the probability of assigning the true age was 49%, with a 51% probability of under ageing by one or more years.