Beyond the outer rim: Estimating scale-based age underestimation in long-lived cypriniform fish

Information on the age structure and growth rate of fish populations is essential for providing management advice, including estimating maximum sustainable yield or developing conservation management strategies for focal species. For the latter, non-lethal indicators of age structure, such as scale annuli are widely used; however, their accuracy is believed to decline with increasing age. Despite this, long-term evaluations of the accuracy of scale-based age determination remain scarce. Here, a 10-year study on a population of a medium to long-lived cypriniform fish, asp Leuciscus aspius, was used to evaluate a bias between the estimated and actual age of the fish. A total of 1005 females, 1515 males and 244 juveniles were individually PIT tagged and recaptured at least once one or more years after tagging, with scale samples collected on every capture occasion. Since the year of first tagging was known, the bias between the initial age determination and subsequent age determinations was established. In about 17% of females and 15% of males, the determination of the year in which the fish was born was higher in subsequent recaptures than in the first scale estimation, indicating underestimation of the number of growth rings. This was more likely to occur in older individuals, while this issue affected only 0.03% of juveniles (0–4 years) (calculated as a weighted average across all juvenile individuals and recapture events, not age-specific values). This study demonstrates an increasing divergence between the age estimated from scales and the actual age. Although the scalimetric approach is a valuable, non-lethal method for age-determination, the ages obtained should be treated with caution, especially for older individuals. Corrections should be assessed and applied in populations of conservation interest, where scale samples are the only available non-lethal option.