Scale reading validation for estimating age from tagged fish recapture in a brown trout (Salmo trutta) population

This study presents the validation of a scale-reading method for age estimation of brown trout (Salmo trutta). Age estimated from the scale reading was compared with the true age from recaptured fishes pit-tagged at the 0+ stage. Recapture was performed via electrofishing or trapping during migration up to 6 years later. A sample of 375 fish was analysed, including freshwater (363 fish) with two sub-groups (resident and migratory) and anadromous forms (12 fish). Average percentage of error in the scale reading was low (6.4%) in the whole sample. Error rate dramatically increases with age. There was no error in the sea trout sample. When considering freshwater fish alone, average error rate remained low (6.6%) and was higher in resident fish (older individuals) than in migratory fish (younger individuals). No statistical difference was observed between the true age and the age estimated from scales, thus confirming the reliability of scale reading for age estimation.