Comparative study of three alternative methods of aging Baltic flounder (Platichthys flesus)

Age reading is a major source of uncertainty in the assessments of fish stocks. The paper presents preliminary results of alternative approaches to estimating the age (year-class) structure of Baltic flounder, Platichthys flesus. The size structure of catches and the weight distribution of otoliths were analysed using Bhattacharya's method to distinguish between different cohorts (year classes). The traditional counting of annual growth zones on otoliths yielded the greatest number of age groups (12). Age determination by Bhattacharya's analysis of otolith weight frequencies enabled to distinguish between 7 cohorts. Bhattacharya's analysis of length frequencies of the same material allowed distinguishing 5 or 6 different groups. Theoretical weight distributions of otoliths, calculated by Bhattacharya's method for age groups 1 and 6, were not significantly different from the corresponding distributions of weights of otoliths obtained by the method of counting annual rings. The importance of random selection of individuals for the determination of the age structure of catch by annual rings of otoliths is emphasized.