Age validation of juvenile cod in the western Baltic Sea

The methods routinely used to estimate fish age are often un-validated and susceptible to errors and uncertainties. Despite numerous attempts, age determination of western Baltic cod (WBC, Gadus morhua) using otoliths is still unreliable, predominantly due to inconsistent interpretation of the first translucent zone (TZ). Length-frequencies of undersized (<38 cm) cod collected during 2013–2016 from pound nets near Fehmarn Island were analysed to understand TZ formation patterns. A clear minimum separated two cohorts within the lengthfrequency samples every year. The length-frequency information was combined with otolith edge analysis to observe the development of TZs in age-0 and age-1 cod otoliths, and to validate the timing of TZ formation, which was consistently completed between September and December. Mean TZ diameters of 4 917 juvenile cod otoliths varied between cohorts (mean diameters of the first TZ: 2.0 ~c 0.5 mm; second TZ: 3.9 mm ~c 0.5) and TZ diameter variation was found to be related to individual growth rate. The timing of formation of the first TZ was positively related to water temperature, and was confirmed as a “summer ring” rather than a “winter ring”. TZ formation and shallow-water occupancy suggest an influence of peak summer water temperatures on WBC ecology. An age reading guide for juvenile WBC otoliths is provided.