The number of opaque zones for each otolith was counted twice, without reference to their length or otolith weight with a minimum time interval of 7 days. When the two readings of the same otolith resulted in different age estimates, the slide was considered unreadable (Labropoulou and
Papaconstantinou, 2000; Rico et al., 2001). The distance from the focus to the outer margin of each opaque zone (rn) and the radius of the otolith (R) were measured with an ocular
micrometer, along the ventral tip of sulcus acusticus (Fig. 2).