The sagittae otoliths were sectioned along the transverse axis (cross-sectioning), this involves embedding the otoliths in black resin blocks, removing a thin section from the transverse midplane. Images were taken of each otolith again using Leica IC80 HD (Digital Camera Module by Leica Microsystems) under a stereomicroscope at 10× magnification using a reflected light setting, with resolution 2048 x 1536 pixels. The age determination and annotation of cod otoliths in this study were carried out by highly experienced otolith reader. The otolith sizes at the mid-point of respective annual translucent zones were then marked along a transect from the core to the outer distal edge of the otolith, with the transect being approximately perpendicular to the width of the otolith (following Li et al. , Fig 2). Only the first five years of growth were measured. Fish growth slows with age and accurate estimates of annuli growth along the distal axis became increasing difficult with age, particularly in the very old individuals.