Element profiles of a 4-year-old, 290 mm long, male Baltic cod from thefirst validation collection, the DECODE samples, where age was obtained from patterns in daily otolith increment widths, including an image of the corresponding otolith viewed under reflected light. Profiles show relative element concentrations (element:Ca ratios divided by the profile mean for each element) from the TET mark to the otolith edge. Among the elements regulated by physiological processes, P varies consistently over the seasons in both validation samples, with Min co-occurring with otolith zones without visible daily increments in DECODE otoliths or, in the case of the TABACOD otoliths, in late winter - early spring. Transversal section of otolith of a 3-year-old Baltic cod from thefirst validation collection, the DECODE samples, where age was obtained from patterns in daily otolith increment widths, viewed under reflected light (bottom right), where the black box indicates the otolith area viewed under 20 magnification and transmitted light (middle image). Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter - early spring when water temperatures are coldest in tagged cod. [Extracted from the article]