Losing track of time : causes and solutions for the problematic determination of Baltic cod age

Age is a fundamental parameter in biology and fishery science to estimate growth and population parameters. Losing track of time, in the sense of lacking reliable estimates of age and growth creates substantial problems for managing difficult-to-age-species. This thesis explores the use of fish otolith chemistry to improve age determination. Otoliths produce annual growth zones in the form of alternating opaque and translucent rings, corresponding to fast growth in summer and autumn and slow to no growth in winter. For the eastern Baltic cod (EBC), degraded environmental conditions led to a worsening of visual contrast in their otoliths. The EBC served as a model species to test new methods of aging, taking advantage of seasonally driven patterns in otolith chemical constituents. Experiments demonstrated that patterns of physiologically regulated elements significantly improved agreement among untrained chemical readers when compared to experienced readers using standard visual techniques, and that Mg:Ca and P:Ca showed the clearest patterns. Validations demonstrated mechanistic correlation with fish growth, condition, temperature, parasites and proximity to visible winter zones identified by disappearance of daily rings. EBC otoliths from the Stone Age to the present provided historical perspective. Mn:Ca was correlated with hypoxia and water temperature, indicating highest levels in the 2010s. Metabolic status, proxied by Mg:Ca was highest during the 1980s and 1990s, to thereafter decline. Predicted mean length at age by decade showed highest growth in the 1990s and a dramatic decline in the 2010s. The results from this thesis fill crucial knowledge gaps and provide current and historical insights on how EBC biology has changed over time that could improve the management of the threatened population