Otolith microchemistry helps to unlock the chronology of age determination

The chronological differentiation of hard structures in the skeleton of fishes has provided important insights into age and growth of many species of fish (Campana 2001). This field of research is so important that it has spawned huge interest in the fish biology and fisheries community. Chemical signatures represented by trace elemental and isotopic composition of otoliths or scales have been used to determine the use of different habitats/water masses at different life-history stages for a considerable number of species (e.g. Cambie et al. 2016). Different bodies of water differ in their chemical signatures, which are incorporated in the bony structures of fish, and provide a potential fingerprint of metabolic activity through the year (higher with warm water temperatures and lower at cooler temperatures) (Limburg et al. 2018). Heimbrand et al. (2020) studied the microchemistry of the Eastern Baltic Sea cod stock, notoriously difficult to age, to determine whether the microchemistry of the fishes' otoliths would provide greater clarity of age determination compared to the traditionally used visual aging techniques. They used ratios of Mg:Ca and P:Ca as proxies of growth and metabolic activity as these showed the greatest sensitivity to changes in metabolism (likely linked to water temperature). The chemical technique gave more precise results when compared to standard visual techniques applied by expert otolith readers. The quantitative nature of using otolith microchemistry to produce a microchemical map of the otolith provides some potentially interesting opportunities. In the future, the reading of the microchemical otolith ‘map’ might be undertaken using artificial intelligence and neural networks, which would enable automation of the entire process. This exciting development could revolutionise the way in which fish are aged, leading to higher sample sizes and reducing error rates in age-determination and subsequent population assessments.