Are hake otolith macrostructures randomly deposited? Insights from an unsupervised statistical and quantitative approach applied to Mediterranean hake otoliths

Individual fish age data are crucial to fish stock assessment, so their accuracy and precision are vital. The acquisition of age data most often relies on interpreting fish otoliths, a complex task in which expert subjectivity increases with the complexity of the structural patterns of the otoliths. The question arises for certain fish species (e.g. hake, Merluccius merluccius) whether the deposition of otolith macrostructures is meaningful for age estimation. A quantitative method based on the evaluation of otolith similarity in terms of structural patterns is presented to investigate this issue. It relies on the determination of a typology of otolith macrostructures from an unsupervised statistical analysis of the distributions of their characteristics. This typology provides a basis for analysing and comparing structural patterns of otoliths through evaluation of structural otolith similarities. Application to a set of Mediterranean hake otoliths discriminates three types of macrostructure, one likely associated with fish responses to environmental or endogenous factors, and the other two meaningful at a population or group level. Comparisons of structural patterns based on the proposed structural similarity measure over two successive years support the assumption that otolith patterns are stable over time, although male and female otoliths differ significantly in structural pattern. The results bring new evidence that hake otolith patterns are not random and may be relevant to age estimation.