Relationship between Otolith and Fish Sizes: Validation Using Their 3D Shape Analysis

Abstract

Nearly one million otoliths, the calcified structures of the inner ear in teleost fish, are sampled annually in the world for the stock assessment. The analysis of internal otolith measurements is based on the idea that there is a relationship between fish size and otolith size and that this proxy is representative of the whole shape of the otolith. In this study, the relationship between fish total length (TL) and five morphometric parameters extracted to the 3D shape otolith analysis, are examined: length (OL), width (OW), height (OH), area (OA) and volume (OV), based on a set of 104 otoliths belonging to 22 species from the English Channel and the North Sea. The otoliths were scanned in three dimensions using an X-ray microtomography, and the relationships were modelled using an allometric log-linear model. The results reveal significant relationships for all metrics (p < 0.001), with coefficients of determination (R 2 ) ranging from 0.500 to 0.656. In the multi-species pooled model, otolith width (R 2 = 0.656) proved to be the most robust predictor of fish size, slightly outperforming 3D metrics such as area (R 2 = 0.617) and volume (R 2 = 0.580). Otolith length (R 2 = 0.556) was less accurate than three-dimensional metrics. But given the limited sample sizes per species, these allometric trends should be viewed as preliminary, necessitating larger datasets for species-specific validation. The usual morphometric parameters, such as otolith length, are significant proxies of the whole shape of the otolith. These results suggested the common idea that otolith length is a useful indicator, but otolith width must be considered for greater accuracy and 3D metrics for more detailed shape information. However, the combination of species with varying ecologies and growth rates influences data dispersion and highlights the need for species-specific calibrations. These findings confirm the relevance of otolith metrics for estimating fish size and strengthen their utility in trophic ecology, paleoecology, and fisheries management studies.