Age affects otolith shape in a coastal pelagic fish (Scomber colias Gmelin, 1789)

Abstract

Otolith shape analysis is a common and efficient technique to study the population structure of fish species. This approach compares the overall otolith shape among individuals collected along the species´ distribution range. Otolith shape may change during ontogeny; thus, comparing otolith sample collections comprising multiple and variable age classes may lead to biased results. Therefore, it is crucial to test for possible effects of age on the otolith shape for a given species before interpreting potential differences in otolith collections. Here, we explored the effect of age on the otolith shape of a coastal pelagic fish species with a wide geographical distribution and fast growth rate, the Atlantic chub mackerel Scomber colias (Gmelin, 1789). This effect was tested across different years using different age assignment techniques (direct age reading and age back-calculation) and otolith shape descriptors (Elliptic Fourier and Discrete Wavelet). The results show that the age effect on otolith shape is most noticeable between age-1 and older fish, possibly due to rapid growth during the first year of life. Also, the age effect on otolith shape depended on the descriptor used, being more detected when using the Discrete Wavelet descriptor compared to Elliptic Fourier. Our results highlight the need to carefully examine age effects on otolith shape before conducting population structure analysis on mixed-age sample collections.