Stock identification of the Mediterranean horse mackerel (Carangidae: Trachurus mediterraneus ) in the Marmara and Black Seas using body and otolith shape analyses

The Mediterranean horse mackerel ( Trachurus mediterraneus ) has been managed as a single stock in the Black Sea and adjacent regions, and its genetic structure has been under debate for decades, emphasising the necessity for additional research to validate the stock structure. This study used geometric morphometrics for body shape, shape indices, and elliptical Fourier analysis for otolith shape to explore their morphological differences in the Eastern Black Sea, Middle Black Sea, and Sea of Marmara. The findings revealed considerable differences in body and otolith shapes across the examined regions, suggesting the existence of three separate stocks. The geometric morphometrics revealed evident differences in the snout, followed by the caudal peduncle and body depths. Both otolith shape indices and elliptic Fourier analysis revealed significant directional bilateral asymmetry between the left- and right-sided otoliths. The elliptic Fourier analysis revealed a non-overlapping surface difference of 2.12% between the reconstructed left and right otolith outline shapes. However, no significant differences in otolith shape were observed between males and females using either otolith side. In addition, otolith shape analysis using shape indices and elliptic Fourier analysis reliably supported the presence of three distinct stocks of T. mediterraneus. This study provides evidence supporting the recognition of multiple phenotypic stocks of T. mediterraneus.