Morphological structuring of Atlantic bonito (Scombridae: Sarda sarda) stocks in the southern Black Sea revealed by integrative phenotypic analyses

Abstract

Atlantic bonito ( Sarda sarda ), a commercially important and highly migratory species, has long been managed as a single stock in the Black Sea and adjacent waters. This approach has historically been supported by genetic studies indicating widespread homogeneity. The present study provides an integrative phenotypic assessment of this assumption from a morphological perspective, incorporating: geometric morphometric analysis of body shape, traditional otolith shape indices, elliptical Fourier-based otolith shape analysis, and von Bertalanffy growth modelling. Specimens collected from the Eastern, Middle, and Western Black Sea (∼41–42°N) exhibited consistent morphological structuring, supporting the presence of at least two phenotypically differentiated stocks: one comprising the Eastern–Middle region and the other comprising the Western region. Traditional otolith shape indices failed to detect this structure, whereas elliptical Fourier analysis discriminated the Eastern–Middle phenotypic stock from the Western phenotypic stock. Growth modelling further supported phenotype-based stock differentiation, revealing significant differences in growth trajectories between the Western and Eastern–Middle phenotypic stocks, with no significant separation between the Eastern and Middle regions. This study also provides the first evidence of directional asymmetry in Atlantic bonito otoliths, a potential indicator of environmental stress. These findings advocate for a reassessment of single-stock management strategies and highlight the need for further investigation across broader regions, emphasising the value of integrative phenotypic assessment in identifying and refining management approaches for phenotypically differentiated stocks.