Assessing the population structure of Plagioscion squamosissimus (Teleostei, Perciformes, Sciaenidae) from the São Francisco River, Bahia, Brazil, using body morphology and otolith shape signatures.

The south American silver croaker, Plagioscion squamosissimus, holds significant importance for the artisanal fisheries in the sub-middle and lower courses of the São Francisco River, located in northeastern Brazil. To investigate the population structure of this species along its longitudinal profile, a total of 150 individuals (50 fish per site), measuring between 35 and 45 cm in standard length, were collected from three fishers' associations (Olhos D'água do Casado, Petrolândia and Rodelas) using gill nets from September 2023 to March 2024. Morphometry analyses of body shape were conducted using two landmark-based geometric morphometrics: Procrustes coordinates (PCs) and truss networking (TD). Additionally, otolith shape was examined through two otolith contour analyses: wavelet transform coefficients (WTCs) and elliptic Fourier descriptors (EFDs). Both univariate and multivariate statistical approaches were employed to assess the data. The findings revealed a significant secondary sexual dimorphism in otolith shape that was not observed in body shape. This dimorphism may be linked to sex-specific hearing adaptations associated with ecological or reproductive behavioural strategies, although further research is required to test this hypothesis. Despite this sexual dimorphism, the overall reclassification success rate of individuals to their original sites was notably high, ranging from 91% to 94% using body morphometry (TD and PCs, respectively), and reaching 100% accuracy with otolith shape analysis (EFDs and WTCs). The phenotypic differences observed among local populations likely resulted from distinct environmental conditions between sites, probably caused by the construction of two hydroelectric plants that disrupted fish connectivity along the São Francisco River. The results suggest that P. squamosissimus populations captured in the sub-middle and lower courses of the river constitute distinct population-units. This distinction has critical implications for the species' management and conservation strategies.