Otolith shape has been widely used as a tool to investigate the population structure of marine fish, as it reflects environmental, physiological, and genetic influences throughout the lives of individuals. The morphological variability of Pomatomus saltatrix sagittal otoliths along the southeastern–southern coast of Brazil was assessed using Elliptic Fourier Descriptors (EFDs) and Wavelet Transform Descriptors (WTDs). A total of 74 individuals from Rio de Janeiro (RJ, N = 25), São Paulo (SP, N = 24), and Santa Catarina (SC, N = 25) were analyzed. The coefficients obtained from both methodologies were subjected to multivariate analyses, including Permutational Multivariate Analysis of Variance (PERMANOVA) and Flexible Discriminant Analysis (FDA), combined with Jackknife Cross-Validation (JKC). The results showed high similarity in otolith shape among the sampled locations. Overall, PERMANOVA detected significant spatial differences for both EFD and WTD, although the proportion of variation explained by sampling location was low for both descriptors (<5%). After Bonferroni correction, no pairwise differences remained significant for EFD, whereas WTD indicated significant differences between SC and the other two sampling locations. Nevertheless, FDA indicated partial discrimination among sampling sites, with some overlap among groups for both descriptors. Despite this, JKC rates were high, reaching 92% for EFD and 87% for WTD. Overall, the results point to a pattern of weak spatial structuring in P. saltatrix along the southeastern–southern coast of Brazil. Although subtle morphological heterogeneity was detected among the analyzed locations, the evidence does not support the existence of completely isolated stocks. The observed patterns indicate weak spatial phenotypic differentiation and may be consistent with population connectivity; however, connectivity cannot be directly inferred from otolith shape alone and represents only one possible interpretation, as similar environmental conditions, shared growth histories, or limited discriminatory power of otolith morphology could also contribute to the observed scenario. Thus, the study reinforces the utility of otolith morphometry as a complementary tool for population studies and highlights the importance of integrating it with other phenotypic and genotypic approaches for a more comprehensive understanding of the structure of the species’ stocks.

Introduction: Understanding population structure is essential for effective fishery management, and otolith shape analysis provides a robust framework for detecting spatial variation in marine fish populations. The bluefish (Pomatomus saltatrix), a commercially important and widely distributed species along the southeastern and southern Brazilian coast, may exhibit subtle population structuring that is still not fully resolved. Methodology: Otolith contour variation was analyzed using two complementary approaches: Elliptical Fourier Descriptors (EFD) and Wavelet Transformed Descriptors (WTD). A total of 75 individuals (25/site) were sampled from Rio de Janeiro, São Paulo, and Santa Catarina, with total lengths ranging from 33.2 to 45.5 cm. Multivariate analyses included Permutational Multivariate Analysis of Variance (PERMANOVA), pairwise Hotelling’s t-tests (HT²), Flexible Discriminant Analysis (FDA), and jackknife reclassification matrices (JKC). Results: For the EFD approach, PERMANOVA showed no significant differences among localities, while FDA revealed partial overlap among groups and a JKC overall reclassification accuracy of 46%. In contrast, the WTD approach detected significant spatial differences, with PERMANOVA indicating overall variation among localities and HT² identifying significant differences between Santa Catarina and the other regions. FDA improved visual separation of Rio de Janeiro samples, although the JCK accuracy decreased to 35%. Conclusion: The combined results suggest the presence of weak to moderate spatial structuring in P. saltatrix along the studied coastline. However, inconsistencies among analytical approaches and relatively low reclassification success rate to the original site indicate that the observed differentiation is insufficient to conclusively define distinct population units, remaining compatible with either a single stock exhibiting spatial heterogeneity or weakly differentiated subpopulations.

Understanding the population structure of highly migratory marine species is essential for effective fisheries management and conservation. The bluefish, Pomatomus saltatrix, is a widely distributed pelagic fish of great ecological and economic importance in the Southwest Atlantic; however, its population structure in this region remains poorly understood. This study investigated the population structure of P. saltatrix along the southeastern and southern coasts of Brazil using a landmark-based truss network morphometrics approach. A total of 109 adult individuals were sampled at three sites along the Brazilian coast—Rio de Janeiro, São Paulo and Santa Catarina—between August and October 2024. Body morphology was assessed using a truss network system composed of 25 morphometric transformed distances derived from 12 landmarks. One-Way ANOVA or Kruskal–Wallis tests revealed significant differences among sampling sites for 18 of the 25 transformed morphometric distances. PERMANOVA revealed significant overall differences in body morphology among the three sampling sites, with all pairwise comparisons being significant. Moreover, the FDA showed a clear spatial structure, despite partial overlapping between the three sampling-site groups. Overall jackknife reclassification success was 87.2%, with the highest accuracy achieved for Rio de Janeiro, followed by São Paulo and Santa Catarina. Despite the high reclassification accuracy, the observed morphometric differences may be compatible with a shared stock exhibiting site-specific phenotypic variation, although stock boundaries cannot be established from morphometric evidence alone. Overall, these findings demonstrate that landmark-based truss network morphometrics is an effective tool for detecting spatial phenotypic structuring in P. saltatrix, providing valuable information to support future studies and fisheries management in the Southwestern Atlantic.

Introduction: Understanding population structure is essential for the effective management of marine resources. The bluefish (Pomatomus saltatrix) is a widely distributed and commercially important species along the Brazilian coast, yet its population structure in the South–Southeastern region remains unclear. Objectives: This study aimed to assess the body morphological variation and to infer the population structure of P. saltatrix along the South and Southeastern coasts of Brazil using a landmark-based geometric morphometrics method. Methodology: Individuals were collected between August and October 2024 from artisanal fisheries in Rio de Janeiro (n = 46), São Paulo (n = 37), and Santa Catarina (n = 29). A total of 12 landmarks were used to derive a truss network and extract 25 transformed distances (DTs). Data were analyzed using univariate (one-way ANOVA, followed by Tukey’s test if needed, or Kruskal–Wallis followed by Dunn’s test if needed) and multivariate statistics (PERMANOVA and Flexible Discriminant Analysis (FDA)). Results: Significant regional differences were detected for most DTs, except DT3, DT4, DT6, DT9, DT13, DT24, and DT25. PERMANOVA revealed significant differences among all sampling locations, including pairwise comparisons (p < 0.05). FDA achieved an overall reclassification success of 93% of individuals to their original location. Conclusion: Despite the high reclassification accuracy, the results support the existence of a single stock with spatial structuring rather than completely distinct population units, indicating that P. saltatrix in this region is not homogeneous.