Population Structure and Spatial Connectivity Inferred from Spatial Phenotypic Differentiation of the Bluefish Pomatomus saltatrix (Teleostei: Pomatomidae) Along the Southeastern–Southern Brazilian Coast Using Landmark-Based Truss Network Morphometrics

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.