Genetic and morphological evidence of asymmetric hybridization between sympatric Rastrelliger brachysoma and R. kanagurta: Implications for sustainable marine fisheries management

The sympatric mackerels Rastrelliger brachysoma and R. kanagurta exhibit extensively overlapping distributions throughout Southeast Asia and are frequently harvested in mixed schools. The recent observation of individuals displaying ambiguous and intermediate morphological characters obscure taxonomic boundaries and suggests the occurrence of natural hybridization. As prolonged hybridization could threaten biodiversity and sustainable fisheries practices, this study aimed to clarify morphological differences and investigate the extent of genetic admixture among R. brachysoma , R. kanagurta , and a phenotypically unknown group localized in the middle Gulf of Thailand using 24 morphological characters and 21 microsatellite loci. Cluster analysis based on morphological characters clearly differentiated R. brachysoma and R. kanagurta , whereas the unidentified group formed a cluster overlapping with both distinct species. Microsatellite analysis indicated substantial genetic diversity ( H o = 0.592 ± 0.029; AR = 8.142 ± 3.507) within the middle Gulf of Thailand populations. Furthermore, while the two reference species exhibited distinct genetic clustering, the unidentified group displayed signatures of genetic admixture ( Q -value = 0.1–0.9). Notably, the morphological characteristics and genetic profile of the unidentified group suggested a higher contribution from R. kanagurta , suggesting asymmetric introgression. These findings provide insight into evolutionary dynamics of Rastrelliger species and offer critical data for fisheries management strategies aimed at sustaining economic development and biodiversity.