International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.3, 123-133 http://ecoevopublisher.com/index.php/ijmec 130 river sections of the same basin, and the need to formulate management measures for different regions; at the same time, it reminds that breeding practices should pay attention to the introduction of wild bloodlines to maintain genetic diversity 6.2 Gene flow of migratory catfish in the Amazon of South America The whip catfish (Brachyplatystoma vaillantii) in the Amazon River of South America is known for its long-distance migration. Formiga et al. (2021) collected mitochondrial control region sequences from 150 whip catfish from five locations within a thousand kilometers of the Amazon River. The results showed that all samples were mixed to form a single unstructured population. The genetic differences between the collection points were minimal, and almost 100% of the variation in the molecular variance analysis came from individuals rather than between populations. This case clearly shows that the gene flow of whip catfish is free throughout the main Amazon river: adult fish spawn at the estuary, juvenile fish swim downstream to the ocean near the estuary to feed, and then subadults and adults swim upstream thousands of kilometers to return upstream to spawn. This cycle allows individuals throughout the basin to continuously exchange genes. The high gene flow of the whip catfish population also leads to its extremely high genetic diversity throughout the basin, and almost every fish has a unique haplotype. This result is of great significance for fishery management. This case highlights the genetic connectivity and management challenges of ultra-long-distance migratory fish, and is one of the few examples in the world that can prove that "large river fish populations are mixed on a scale of thousands of kilometers." 6.3 Impact of artificial introduction on the genetics of local catfish The walking catfish (Clarias batrachus) native to Bangladesh was originally a common small catfish in the country, but its resources have declined due to overfishing and habitat changes. At the end of the 20th century, the faster-growing African catfish (C. gariepinus) was introduced from Thailand and Africa for farming. Although the government subsequently banned the farming of African catfish to protect local species, many farmed African catfish have escaped into natural water bodies. Parvez et al. (2022) compared the sequences of "walking catfish" collected in the wild in Bangladesh with those of walking catfish in other Asian countries and African catfish through mitochondrial COI and Cyt b gene analysis. The results were shocking: the "walking catfish" samples collected in Bangladesh did not cluster with the walking catfish in India and Thailand, but clustered with African catfish. In other words, these individuals collected locally are genetically closer to the invasive African species. Further analysis revealed that some of these "walking catfish" samples are actually hybrid offspring that are morphologically similar to local species (Figure 2): their mitochondrial sequences are almost identical to those of African catfish, suggesting that the maternal genome of African catfish has infiltrated the local population. Figure 2 Identification of native Clarias batrachus and suspected hybrid based on morphological characters (Adopted from Parvez et al., 2022)
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