International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 82 5.3 Secondary diffusion paths caused by human activities In addition to natural geographical spread, human activities have also promoted the secondary diffusion and introduction of the genus Spanish mackerel in modern times. This influence mainly occurs in two ways: one is the opening of canal waterways, and the other is the transplantation and introduction of species. The aforementioned Suez Canal case is the first. The artificial canal has opened up barriers between the Red Sea and the Mediterranean, allowing Red Sea fish such as narrow-band Spanish mackerels to enter. This "Lessepsian migration" is an unintentional artificial spread, but has a significant impact. For example, the Panama Canal (navigated in 1914) connects the Pacific Ocean and the Atlantic Ocean. Although it is mediated by freshwater lakes, it is generally believed that marine species are difficult to cross directly, but occasionally reports indicate that "unusual" fish appear in the waters near the canal. If the climate warms in the future, the new possibility of Spanish mackerel spreading through the canal cannot be ruled out. The second pathway is direct proliferation, release or transplantation. Compared with salmon, Spanish mackerels have not been widely transplanted and cultivated, but there are proliferation and release practices in some countries. These release juvenile fish may spread to other adjacent waters, thereby changing the local population structure (Malik and Muzaffar, 2024). In the United States, there are also coordination of proliferation projects for Atlantic Spanish mackerel and interstate release management. Furthermore, with the development of global marine trade, ship ballast water has become a new medium for the trans-ocean transmission of marine organisms. The larvae or egg may be sucked into the chamber compartment and released when discharged from other ports. If a new environment is suitable, they have the opportunity to build populations. Although the probability of large predatory fish diffusing through ballast water is low, it cannot be completely ruled out. In addition, the "invasion" of populations originally distributed adjacent to each other in the context of climate change can also be regarded as part of secondary diffusion. For example, in recent years, reports of narrow-band Spanish mackerels in the Indian Ocean entering the southwestern Atlantic Ocean along the southern end of Africa have gradually increased, which may be related to the increase in water temperature. 6 Population Structure and Genetic Diversity 6.1 Regional population division and gene flow pattern The wide-area distribution and highly migratory life history of Spanish mackerel make its population structure characterized by "large-scale weak differentiation", but there are also substructures in the local area. Genetic studies usually analyze the degree of gene communication and differentiation levels between Spanish mackerels in different sea areas through mitochondrial DNA sequences, multi-site microsatellites and single nucleotide polymorphisms (SNP) markers. The results generally show that Spanish mackerels exhibit significant gene flow: populations hundreds or thousands of kilometers apart tend to share the same haplotype or alleles, and the F<sub>ST</sub> isovariate index is extremely low, making it difficult to distinguish independent population units (Broderick et al., 2011). For example, the genetic distance between Brazilian horse Spanish mackerels collected at eight sites along the Brazilian coast was small, and AMOVA analysis failed to detect significant intergroup differences, supporting it as a single pan-MiG population. For example, the microsatellite analysis of Japanese Spanish mackerels along the coast of China also found that the samples in the Yellow Sea, East China Sea and South China Sea did not cluster clearly, and the genotype frequency differences in different places were not obvious. These are all attributed to the strong swimming ability and long-distance migration of Spanish mackerels, allowing individuals in different regions to cross geographical barriers and undergo reproductive mixing. This situation is particularly evident in open waters and is called "genetic homogeneity". However, not all cases are completely structured. In some oceanic islands, semi-enclosed seas or environments with obvious ecological differences, Spanish mackerels may form relatively independent subgroups. For example, due to shelf restrictions and monsoon circulation characteristics, the Arabian Sea and the Bay of Bengal groups have experienced certain genetic differentiation. Even within the same species, weak structures may be caused if multiple spawning grounds exist and adults have certain re-entry habits. For example, narrowband Spanish mackerels are separated from the spawning grounds on the east and west coasts of Australia, and the two groups have weak allelic frequency differences.
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