International Journal of Marine Science, 2025, Vol.15, No.4, 186-198 http://www.aquapublisher.com/index.php/ijms 190 4 Comparison of Mackerel Population Structure in Different Sea Areas Around the world 4.1 Differences in the Indian Ocean and Western Pacific populations The Indian Ocean and the Western Pacific waters are integrated, but they contain a complex semi-closed marginal sea and island system. For highly migratory fish such as mackerel, the population structure in this vast area has always attracted much attention. Through the aforementioned high resolution studies, we have known that the narrowband mackerel (S. commercial) has a distinct genetic differentiation pattern in the northern Indian Ocean. It is reported that narrowband mackerels sampled in the North Indian Ocean-East Indian Ocean can be divided into at least four genetic groups. Among them, groups in the northeastern Indian Ocean (the Gulf of Bengal), the eastern (near the Malay Peninsula), and the central and northern Indian Ocean were all identified as belonging to different gene clusters, and the F_ST analysis also supported significant differences between the groups (Vineesh et al., 2018). In particular, the study found that population genetic differences are positively correlated with geographical distance, showing a clear isolation by distance pattern. This means that even without absolute geographical barriers, distant groups gradually accumulate differences in gene frequencies due to limited migration individuals. 4.2 Genetic pattern of the Atlantic and Caribbean Compared with the high diversity of the Indo-Western Pacific, there are fewer species of mackerel in the Atlantic, and their population structure studies are mainly focused on kingfish and Sierra mackerel along the Western Atlantic coast. Previous studies of S. brasiliensis have shown that the species may constitute a genetic unit throughout the southwestern Atlantic Ocean (Venezuela to southern Brazil). Mitochondrial analysis found that samples from different sampling sites (Cumana, Venezuela, northeastern Brazil, southeastern Brazil) shared most haplotypes, with the interpopulation differentiation index Phi_ST and the AMOVA results were not significant. Haplotype networks have a star topology, suggesting that population expansion has been experienced historically (Figure 2) (da Cunha et al., 2020). All these evidences support that the Western Atlantic Sierra mackerel is a single genetic population. However, the application of nuclear DNA markers has revised this conclusion. By comparing microsatellite or SNP data from north-south Brazilian samples, there are signs of slight allelic frequency differences between the southern Brazilian population and the northern Brazilian population (Soeth et al., 2022). The reason may be related to the current system along the Brazilian coast: the North Brazilian warm current and the Malvinas cold current intersect in southern Brazil, forming a hydrological barrier, which restricts the exchange of fish in the north and the south. Similar phenomena may also exist on kingfish (S. cavalla). Kingfish is widely distributed in the East Coast of North America and the Gulf of Mexico, and traditional views regard it as a single management unit. However, trace element trace element trace and microsatellite studies suggest that there is a certain separation between the king fish in the Gulf of Mexico and the king fish along the Atlantic coast (Gold et al., 2010), and may each lay eggs and return to its original position, thus forming two relatively independent subgroups. 4.3 Localized population characteristics of coastal waters in Southeast Asia and China Southeast Asia and China's offshore in the western Western Pacific are important spawning and fishing areas for mackerels, and are also one of the hot spots for population structure research. The population structure of the major species, Japanese mackerel, has been controversial along the coast of China. Traditionally, it is divided into the Yellow-Bohai Group and the East China Sea Group according to the spawning ground and the fishing flood. There are certain differences in the spawning time and growth parameters of the two. Yang Linlin and others analyzed the otolithic morphology of Japanese mackerels in different spawning sites in China. The results showed that the shape differences in the breeding groups of the three major sea areas of the Bohai Sea, the Yellow Sea and the East China Sea were extremely significant. This shows that Japanese mackerels living in different sea areas have ecologically measurable differentiation characteristics, supporting their relative independence of their breeding populations. Further, the otolith morphological discrimination formula was established using discriminant analysis, which allowed the correct distinction rate of Xiangshan Port (East Sea) and Laizhou Bay (Huang and Bohai Sea) samples to reach 71%, providing a morphological basis for population discrimination. Mitochondrial studies of broadband mackerels in the Gulf of Thailand have shown that there is no obvious genetic
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