International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 83 6.2 Application of microsatellite and SNP markers in genetic structure analysis High-resolution molecular markers are a powerful tool for analyzing the structure of fine populations. For high-gene flow species such as Spanish mackerel, structural differences were often difficult to detect in the past single mitochondrial markers. In recent years, researchers have used more microsatellite and SNP markers of multiple loci to increase detection statistical effectiveness. Microsatellites are simple repeat sequences with variable lengths, and their high polymorphisms make them well suited for population-level studies. By developing species-specific microsatellite sites and genotyping different populations, allelic frequency distributions can be obtained for calculating genetic distances and differentiation indicators. As a new generation of markers, SNP is more dense and distributed throughout the genome, which helps capture population adaptability differences. Joy et al. (2020) used ddRAD sequencing to obtain thousands of SNP sites in narrowband Spanish mackerels, which were used to analyze population structures such as the South China Sea and the Java Sea. They found that overall these groups were mixed, but two genetic clusters could still be identified through F<sub>ST</sub> weak values and STRUCTURE clusters, corresponding to the South China Sea-Java Sea and the Bali Sea-Sulawesi Sea, which may be related to the marine circulation pattern. In addition, SNP data can also be used to detect local adaptation signals, such as screening differential sites associated with environmental variables such as water temperature and salinity, to help identify the genetic response of populations to the environment. For a wide range of species like Spanish mackerel, this method can reveal adaptive genetic differentiation in different regional populations, and even if neutral markers show no structure, differences in functional genes may be found. 6.3 The association between gene diversity and ecological adaptability Genetic diversity is the basis for population adaptation to environmental changes and long-term survival. The species of Spanish mackerel are generally highly genetically diverse, which is related to their large populations and extensive genetic exchange. Sequence analysis of mitochondrial control zones showed that most Spanish mackerel populations had high haplotype diversity (>0.8) and medium nucleotide diversity, suggesting that they had experienced population growth in history and maintained a large effective population size. For example, the analysis of mitochondrial COII genes of Spanish mackerel fish at different locations in Indonesian waters has 8 haplotypes, with haplotype diversity reaching 0.89 and nucleotide diversity reaching 0.072, which is a relatively high level (Widayanti et al., 2024). High genetic diversity means that there are abundant genetic variations in the population, which can be selected for natural selection and screening, which is conducive to adapting to environmental changes. Therefore, genetic diversity is often positively correlated with ecological adaptability. On the one hand, diverse genotypes improve populations’ ability to respond to different environmental conditions. For example, traits such as temperature tolerance and disease resistance are often regulated by multiple genes. Population with high genetic variation is more likely to contain genotypes that are tolerant to extreme conditions, thus having a higher survival rate when the environment changes dramatically. On the other hand, genetic diversity also reflects the effective scale and mobility of the population in history. Large populations with high connectivity are usually more robust and less likely to become extinct due to local disasters. Large-scale gene exchange of Spanish mackerels has buffered the overall impact of local resource fluctuations to a certain extent. However, there are also studies that local genetic differentiation may promote the evolution of special traits when populations adapt to specific niches. For example, two narrow-band Spanish mackerels along the coast of Australia have evolved different spawning sequences despite overall genetic exchange to adapt to local temperature and seasonal changes. 7 Global Fisheries Development Discovery Status and Management Challenges 7.1 The economic status of Spanish mackerel in fisheries in various countries 7.1.1 Comparison of major catch countries and yields Spanish mackerel is an important part of the marine fishing industry in many countries due to its excellent meat quality and high market value. In Asia, China, Japan, Indonesia and other countries have long been among the forefront of the world in terms of Spanish mackerel fish catches. The Japanese Spanish mackerel fishery along the coast of China has a long history. The Spanish mackerel production was once very considerable in the 1980s and
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