International Journal of Marine Science, 2025, Vol.15, No.4, 186-198 http://www.aquapublisher.com/index.php/ijms 193 decrease in haplotypes). Although the decline is not large at present, long-term high-pressure fishing may trigger genetic bottlenecks (Gwak, 2025). Fishing also tends to remove larger mature fishes in individuals, disrupt the original mating system, and make the actual gamete contribution more tilted to a few small, high-reproductive individuals, which will also reduce the effective transmission of alleles. Figure 3 Morphological features, worldwide occurrences and sampling location of chub mackerel (Adopted from Lee et al., 2023) Image caption: (a) Morphology of chub mackerel provided from the Marine Fish Resource Bank of South Korea (MFRBK). (b) Locations of worldwide occurrences of chub mackerel. (c) Local map of the sampling location of the chub mackerel individual of fScoJap1 assembly marked as a blue star mark in South Korea (34°46′15.8″ N, 128°23′54.0″ E). Each red dot on the map represents an occurrence location. Some dots were shaded (30% transparency) to display overlapping dots (Adopted from Lee et al., 2023) Secondly, artificial isolation refers to human activities that change habitat connectivity. For example, large-scale offshore projects and increased shipping noise may hinder the traditional migration routes of fish, and the actual effect is similar to that of geographical barriers. Although mackerel has strong swimming ability and no evidence has been shown that human maritime activities have significantly prevented its migration, with the intensification of offshore development, it is not ruled out that local populations have reduced their interactions with other groups due to avoiding human interference, thus gradually genetic differentiation (Cheng et al., 2015). Another human factor is release and breeding escape. If fish from non-homologous groups are artificially introduced into a region and cross with local fish, it may change the genetic structure. Although mackerels usually do not undergo artificial proliferation and release, it is not ruled out that future breeding or breeding trials can cause escape, such as artificially cultivated broadband mackerels escape into the wild and mated with wild groups, which will introduce artificially selected genotypes and "dilute" the wild gene bank. Studies have confirmed the effect of this infiltration on the genetic diversity of wild populations in other fish.
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