International Journal of Marine Science, 2025, Vol.15, No.4, 186-198 http://www.aquapublisher.com/index.php/ijms 192 considered to be one of the six obstacles. For mackerel, Japanese mackerel spawns mainly in nearshore low-salt water environments (such as Hakka Bay). Once a juvenile fish enters the range of flushed water, it is difficult to go further north, thus forming a partially isolated breeding group in the Yellow Sea and the East China Sea. In addition to freshwater fronts, the intersection of hot and cold flows can also form similar barriers (Yu et al., 2022). 5.1.2 Dynamic regulation of population structure by seasonal changes of ocean currents Many ocean currents have seasonal characteristics and periodically change with monsoon or climate factors. This seasonal change can have a dynamic impact on fish migration pathways and gene flow. Taking China's coastal areas as an example, in winter, the northeast monsoon is strong, the cold current goes south, the main body of the black tide is eastward away from the shelf, and the water exchange between the Yellow Sea and the East China Sea weakens; in summer, the southwest monsoon prevails, and the warm current goes north to the coast, promoting the connection of water clusters in the two sea areas. Correspondingly, most Japanese mackerels retreat to the East China Sea to overwinter in winter, and go north to the Yellow Sea to lay eggs in summer. Seasonal changes in ocean currents just guide this migration cycle, causing fish in the two sea areas to mix briefly in summer and return to their respective positions in winter (Cheng et al., 2015). This periodic intersection helps maintain genetic exchange between groups without completely confusing the population, because it is not consistent throughout the year. Similarly, in the northern Indian Ocean, the currents in winter and summer reverse under the monsoon drive. Narrow-band mackerels may follow ocean currents and travel to the Arabian Sea and the Bay of Bengal in different seasons to feed or breed. During the alternation of monsoons, fish from different regions may converge in open waters to exchange genetic material (Gwak, 2025). 5.2 Genetic response to changes in marine environment Differences and changes in marine environmental factors will have an impact on the genetic structure of highly migratory fish such as mackerel. On the one hand, different environmental conditions may drive local populations to produce adaptive genetic differentiation; on the other hand, changes in marine environment caused by global climate change will also change population distribution and gene communication patterns. Temperature is one of the important factors. The tolerance range of water temperatures of various species of the genus Mackerel is slightly different. For example, Japanese Mackerel can tolerate lower temperatures in the Yellow Sea for winter, while broadband Mackerel prefers warmer waters. In the long run, temperature gradients may become genetic selection pressures, causing populations of different temperature bands to accumulate different allelic frequencies. For example, Japanese mackerels in higher northern latitudes may present selectable markers on genes associated with cold resistance, distinguishing them from southern populations (Guo et al., 2021). Although this "adaptive differentiation" may not be obvious in neutral markers, it may be detected by genome-wide scans (Figure 3) (Lee et al., 2023). Similar findings have been observed in related groups such as tuna. Salinity is also an influencing factor. Mackerels usually do not enter fresh water, but lay eggs in low-salt areas of the estuary. The low saline environment at the Yangtze River estuary has different selection pressures on embryonic development compared with the outer sea, which may lead to differences in the genetic composition of the egg-laying population in the Yangtze River estuary with the population in higher salinity waters. The spatial differences between environmental factors such as oxygen content and pH in the distribution of mackerel are not extreme, so it is generally believed that the impact is small. 5.3 The interference of fishing pressure and artificial isolation on genetic structure The impact of human activities on the genetic structure of marine fish has attracted widespread attention in recent years. For mackerels, overfishing and man-made isolation are two main human interference factors that may have important implications for their population genetic structure. First, overfishing will lead to a sharp decline in population size and a change in age structure. From a genetic point of view, it is equivalent to a significant reduction in the effective population size (N_e), aggravate incomparison and genetic drift, thereby reducing genetic diversity. For example, Gwak (2025)'s research pointed out that as the number of Sierra mackerels increases in southern Brazil, the genetic diversity indicators of local populations have a downward trend (such as a
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