International Journal of Marine Science, 2025, Vol.15, No.4, 186-198 http://www.aquapublisher.com/index.php/ijms 191 differentiation between the populations in the Gulf of Thailand, which may be due to the fact that the bay is a semi-enclosed water body, and the internal water mass circulation causes the fish to mix (Radhakrishnan et al., 2018). Figure 2 (A) Map of South America showing the eight sites from which the specimens of S. brasiliensis analyzed in the present study were obtained: 1- Cumana; 2- Isla de Margarita; 3- San Fernando; 4- Las Cuevas; 5- Erin; 6- Macapá; 7- Fortaleza and 8- Paranaguá. (B) Neighbor-Joining and Maximum Likelihood trees for the Scomberomorus brasiliensis haplotypes of the MT-ND4 gene. Each haplotype is represented by a different color, yellow circles = 1- Cumana, red squares= 2- Isla de Margarita, blue = 3- San Fernando, brown = 4- Las Cuevas, green = 5- Erin, gray = 6- Macapá, pink = 7- Fortaleza and black = 8- Paranaguá (Adopted from da Cunha et al., 2020) 5 Factors and Ecological Driving Mechanisms of Gene Flow 5.1 The influence of ocean currents and migration pathways on gene communication 5.1.1 The dual role of ocean currents as population connection channels Ocean currents are a key factor affecting the gene flow of marine fish populations. For mackerels, ocean currents can not only act as "highways" to promote long-distance gene exchange, but also may form environmental barriers and limit the mixing of different groups. In promoting gene flow, strong currents can carry floaty eggs and juvenile fish, making them cross distances that were originally difficult to cross independently. For example, the Kuroshima and its tributaries flow through the East China Sea and Taiwan Straits, bringing the Japanese mackerel fish produced in the southern part of the East China Sea to the northerly Yellow Sea, thus emerging new individuals supplemented by the East China Sea (Sassa and Tsukamoto, 2010). However, ocean currents also have a barrier effect. When there are water currents of very different natures between the two sea areas, the migration and early life history stages of fish are restricted. Studies show that in cases of low genetic connectivity among populations in the Yellow Sea and East China Sea, the Yangtze River flush water may be an important factor. There are large areas of low-salt water mass in the Huangdonghai summer, separating the northern and southern waters at the plankton level. Through the review of 137 literatures, it was found that more than 65% of the cases believed that there was significant genetic differentiation between the Yellow Sea and the East China Sea marine protection zone, and the freshwater front of the Yangtze River was
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