International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 80 high-protein foods. This “high predation-high metabolism” strategy requires strong circulatory and respiratory support (Planas et al., 2017). The gill filaments of Spanish mackerels have a large area and high oxygen capacity in the blood, which allows them to meet their oxygen needs when they move at high speed. Again, Spanish mackerels showed obvious seasonal migration behavior. Take Japanese Spanish mackerel as an example. Every spring, as the sea water heats up, it migrates from south to north to the Yellow and Bohai Sea to lay eggs, and goes south to overwinter in autumn and winter (Fauvelot and Borsa, 2011). This migration model is conducive to the use of highly productive sea areas in each season. The migratory ability of Spanish mackerels is partly derived from their perception of environmental cues such as geomagnetic fields, ocean currents, and temperature gradients, as well as the guidance of group behavior. Genetic studies have shown that high migration leads to gene mixing among different egg-laying populations, reducing geographical differentiation. 4.3 Reproductive strategies and ecological adaptation of reproductive cycles Spanish mackerel has r-countermeasure reproductive characteristics, namely high egg laying rate and long-distance egg laying migration to increase the survival chances of offspring. Most Spanish mackerels mature early and have strong fertility. For example, Japanese mackerel females mature sexually at about 2 years old and can lay eggs at about 40 cm in length. The spawning season usually chooses summers with suitable water temperatures and abundant baits, and is carried out in warm waters along the coast or near islands. Spanish mackerel is a multiple-spawn type, and ovulates multiple times in batches throughout the breeding season. Each mature female can lay hundreds of thousands to millions of eggs in one season (Weng et al., 2020). These eggs are floating eggs with a diameter of about 1 mm, and contain oily balls that can spread to a wide area with the current, thereby increasing the distribution range of juvenile fish. High egg laying number and diffusion at the slab stage are a kind of adaptation of Spanish mackerel to unstable marine environments - even if locally unfavorable conditions are present, drifting larvae may still reach better sea areas, thereby ensuring population continuation. However, this strategy also leads to possible confounding of offspring between spawning grounds, reducing the isolation of population structure. For example, a study on the microchemistry of larvae otoliths of East China Sea and Yellow Sea Spanish mackerel found that juvenile fish hatched in different spawning grounds could share the same fishery and were jointly supplemented into multiple local populations rather than independently supplemented. This shows that the breeding strategies of Spanish mackerel tend to be "source-aggregation" dynamics, that is, multiple spawning grounds serve as sources to transport young fish to a wide range of habitats. In addition to high egg laying, Spanish mackerel also exhibits synchronization of environmental cycles in the reproductive cycle. For example, when the water temperature of Japanese Spanish mackerel rises above 20 ℃ from April to June each year, the peak is in early summer, which is consistent with the peak of bait small fish during this period, which is conducive to improving the survival rate of juvenile fish. After laying eggs, adult fish will return to the fattening grounds (such as the northern waters) in time to recover their strength. Reproductive input and feeding growth alternately to maintain a healthy body condition cycle. In addition, some low-latitude species may lay eggs multiple times throughout the year, but there is still a tendency to reproduce concentratedly during periods of good conditions such as rainy season and estuaries. 5 Global Distribution Pattern and Migration Route 5.1 Native populations of the Western Pacific and Indian Oceans The Western Pacific-Indian Ocean region is considered to be the origin and differentiation center of the genus Spanish mackerel, and is also the area with the most diverse species and the most prosperous population today. In the Western Pacific, China's East China Sea, South China Sea, Japan's offshore and Southeast Asian Archipelago waters are inhabited by several Spanish mackerel species, especially those represented by Japanese mackerel (S. niphonius) and narrow-band Spanish mackerel (S. commercialson) (Vineesh et al., 2018). Japanese mackerel is mainly distributed in the coasts of China, Japan and South Korea in the northwest Pacific, and is an important fishery resource in this sea area in spring and summer. Its typical migration route is to overwinter in the East China Sea and the northern part of the South China Sea in winter, and to migrate northward to the Yellow Sea and Bohai Sea as the water temperature rises, and then return south to overwinter in late summer and early autumn. This migration period leads to obvious abundance fluctuations in populations in different sea areas at different
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