International Journal of Marine Science, 2025, Vol.15, No.5, 277-286 http://www.aquapublisher.com/index.php/ijms 282 phenomena have also been reported in the breeding of other fish. For example, the body length and mass of the yellow croaker F₂ are only 0.029 and 0.134, respectively, which is significantly lower than the previous generation's 0.26 and 0.18. This suggests that after a generation of strong selection, favorable variation in the population has been partially fixed, with the potentially reduced selectable variation, resulting in weaker selection responses in subsequent generations (Wang et al., 2022). Faced with this situation, we took two measures: First, appropriately reduce the selection intensity of the second generation to avoid excessive elimination and exhaustion of mutations; second, a small number of wild individuals from unrelated groups were introduced in the F₂ generation and hybridize with the breeding group to restore some genetic diversity. Practice has proven that these measures help to continue genetic gain, causing new signs of improvement in the growth traits of the third generation. Figure 2 Growth condition of cage-cultivated large yellow croaker from Jiaocheng (JC), Shacheng (SC) and Xiapu (XP), Ningde City (Adopted from Chen et al., 2020) 6.3 Case-specific factors affecting breeding effect During the breeding process of Hainan mackerel, we also observed the influence of some case-specific factors on breeding effects. First, there are environmental fluctuations factors. During the experiment, a strong El Niño incident happened, and the water temperature near Hainan was about 2 ℃ higher than usual. This led to the decline in feeding and impeded growth in both groups in summer, partially covering up the growth advantages of breeding groups. Secondly, the evaluation error may be caused by a relatively small family coefficient. Due to the relatively concentrated breeding season of mackerels, only 20 and a half-compatriot families were constructed in the first batch, which to some extent limited the accuracy of genetic parameter estimation. The number of families was subsequently increased to 31 by extending the egg-laying induction period, making heritability estimates more reliable (He et al., 2018). As a high-value large predator fish, mackerel is sensitive to management factors such as feed nutrition and breeding density. If the feed supply is insufficient or the density is too high, the growth advantage of the breeding group will be weakened (Yu et al., 2024). This reminds us that the high yield potential of good varieties can only be fully utilized under optimized breeding conditions. Therefore, while promoting good varieties, it is necessary to improve breeding management, such as providing high protein feed and reasonably controlling stocking density. 7 Case Inspiration and Promotion Value 7.1 Reference significance for regional breeding models The practice of selective breeding of mackerels in Hainan provides useful inspiration for regional marine fish breeding models. The case highlights the significance of carrying out breeding of characteristic varieties according to local conditions. In the past, China's aquatic breeding work focused on freshwater varieties such as grass carp and tilapia, while Hainan took the lead in achieving breakthroughs in seawater famous varieties such as mackerel,
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