International Journal of Marine Science, 2025, Vol.15, No.5, 277-286 http://www.aquapublisher.com/index.php/ijms 281 5.2 Genetic correlation between body length, weight and growth rate There is often a genetic correlation between growth traits, i.e. genetic improvement of one trait may be accompanied by changes in another trait. This study analyzed the genetic correlation between body length, weight and growth rate of mackerel. The results showed that body length was highly positively correlated with body weight, with its genetic correlation coefficient close to 0.9, and its phenotypic correlation coefficient was about 0.8, and its correlation reached a very significant level (P<0.01). This means that by selecting parents with larger weight, offspring will often increase in size accordingly. Therefore, weight can be used as the main selection indicator in breeding practice, and body length will be improved simultaneously. High correlation is also reflected in most farmed fish, such as the genetic correlation between Oriental Snake body length and weight is as high as 0.92 (Wang et al., 2022). In contrast, the genetic correlation between growth rate and final weight may be more complex. If the measured growth rates are not correlated with weight at harvest at different times, choosing early rapid growth families may not guarantee adult weight maximization (Oliveira et al., 2016). 5.3 Population differentiation and variability under selection pressure When a population continues to experience directional selection pressure, the frequency of alleles within it will change directionally, causing the population's genetic structure to deviate from its original state. This population differentiation is the expected result of selective breeding, but it is wary of the consequences of a significant decline in genetic diversity (Imron et al., 2015). In this study, we compared the differences in neutral molecular markers between selected groups and unselected natural groups. It was found that after two generations of growth trait selection, the allelic diversity of the breeding population decreased slightly, and specific allelic frequency shifts occurred at some growth-related loci (Landguth and Balkenhol, 2012). This suggests that targeted selection has caused a certain degree of genetic differentiation among breeding populations. Some model prediction studies have also shown that continuous artificial or fishery selection will drive the evolutionary changes in fish populations in genetic life history characteristics, such as early maturity or growth rate changes. In breeding practice, we hope to accumulate favorable variation, but we must prevent the population genetic basis from being too narrow. To this end, we maintained a relatively large effective population size at each generation of breeding and introduced new wild individual ancestry for periodic hybridization to enrich the genetic background. 6 Hainan Case Results and Analysis 6.1 Comparison of growth performance between selected breeding groups and natural groups By comparing the growth traits of Hainan mackerel breeding groups (two generations in succession) and natural groups, the results achieved by artificial selection can be clearly seen. The results showed that the breeding population performed better than the control natural population throughout the breeding cycle. By 18 months of age, the average weight of individuals in the breeding population was about 15% higher than that of the control population and the average body length was about 7%. The growth curves of the two groups were significantly separated in the middle and late stages, and the breeding groups showed higher growth rates in the late stages of breeding. This result is similar to the breeding experiment of yellow croaker. The growth rate of body length and weight of the selected offspring in the later stage exceeded that of the unbred control (Figure 2) (Zhou et al., 2019; Chen et al., 2020). It can be seen that the growth potential of mackerel has been successfully tapped through directional selection, which has greatly improved the growth performance of the population. Considering that there are many restrictions on the natural sea area environment, the actual increase in yield of artificial breeding lines under breeding conditions may be more significant. This means that promoting good varieties is expected to reduce dependence on wild mackerel fishing and contribute to resource conservation. 6.2 Differences in genetic gain in different breeding generations Genetic gain between different breeding generations is an important basis for evaluating breeding effects. In this study, we observed that the average body length and weight of mackerels were significantly improved after the first generation (F₁), but the genetic gain amplitude of the second generation (F₂) was reduced. Specifically, F₁ has increased weight by about 10% compared with the unsuccessful population, while F₂ has only increased by about 5% further compared with F₁, and the second generation has almost no advantage in terms of body length. Similar
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