A novel morphological index applied to genetic improvement of body shape in grass carp Ctenopharyngodon idella

Body shape is an economically important trait in grass carp (Ctenopharyngodon idella) that directly influences consumer acceptance and farmer profitability. It is essential to incorporate shape traits into the ongoing breeding program, as this would be expected to achieve rapid growth and maintain a slender shape for cultured populations. Thus, the aim of this study was to explore a suitable morphological index for genetic improvement of shape traits in grass carp. The viscerosomatic index (VSI) was introduced as a quantitative indicator of individual slenderness for the evaluation of various morphological ratios. Then, based on correlation and path analysis among body measurements (BMs) and weights of 290 recorded individuals, nine representative ratios that can reflect body slenderness from different angles were employed in this study, including Fulton's condition factor (K), Richter's condition factor (B), and other length ratios of different BMs (S1-S7). The results showed that each of these ratios exhibited a favorable relationship with VSI, confirming that more slender individuals always have a lower VSI. Of these ratios, S1 (body height-to-body length ratio, BH/BL), S2 (body height-to-caudal peduncle height ratio, BH/CPH), and S3, calculated as (BH-CPH)/BL, had the top three correlation coefficients with VSI (r = 0.501, 0.558, and 0.591, respectively). Furthermore, the S3 index reached the highest correlation (r = 0.732) in the sample group (n = 50) of mean body weight of 1560.8 g, suggesting that a quantitative comparison of individual slenderness would be more accurate within this fish size range. In addition, 636 pedigreed fish from 21 full-sib families were used for estimates of genetic parameters via an animal model. In comparison with the low heritability of K, the three ratios showed moderate to high heritability (h2 = 0.47 +/- 0.14, 0.20 +/- 0.08, and 0.31 +/- 0.11, respectively), indicating significant potential for their genetic improvement. However, there were weak but undesired phenotypic and genetic correlations between body weight and the three ratios (0.13 < rp < 0.19; 0.23 < rg < 0.50), predicting that selection for rapid growth may slowly lead to the development of a rotund shape. In conclusion, the novel S3 index was suggested as the preferred candidate of shape traits for selective breeding in grass carp, especially because its expected relative genetic gain (4.07% at 10% selection intensity) was higher than those of the others (1.44%-3.99%).