Morphological Differences between Natural Hybrids of Silver Carp and Bighead Carp in the Xiangjiang River and Artificially Hybridized and Self-bred Populations

Purpose The aim of this study was to further confirm the suspected natural hybrid population of silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) discovered in the Xiangjiang River (hereafter referred to as the “ZJ”).

Methods Based on fish morphology and four statistical analysis methods, the traditional and geometric morphological differences among five populations were compared: the self-bred silver carp (LL) and self-bred bighead carp (YY) from the original stock of the Xiangjiang River, the reciprocal hybrid populations of silver carp and bighead carp bighead carp♀ × silver carp♂ (YL), silver carp♀ × bighead carp♂ (LY), and the ZJ population.

Results The gill rake structures of ZJ, YL, and LY were similar and distinctly malformed. One-way ANOVA revealed that 19 of the 27 morphological traits differed significantly (P<0.05) between the LL and YY populations. Multiple traits of the LY, YL, and ZJ populations exhibited intermediate characteristics between LL and YY, with head length and abdominal keel length being able to clearly distinguish among the different populations. Principal component analysis (PCA) showed that PC1 and PC2 effectively separated the LL, YY, and ZJ populations, while the LY and YL populations showed overlapping distributions. Discriminant function analysis indicated high classification accuracy for all populations; cross-validation enabled complete discrimination of ZJ and YY, with the lowest discrimination rate (84.6%) for YL. Cluster analysis showed that the LY and YL populations first clustered into a small branch, then joined with YY; ZJ clustered with LL. The four morphological multivariate analyses yielded consistent, complementary, and mutually corroborative results.

Conclusion The analytical results from four different methods complement and corroborate one another, confirming that the specimen ZJ collected from the Xiangjiang River is indeed a natural hybrid between silver carp and bighead carp. The findings facilitate the identification and management of possible hybrid resources between silver carp and bighead carp in natural waters, and provide a reference for the conservation of their original stocks and the sustainability utilization of their resources.