Comparative studies on age identification materials for Gymnodiptychus pachycheilus from the middle reaches of Yalong River

Gymnodiptychus pachycheilus is an endemic schizothoracinae fish in China, which is mainly distributed in the upper reaches of the Yellow River and the middle and upper reaches of Yalong River (tributary of the Jinsha River). In recent years, the wild resources of G. pachycheilus have declined seriously, and it has been listed as a second-class state-protected animal of China since 2021. In order to determine the suitable age identification materials for G. pachycheilus, we studied seven age identification materials of G. pachycheilus (including lapillus, asteriscus, vertebrae, anal scale, operculum, dorsal fin spine, and cleithrum) by using 83 samples from the middle reaches of Yalong River in October-November 2020. Preliminary observations showed that there was no obvious characteristics of annual rings on the dorsal fin spine and cleithrum, and the rings on asteriscus were too dense and complex to be counted. Thus, it was concluded that dorsal fin spine, cleithrum, and asteriscus are not suitable age identification materials for G. pachycheilus. Moreover, the lapillus proved the optimal age identification material for G. pachycheilus, which had the highest clarity, as well as the highest coincidence rate (CR: 84.34%) and the lowest index of average percent error (IAPE: 1.38%) between two age readings by the same observer. However, the weight of lapillus was not suitable for age identification of G. pachycheilus. Due to the similar age readings by the vertebrae and lapillus (CR: 96.39%, IAPE: 0.52%), the complementary readings of these two materials could make the age identification results of G. pachycheilus more accurate. In addition, the anal scale was only suitable for the age identification of low-aged G. pachycheilus (aged ≤ 5 years), and the operculum was unsuitable for age identification of G. pachycheilus. Our results have filled the research gap concerning age identification of G. pachycheilus, and provided applicable age identification materials for the basic biological studies on growth, feeding, and reproductive characteristics of G. pachycheilus.