Food organism compositions and feeding characteristics of Gymnodiptychus pachycheilus in the middle reach of Yalong River

Gymnodiptychus pachycheilus, as a native fish species belonging to Schizothoracinae fishes in China, is mainly distributed in the upper reach of the Yellow River and the middle-upper reaches of Yalong River. In order to analyze the food organisms and feeding characteristics (including feeding intensity and its seasonal change, composition and diversity of food organisms, and feeding strategy) of G. pachycheilus in the middle reach of Yalong River, a total of 207 samples were collected from the middle Yalong River, which was located in Ganzi Tibetan Autonomous Prefecture, Sichuan Province, from October 2020 to May 2022. Then, 207, 30, and 53 G. pachycheilus samples were respectively used to analyze the feeding intensity and composition of food organisms among different size, gender, and seasonal groups, measure the intestinal length (IC), and analyze the intestinal contents. The results showed that the overall feeding intensity level of G. pachycheilus was low in the middle reach of Yalong River, and the feeding intensity was the lowest at the stage of reproduction. In this study, the total number of species, which were fed by G. pachycheilus, was 38. The main food organism for G. pachycheilus was the Hydropsychidae larvae, which belongs to benthic macroinvertebrates, and the attached algae of Bacillariophyta (e.g., Diatoma spp., Navicula spp., Cymbella spp., Comphonema spp., etc.) was also fed by G. pachycheilus. The average IC of 30 G. pachycheilus samples was 1.61±0.06, which was at the lower middle level of Schizothoracinae fishes according to the previous studies. The Shannon-Wiener diversity index (H') and Pielou evenness index (J) of the food organisms for G. pachycheilus were 2.10 and 0.58, respectively, which were at the higher level of Schizothoracinae fishes according to the previous studies. In addition, the results based on Amundsen graphic method and Schoener overlap index analysis indicated that the overlap of food organisms was higher among G. pachycheilus samples from different body length and gender groups, while that was lower among those from different seasonal groups. Overall, G. pachycheilus in the middle reach of Yalong River belonged to the euryphagous and omnivorous fish species, which preferred benthic invertebrates, and seasonal changes had a more significant effect on the composition of food organisms for G. pachycheilus compared to body size and sex. The results of present study could further enrich the basic biological data of G. pachycheilus, and provide basic data and scientific basis for the wild resources protection, artificial domestication, and feed research and development of G. pachycheilus.