Schizothorax kozlovi is an endemic and vulnerable fish species found in the upper Yangtze River in China. Over the past few years, the population resources of S. kozlovi have been nearly completely depleted owing to multiple contributing threats. While the complete mitochondrial genomes serve as important molecular markers for phylogenetic and genetic studies, the mitochondrial genome of S. kozlovi has still received little attention. In this study, we analyzed the characterization of the mitochondrial genome of S. kozlovi and investigated the phylogenetic relationships of Schizothorax. The complete mitochondrial genome of S. kozlovi was 16,585 bp in length, which contained thirty-seven genes (thirteen protein-coding genes (PCGs), two ribosomal RNA genes (rRNAs), twenty-two transfer RNA genes (tRNAs)) and two non-coding regions for the origin of light strand (OL) and the control region (CR). There were nine overlapping regions and seventeen intergenic spacers regions in the mitochondrial genome. The genome also showed a bias towards A + T content (55.01%) and had a positive AT-skew (0.08) and a negative GC-skew (−0.20). All the PCGs employed the ATG or GTG as the start codon and TAA, TAG, or single T as the stop codon. Additionally, all of the tRNAs displayed a typical cloverleaf secondary structure, except trnS1 which lacked the D arm. The phylogenetic analysis, based on the maximum likelihood (ML) and Bayesian inference (BI) methods, revealed that the topologies of the phylogenetic tree divided the Schizothorax into four clades and did not support the classification of Schizothorax based on morphology. The phylogenetic status of S. kozlovi was closely related to that of S. chongi. The present study provides valuable genomic information for S. kozlovi and new insights in phylogenetic relationships of Schizothorax. These data could also offer fundamental references and guidelines for the management and conservation of S. kozlovi and other species of Schizothorax.

The study described the length–weight relationships (LWRs) and diversity status of fishes in the midstream of the Jialing River, which is the largest tributary of the upper Yangtze River, China. A total of 4592 specimens from 53 fish species belonging to three orders and eight families were collected from December 2021 to November 2022. The results showed that Culter oxycephaloides, Xenocypris davidi, Hemibarbus labeo, Hemiculter tchangi were dominant fish species in the study region. Twenty-five fish species (IRI ≥ 10) were subjected to LWR analysis, and the regression parameters a and b for fish species varied from 0.006 to 0.333 and 2.129 to 3.391. Eleven fish species were determined to have isometric growth, and 14 fish species were determined to have allometric growth. The diversity analyses suggested that the diversity status of fishes were kept relatively stable during the sampling period and that the fishes suffered moderate disturbance in the midstream of the Jialing River. The present study provided basic biology data for fish conservation and management after the fishing ban in the Jialing River.

Hemibarbus maculatus is a common economic fish in the midstream and downstream of the Jialing River. In order to resolve the difficulties in aquacultural cultivation, we tested the intestinal and liver digestive function of wild and cultured Hemibarbus maculatus. Histological methods and special biochemical staining methods were used to compare the differences of morphological structure, goblet cells, argyrophil cells, lymphocytes and Na⁺/K⁺ATPase in the intestine, and the morphological structure, glycogen and lipid in the liver between the two kinds of Hemibarbus maculatus. The results showed that higher amount of fat was found to attached to the gut, lower Na⁺/K⁺ATPase vitality in the foregut and hidgut (p < 0.01) and lower number of goblet cells in the hindgut (p < 0.01) of the cultured Hemibarbus maculatus when compared to the wild ones. The number of the argyrophilic cells did not show significant differences between the two kinds, but the number of lymphocytes was significantly lower in the segments of gut in cultured. This suggests the absorptive function and intestinal immunity are weaker in the cultured Hemibarbus maculatus. In addition, more glycogen and lipid were found in the liver of cultured fishes, which indicates the decreased digestive function of the cultured Hemibarbus maculatus. In conclusion, the intestinal digestion, absorption and lymphocytes level of the wild are generally better than those of the cultured, and more hepatic lipopexia and glycogen are present in the cultured ones. Future aquacultural activities should consider these changes when facing pragmatic problems.