This study focused on monitoring the dynamics of bacterial communities and assessment of the influences of physicochemical parameters during a culture cycle in black carp polyculture ponds. Our results showed high variation in microbial diversity and microbial composition in the water column during the culture period. Proteobacteria, Actinobacteria, and Bacteroidetes were the most abundant phyla, while the abundances of the phyla Cyanobacteria, Firmicutes, and Bacteroidetes changed in different months. Moreover, 13 abundant genera with significant difference were found between different months. Thus, samples in different months were divided into three groups according to principal coordinate analysis (PCoA) and unweighted pair-group method (UPGMA) clustering results. RDA showed that total nitrogen (TN), total phosphorus (TP), phosphate (PO₄³⁻-P), nitrate (NO₃⁻-N), temperature (T), dissolved oxygen (DO), and pH significantly shaped the microbial community composition in different months. While Pearson correlation coefficient showed that T, SD, and pH were strongly correlated to the dominant genera. Considering some genera are potential pathogenic bacteria, we could manage the black carp pond by quickly monitoring the water temperature and SD in the future.

One of the crucial factors influencing the growth and viability of larvae and juveniles is their opening diets. The objective of this study was to identify suitable initial feed options for M. macropterus larvae and juveniles. A total of 1200 newly hatched M. macropterus with an average weight of 18.3 mg and an average length of 11.58 mm were selected and randomly divided into four groups. The fish were fed with different opening diets, including rotifer, Artemia nauplii, Tubifex, and micro-diet from six days after hatching (dahs), respectively. Growth indices and activities of digestive enzymes were assessed at 10, 15, 20, 25, 30, 35, and 40 dahs. Histological examination of the structure of the digestive tract was performed at 40 dahs, while survival rates were also documented. The results demonstrated that different diets had no effect on the survival rate of larvae and juveniles of M. macropterus. The growth performance indices were ranked as follows: Tubifex group > Artemia nauplii group > micro-diet group > rotifer group. Remarkably, the Tubifex group exhibited superior growth performance, which was also reflected in the structure of the digestive tract and digestive enzyme activity. Therefore, it is recommended to include Tubifex in the diet of M. macropterus larvae and juvenile during the standardized farming process.

It is necessary to determine the optimal temperature for Percocypris pingi growth in recirculating aquaculture systems. To describe the effects of temperature, we evaluated the growth, antioxidant enzyme activity, and gut microbiota structure of P. pingi at different temperatures, including 14, 18, 22, and 26 °C. Results showed that increases in body weight of individuals of the groups subjected to 18 and 22 °C temperatures were considerably higher than those in the groups subjected to temperatures of 14 and 26 °C between 20 and 60 d after the experiment started. Acid phosphatase activity in the liver and kidneys of P. pingi did not differ significantly among the various temperature groups (p > 0.05). A gradual restoration of the alkaline phosphatase and superoxide dismutase activities to variations in the surrounding temperature was observed in the liver and kidney of P. pingi. Interestingly, the water temperature did not affect the α-diversity or composition of the gut microbiota of P. pingi. In conclusion, water temperatures between 14 and 26 °C significantly impacted the growth of P. pingi (p < 0.05) but not the liver and kidney antioxidant capacity or the gut microbiota within 60 d.

Rhinogobio ventralis is a rare fish found in the Yangtze River in China and has significant ecological and economic value. In this study, a bacterial strain (RV-JZ01) was isolated from the livers of diseased R. ventralis. This isolate was identified as Aeromonas veronii based on its morphology, biochemical features and 16S rDNA phylogenetic analysis. The artificial infection of healthy R. ventralis (16 ± 2 cm) with RV-JZ01 resulted in the manifestation of clinical symptoms, in accordance with those of naturally infected fish. The 50% lethal dose (LD₅₀) of RV-JZ01 for R. ventralis was 6.3 × 10⁶ CFU/mL. Histopathological examination revealed various pathological changes in the diseased fish, including intestinal villus swelling and rupture, hepatocyte vacuolization, renal tubular cell nuclear enlargement and pyknosis, and myocardial fiber fracture and atrophy. RV-JZ01 infection significantly reduced the gut flora diversity of R. ventralis, with the relative abundances of Firmicutes and Fusobacteria increasing, and those of the Proteobacteria and Bacteroidetes decreasing. The abundance of Lactobacillus and Streptococcus dramatically increased, and the abundance of Clostridium and Escherichia reduced in the intestinal microbiota of R. ventralis infected with RV-JZ01. Antibiotic sensitivity testing revealed that RV-JZ01 was highly susceptible to 12 antimicrobials, including erythromycin, cefalexin, norfloxacin, furazolidone, sulfonamides, enrofloxacin, doxycycline, piperacillin, florfenicol, gentamicin, and lincomycin. These results contribute to the understanding of pathological alterations in R. ventralis following A. veronii infection, offering valuable data to support the implementation of disease treatment.

This study investigated the aggressive behavior of Hemibagrus macropterus (Bleeker) by examining the effects of size combinations (large vs. large (LL), large vs. small (LS), small vs. small (SS)), sex combinations (male vs. male (MM), male vs. female (FM), female vs. female (FF)), and shelter availability (tiles, pebbles, grass) under varying group densities (1, 3, and 6 fish). The results revealed that H. macropterus exhibited nocturnal behavior, engaging in exploration, territorial defense, and aggressive interactions such as chasing and biting, with significantly higher aggression at night. A social hierarchy distinguishing between dominant and subordinate individuals emerged. Larger fish displayed higher aggression frequencies compared to LS and SS groups, though the presence of shelters reduced aggression, particularly in environments with ample shelters. Females exhibited stronger aggressive tendencies, with FF and FM groups showing higher aggression than MM groups. Shelter preference experiments indicated a preference for tile and pebble caves, with no cohabitation in these shelters, while aquatic plant caves allowed cohabitation. Aggression was significantly higher in competitive (6 fish) versus non-competitive (3 fish) groups. These findings suggest that nocturnal feeding, sufficient shelters, and size-segregated rearing can mitigate aggression and improve cultivation efficiency in H. macropterus.

Schizothorax kozlovi, as an endemic and vulnerable fish of the upper Yangtze River in China, faces many threats. In order to expose the population structure of wild S. kozlovi, the carbon and oxygen isotopic ratios in the otoliths, and the gene sequences of two common mitochondrial markers (Cytb and COI) were investigated in four sampling locations, and then their relationship with ambient temperature was further investigated. In general, it exhibits limited geographic population structuring of S. kozlovi in the upper Yangtze River by both mtDNA and stable isotopes. The values of otolith stable isotope ratios varied from −15.30‰ to −12.37‰ for δ¹⁸O and from −10.10‰ to −6.13‰ for δ¹³C. Significant relationships were revealed between stable isotope ratios and specific mean monthly water temperature variables (from November to March), indicating low temperature effect on otolith stable isotope ratios. Haplotype diversity and nucleotide diversity were 0.928 and 0.00778, both exhibiting high levels. A median-joining haplotype network indicated a mixture of geographical distribution but exhibited two distinct haplotype lineages (Clade I and Clade II). AMOVA detected that the higher percentage of genetic variance was within sampling locations (96.94%) and between two haplotype lineages (72.82%). Most F_ST values between sampling locations showed small levels of genetic differentiation except the differentiation between population SJ (Sanjiangkou) and JP (Jinping). Therefore, two haplotype lineages and population JP of S. kozlovi in the upper Yangtze River are suggested as three management units for conservation due to their moderate-to-great genetic differentiation and isolated habitat.