In August 2019, massive mortalities of cultured Hyriopsis cumingii occurred in a local shellfish breeding experimental base in Chongqing, China. In this report, we identified 14 isolates from cultured mussels in mortality events and demonstrated that these strains caused mortality from 9% to 97%. Significant mussel mortality (p < .05) was observed in association with most isolates after bacterial challenge; however, considerable variation in the virulence of different isolates was noted, with survival ranging from 3% to 88% after 7 days. The most virulent strains (HOP3 and GL1) in haemolymph and tissues were identified as Stenotrophomonas maltophilia and Aeromonas veronii based on biochemical and 16S rDNA sequence analysis. The LD50 values of HOP3 and GL1 were found to be 2.45 × 106 CFU/g (body weight) and 2.24 × 105 CFU/g (body weight) respectively. Histopathology of H. cumingii infected with S. maltophilia and A. veronii displayed degeneration of muscle fibres and gill necrosis. We tested the antibiotic susceptibility of the two isolates and found that HOP3 was multidrug‐resistant. Growth optimization study revealed that both isolates remained surviving across wide environmental ranges. Our data lay the foundation for the pathogenic mechanism of mussel bacterial diseases, and they also suggest that effective control measures are worth further investigation.

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To investigate the effects of different raising environments on the flesh quality, the study set up three groups of silver carp (Hypophthalmichthys molitrix), that is, pond-farmed; short-term stocked; and ranched, grown in a natural water ranch. This study compared the differences in muscle proximate composition and amino acid composition among three groups. The results showed that there was no difference between the ranched and the stocked groups in content of crude protein, ash, total amino acids (TAA), essential amino acids (EAA), and umami-flavored amino acids (UAA), with both groups being significantly higher than the farmed group. This shows that, in terms of flesh quality, the stocked group was closer to the ranched group and better than the farmed group after being stocked in natural water for 30 days. The results suggest that the flesh quality of farmed fish was improved by short-term stocking in natural water. To elucidate the reasons of flesh quality change in the stocked group, this study compared the food composition and intestinal digestive enzyme activity in three groups, demonstrating that the ranched and stocked groups had similar food composition, with their detritus proportion lower than that of the farmed group, consuming easily digestible phytoplankton diatoms rich in amino acids and fatty acids. No significant difference was seen between the ranched group and the stocked group, for which amylase, lipase, and trypsin activities were lower than that of the farmed group. It is hypothesized that differences in environmental and food composition resulted in lower digestive enzyme activity in the ranched group and the stocked group. In summary, the short-term stocking of farmed fish in a natural water ranch can effectively improve the nutrient content, quality and flavor, and promote value of fish products.