Objective Trachidermus fasciatus is a nation's second-class protected species, belonging to a migratory fish with eurysalinity. The study aims to explore a suitable desalination pattern for T. fasciatus before the proliferation and release of the fish.

Method The microstructural changes of the gills of juvenile T. fasciatus under acute freshwater stress, different desalination speeds (the salinity was reduced by 5 concentrations each time, and reduced every 12, 24, 48 h, recorded as A, B, C groups) and different desalination amplitudes (the salinity was reduced once every 24 h, and 2, 3 and 5 salinity concentrations were reduced each time, recorded as D, E and B groups) were investigated by tissue sectioning techniques.

Result The results showed that, when the juvenile T. fasciatus at salinity of 25 was directly put into freshwater for acute freshwater stress, the morphology of gill tissue structure was normal at 1 h; at 12 h, the gill filaments showed a slight epidermal detachment; at 24 h, the hemocytes appeared to be absorbed and swollen; at 3 d, the number of gill filaments and gill lamellae with vacuoles increased significantly; at 7 d, the gill filaments and gill lamellae showed the most serious epidermal detachment, and the length of gill lamellae was increased while the width was decreased, with bent ends, cytolysis and irregular shape. Vacuoles appeared at the base of the branchial lamella and their positions also increased. The number of blood cells decreased and the number of mitochondria-rich cells increased. At salinity of 25, the gill tissues of group A-C did not change significantly. When salinity was reduced to 15, the structural breakage of gill filaments was more obvious in group A than that in groups B and C; when salinity reached freshwater, the gill tissue structure of group A was severely broken, the length of gill lamellae was increased but the width was reduced, the structure was incomplete, the number of mitochondria-rich cells was increased, and the number of hemocytes was reduced compared with that at salinity of 15; the structures of gill filaments and gill lamellae of group A were significantly different than those of groups B and C, with the degree of incompleteness being more serious, and the difference between groups B and C was not obvious. The differences between groups B and C were not obvious, and the gill tissue structure was broken to a similar extent. When the initial salinity was 25 under freshwater acclimation for 1 h, the differences in gill tissue structure were not obvious among group D, group E and group B; at 12 h, the number of vacuoles in gill tissues of the three groups increased; at 24 h, the number of vacuoles in the gills of group B and group D was more than that of group E; at 3 d, the gill tissues of group B had more detachments compared with the other two groups; at 7 d, the number of mitochondria-rich cells and blood cells was more than that of groups B and D. The degree of incompleteness of gill tissue structure was more serious in group B than that in groups D and E.

Conclusion With the increase of freshwater acclimation time, T. fasciatus improves its osmotic capacity by increasing the length of gill lamellae and controlling the number of hemocytes and mitochondria-rich cells to reduce the harm of stress on gills. In addition, lowering the desalination rate and amplitude will help T. fasciatus to better adapt to the salinity change, and it is recommended to choose the desalination pattern with the salinity decreasing by 2 per day to reduce the damage of freshwater acclimation on the gill tissues and structures of T. fasciatus.