Effects of high-temperature stress on the histopathology of gills and liver tissues and serum biochemical indicators of Hucho taimen

Temperature fluctuations caused by climate change and global warming pose significant threats to various species. Understanding the mechanisms of fish tolerance to high temperatures can help develop effective strategies to cope with climate changes in aquaculture. Hucho taimen is an important economic cold-water salmonid. This study designed a temperature acclimation protocol that included gradual warming from an optimal 18 °C to 26 °C, maintaining 26 °C for 7 days, and then reducing the temperature back to 18 °C. The impact of high temperature on taimen was compared and analyzed through respiratory rate, histopathology, and serum biochemical indicators. The results demonstrated that during the gradual temperature increase from the optimal 18 °C to 26 °C, the respiratory rate of taimen accelerated with rising temperature. Histopathological changes in gills and liver tissues at the cellular level progressively worsened, including hypertrophy and rupture of gill epithelial cells, as well as hepatocellular hypertrophy, vacuolization, and necrosis. At 26 °C, the pathological grad reached a severe level (with lesions comprising over 61% of the tissue). Serum levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) activities sharply increased with the rising temperature. Alkaline phosphatase (ALP) activity remained unchanged at 22 °C but significantly decreased at 26 °C. The total cholesterol (TC) content gradually decreased, while the total protein (TP) content initially decreased and then recovered. During the 7-day period of high temperature at 26 °C, the respiratory rate of taimen remained consistently higher than that of control group. Histopathological deterioration of in the gills and liver tissues further intensified, including epithelial necrosis, vascular occlusion, and secondary lamellar fusion in the gills, as well as hepatic congestion, lipid infiltration, and inflammatory cell infiltration. The histopathological grad peaked at 105 h, followed by a gradual decrease in lesions. Serum levels of AST, ALT, and LDH activities initially decreased sharply and then increased, while ALP activity at first increased and then normalized. TC content slowly recovered, and TP content initially decreased and then returned to normal. After returning to 18 °C from 26 °C, the respiratory rate returned to normal levels, gill tissues recovered completely, and liver tissues had partial recovery, though there was moderate lipid infiltration and inflammatory cell infiltration (lesions comprising 31%-60% of the tissue). Serum levels of AST, ALT, and LDH activities decreased but remained significantly higher than that of the control group, while ALP activity showed a sharp increase. TC and TP contents restored to normal levels. The study found that the respiratory rate of taimen increased with rising temperatures, and high-temperature stress caused the respiratory rate to remain elevated above normal levels. This indicates an increased oxygen demand and metabolic rate under high-temperature stress. High-temperature stress induced various types and degrees of lesions in the gill and liver tissues of taimen. After the temperature returned to normal, the gill tissues recovered, while the liver tissues showed partial recovery but remained in a significantly pathological state. High-temperature stress affected the liver functions related to metabolism, synthesis, and storage, with a more significant impact on metabolic capacity. The combined analysis of respiratory rate, pathology, and biochemical indicators suggests that taimen has some adaptive capacity to high-temperature stress. These findings of this research provide a theoretical foundation for the physiological regulatory mechanisms of temperature tolerance in taimen, as well as for the efficient aquaculture and selection of high-temperature resistant strains.