Abstract: To detect the effects of environmental factors (temperature, pH, ammonia, and nitrite) on the virulence of Megalobrama amblycephala infected with pathogen Aeromonas hydrophila, an experiment was conducted based on the orthogonal design L₁₆(4×2¹³). The water temperature (A) was designed with four levels, 20, 24, 28, and 32 ℃; pH (B), ammonia (C) and nitrite (D) were set with two levels respectively: 6.5 and 8.0 for pH, 0.02 and 0.04 mg·L^-1 for ammonia, and 0.1 and 0.3 mg·L^-1 for nitrite. The virulence strength of M. amblycephala infected with A. hydrophila was determined by the fish survival time. The results showed that the interactive effect of pH and ammonia nitrogen (B×C), as well as temperature (A) significantly impacted the virulence of M. amblycephala infected with A. hydrophila (F>F_0.01), the effect of pH (B) was significant (F>F_0.05), while the effects of ammonia nitrogen (C) and nitrite (D) were not significant. Based on the statistical analysis, the fish survival time y (h) presented a significant linear correlation with environmental factors (temperature, pH and ammonia nitrogen), i.e., y=164.713-6.399A+14.367B-11.914(B×C). According to the equation, M. amblycephala shows weakest virulence to A. hydrophila infection and can survive longest time (149.76 h) when the temperature is 20 ℃, pH=8.0 and ammonia nitrogen 0.02 mg·L^-1. The experimental results provide reference for the prevention and control warning of hemorrhagic disease caused by A. hydrophila in M. amblycephala breeding.

Key words: dissolved organic matter, naphthalene, microbial biomass, soil respiration, subalpine forest.