CHARACTERISTICS AND INFLUENCING FACTORS OF NITROGEN AND PHOSPHORUS EXCRETION RATES OF 10 COMMON FISH SPECIES IN LAKES OF THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER

To investigate the nitrogen and phosphorus excretion rate characteristics of fish in the middle and lower reaches of the Yangtze River and their influencing factors, samples of the nitrogen and phosphorus excretion rates and ecological stoichiometric characteristics of 10 common fish species in the middle and lower reaches of the Yangtze River were collected and analyzed from May to June 2023. Models were employed to analyze the key influencing factors of nitrogen and phosphorus excretion rates. The results showed that the nitrogen excretion rate of common fish species in these lakes ranged from 0.15 to 16.24 mg/h, and the phosphorus excretion rate ranged from 0.01 to 5.73 mg/h. Fish nitrogen and phosphorus excretion rates were significantly positively correlated with body weight (<italic>R</italic>²=0.86, <italic>P&lt;</italic>0.001; <italic>R</italic>²=0.87, <italic>P&lt;</italic>0.001). Larger fish excreted more nitrogen and phosphorus than smaller ones, yet the mass-specific nitrogen and phosphorus excretion rates were higher in small-sized species. There were significant differences in mass-specific nitrogen and phosphorus excretion rates among fish with different trophic guilds (<italic>P&lt;</italic>0.001), and planktivorous fish had relatively high mass-specific nitrogen and phosphorus excretion rates. The fish nitrogen excretion rate was significantly negatively correlated with the body nitrogen content (<italic>R</italic>²=0.10, <italic>P&lt;</italic>0.001), while the phosphorus excretion rate was not correlated with the body phosphorus content. Model results indicated that body size was the key factor in predicting fish nitrogen and phosphorus excretion rates. Although diet and fish body nitrogen and phosphorus elemental content also affected these excretion rates, their prediction in the model was poor. The ecological metabolism theory outperformed the ecological stoichiometry theory in predicting fish nitrogen and phosphorus excretion rates, but the effect of species classification on the model requires consideration. The results of this study are important for understanding the role of fish in the nutrient cycling process in lake ecosystems.