Abstract: Levels of Polycyclic aromatic hydrocarbons (PAHs) metabolites in biles of Nile tilapia (Oreochromis niloticus) from aquafarms around the Tai Lake and of crucian carp (Carassius auratus) from aquafarms around the  Hongze Lake were determined with the fixed wavelength fluorescence method (FF method) to explore for appropriate dilution rate and optimum sample size to optimize this determination method through inner filter effect and variation analysis, and further to perform source apportionment of the PAHs. Results show no significant correlation between fluorescence ratio of the PAHs in biles of the crucian carp and Nile tilapia and biliverdin, indicating that there was no inner filter effect existing in bile; however, the fluorescence ratios of various PAHs all displayed obvious rising trends with increasing dilution rate and the variation between the various PAHs also intensified. Intensified variation would lead to lower effect value and bigger sample size. It is, therefore, proper to control the dilution rate at around 1:1000, and correspondingly, the optimum sample size of crucian carp and Nile tilapia would be 6-7 and 27-62, respectively. The PAHs pollutants in the water body around the aquafarms in the two lake regions are all petroleum-sourced, which is possibly the result of fuel leakage of vessels sailing in the lower reaches of the Yangtze River and of the waste discharge from the developing industry in the regions. The findings of this research may provide some reference for rational use of FF method in bio-testing PAHs.

Key words: risk of biodiversity, risk assessment, biodiversity management