2.3. Methods
2.3.1. Investigation of aquacultural ponds
Basic information of the selected aquacultural ponds was investigated by field research, including the size and condition of ponds,cultivated breeds and feed addition. Water quality in the selected ponds was detected every week from July to August and the average value was adopted. The determination of water quality index including DO, pH, transparency, CODCr, TN, NH4+-N NO3−-N and NO3−-N, were followed by standard method (SEPA, 2002). Pollution intensity in aquacultural ponds was then calculated according to Xu (2003).