Reclamation of wastewater for polyculture of freshwater fish: Fish culture in ponds

A series of eight pilot-scale ponds (Au Tau pond system) were constructed at Au Tau Fisheries Office, Hong Kong Agriculture and Fisheries Department. Water from a polluted river (Kam Tin River) was pre-treated in the first four ponds by providing sedimentation (S1, S2) and aeration (A1, A2). The effluent from A2 was introduced into the latter four ponds (C1-C4) for polyculture of six species of freshwater fish (silver carp, big head, common carp, grass carp, tilapia and black bass) and supplementary fish feed (peanut cake) was provided. All fish (except grass carp) grew to marketable size within one year with a total yearly net yield of 5 t ha^-1 yr^-1. In addition to the supplied fish feed, fish also fed on the natural food developed in the ponds and fish growth (except common carp and tilapia) was found to be significantly (p < 0.05) correlated with the concentrations of organic matter and nutrients in water. Biological oxygen demand, inorganic N and PO₄^3- were responsible for 60-80% variations of fish growth according to multiple regression analysis, which suggested that organic matter and nutrients in ponds affected fish growth through the food web developed in ponds. The N conversion to fish flesh from total N input was not more than 3.5% which was below the range reported in other studies (5-25%). The total carbon contents in the sediments of fish ponds did not change significantly (p > 0.05), but the significant increase of N (p < 0.05) seemed to be derived from the input of fish feed and algal precipitation.