One farmer in Petchaburi province polycultured milkfish with shrimp, with the
purpose of using milkfish in improving water quality in shrimp ponds and in cleaning up
waste from the pond bottom. However, Baylon (1996) reported that polyculture of shrimp
with milkfish resulted to significantly higher phytoplankton content compared to
monoculture, but no significant differences in biological oxygen demand, total suspended
solids, phosphate, nitrate and ammonia contents of pond water between polyculture and
monoculture. Baylon (1996) also undertook a crop rotation experiment to determine the
effect on sediment quality of shrimp ponds by culturing milkfish immediately after shrimp
harvest. The results showed that sediment sampled from the ponds where milkfish had been
cultured had shown a decrease in organic matter for three ponds only, while the phosphorus
content decreased in only two ponds. Tian et al. (2001a) reported that most varieties of
tilapia are omnivorous in feeding habit and are filter feeders in the main, thus they are
capable of improving water quality in ponds (Diana et al., 1991; Ruan et al., 1992, 1993;
Zhang et al., 1999).