Although the latter two products are largely used for fertilization, their potential use as feed ingredients is high and is currently receiving considerable research attention. It should be mentioned that plant residues until now have had little use as fertilizers for aquaculture, in contrast to their application in agriculture. This is because of the fundamental physico-chemical differences between water and soil as cultivation media. However, some fish ponds are occasionally or regularly fallowed. The use of organic residues in aquaculture is, to a certain extent, dependent on competition for these residues by agricultural production systems.
Manures produced by fish in a polyculture pond immediately enter a detritus food chain.
A portion of this detritus is recycled into higher trophic levels, in this case, table fish. In the latter, manure must be collected and distributed with an attached expenditure of energy and human labour. Sewage utilization in aquaculture is conditioned by cultural, sanitary, and economic constraints. Disease and parasite transmission, while still a consideration, is often over-rated. Adequate pond management, as discussed earlier, and careful cooking can overcome most of these potential problems.