Constraints to fish production in buffalo-manured systems supplemented with inorganic fertilizers

This study was conducted to investigate nutrient limitation, light limitation and poor water quality as possible factors constraining primary productivity and fish production in a buffalo manured system supplemented with inorganic fertilizers. A buffalo manure nutrient release experiment showed that about 90 % of the initial manure total Kjeldahl nitrogen (TKN) of 17 mg N/g dry matter (DM) and 35 % of initial manure total phosphorus (TP) of 1.3 mg P/g DM were released as soluble nitrogen and soluble reactive phosphorus, after 3 and 4 days of manure application, respectively. A 51-day monosex Nile tilapia (Oreochromis niloticus) grow-out experiment was carried out in 18 tanks. Each tank received six levels of buffalo manure (0, 12.5, 25, 50, 100 and 200 kg DM/ha/day) with supplemental urea and triple super phosphate, added to all treatments to give a total nitrogen input of 3 kg and a phosphorus input of 1.5 kg/ha/day, assuming 90 % of manure TKN and 35 % of manure TP were available as N and P from manure, respectively. An experiment to study the degradation characteristics of water colour in the buffalo manured tanks was also conducted, since the typical dark brown colour of water in these systems was suspected to be a major factor inhibiting light penetration. The data suggested that light limitation, low dissolved oxygen and high tannin content were probably all major factors inhibiting primary productivity and fish production, particularly at manure was a poor source of nitrogen, since about 93 % of the soluble nitrogen released from manure with 1.74 % average TKN, was in organic form, the availability of which to phytoplankton is not known. It was also a poor source of phosphorus with an average content of 0.13 %. Therefore, it may be worth applying buffalo manure to fish ponds when carbon is limiting in water, if further investigation proves its value as a good source of carbon. The colour degradation experiment showed that the brown colour of water is degradable and decreased by 37 % within 22 days. Therefore, problems of low light penetration and poor water quality can probably be overcome either by applying the manure at intermediate levels at less frequent intervals or at lower levels at more frequent intervals although these were not investigated.