Field and laboratory evaluation of the effects of C:N manipulation on culture of Penaeus monodon shrimp with zero and limited water exchange model

摘要

One of the main obstacles to development of intensive aquaculture is the accumulation of toxic inorganic nitrogen which should be kept very low by frequent water exchange or recycling of the water through a biofilter. This thesis describes another method of removing inorganic nitrogen pioneered by Avnimelech et al. (1988;1989;1992a;1994) and Avnimelech (1999) using heterotrophic bacteria population of which was augmented by the addition of a carbonaceous substance, molasses, to increase the feed C:N ratio under both field and laboratory conditionsThe principal aims of this field and laboratory studies were (1) to investigate the water quality and production variables in shrimp culture of zero water exchange model (ZWEM) and limited water exchange model (LWEM) using molasses, (2) to establish correlation C:N ratio level with levels of ammonia, nitrite, dissolved oxygen, pH and shrimp growth in shrimp farming of ZWEM and L WEM. The study was also designed to experimentally evaluate (1) effect of molasses application frequency on water quality and production variables in shrimp culture of ZWEM, (2) effect of stocking density on water quality and production variables in shrimp culture of ZWEM using molasses as a carbon source and (3) effect of reducing conventional feeding rate on water quality and production variables in shrimp culture of ZWEM using molasses as a carbon resource.It was found that:1. Addition of molasses to shrimp farming with ZWEM and ZWEM tended to have role in removing ammonia and nitrite. Also, application of molasses to laboratory tanks increased the population of heterotrophic bacteria and concentrations of total suspended solids, but led to a reduction in levels of ammonia, nitrite and dissolved oxygen, pH and chlorophyll a.2. Treatment with molasses and limited water exchange (5 % weekly) was more effectives in the removal of inorganic nitrogen those that of molasses and zero water exchange.3. Laboratory experiments showed that molasses added to give a C:N ratio level of 20.0:1 (in shrimp culture of zero water exchange model); 17.5:1 (in limited water exchange model ; 5 % fortnightly) and 15.0:1 (in limited water exchange model; 5% weekly) was the most effective in terms of in the removal of inorganic nitrogen and shrimp production variables.4. Frequency of molasses application once a day with C:N ratio level of 20.0:1 was the best in shrimp culture with ZWEM5. Laboratory studies have shown that ZWEM with the addition of molasses to give C: N ratio level of 20.0:1 could only work when stocking density was 30 shrimp m-1.The results pose many questions for future investigations.