This work describes the immobilisation of ammonia using high feed carbon to nitrogen (C:N) ratios, supplemented with wheat powder, to juvenile Penaeus monodon prawns. The pra\Vns were grown in tanks located in the out door hatchery, Charles Darwin University, under zero-water exchange system. It was found that at all C:N ratios tested (eg, 6.1, 7.5, 15, 22.5) the levels of ammonia were low, ranging from 0.15 to 0.35 mg r1 • However, the levels of nitrite were 73.0, 61.9, 12.6 and 0.7 mg r1 at the end of eight weeks of culture in C:N ratios of 6.1, 7.5, 15 and 22.5, respectively. The results suggest that at low C:N ratios less than 15, the excess ammonia was converted to nitrite by nitrifying bacteria. At C:N ratios greater than 15 all ammonia produced from the prawn excretion and the breakdown of feed was immobilised in the biomass of heterotrophic bacteria, which was formed in response to the high feed C:N ratio. C:N ratio of 22.5 was found to be very effective with virtually no accumulation of ammonia and nitrite. As a result of low concentrations of ammonia and nitrite, growth rates and food conversion ratios of P. monodon were much improved when they were grown in high C:N ratio treatments. In addition to the immobilisation of ammonia, heterotrophic bacteria also provided extra food for the juvenile prawns resulting in high feed conversion ratio. The results agree with those in literature and confirm that carbonaceous substrates can be used to augment the C:N ratios of feed to a level that stimulates the growth of heterotrophic bacteria and immobilise inorganic nitrogen produced during prawn aquaculture. In other words, the hannful nutrients produced can be lowered in situ without having to exchange them with the environment by dilution method. This means that aquaculture activities can become environmental-friendly if this practice is adopted by the industry on a broader scale.
|Date of Award||Jun 2004|
|Supervisor||Jim Luong-Van (Supervisor) & Lindsay B. Hutley (Supervisor)|