Abstract:
Urea, a nitrogen-containing chemical product, has been widely used in aquaculture due to its relatively cheap source of nitrogen and being an important source of nitrogen for microorganisms in the ecosystem of aquaculture pond. However, a knowledge of using urea in the shrimp closed culture system has not been reported. In this study, four sets of experiments were performed to determine the effects of using urea on black tiger shrimp (Penaeus monodon) when reared in closed system. The first experiment was to clarify a safety concentration of urea to post-larva 15. Six urea concentrations of 0, 5,000, 10,000, 15,000, 20,000 and 25,000 ppm were tested. The results showed that 50% lethal concentration (LC50) of urea was 20,806 ppm and the safety concentration was 2,081 ppm for post-larva 15 of P. monodon. The second experiment was to determine the optimal concentration of urea on the changing of water qualities and growth of the shrimps. Shrimps of an average body weight 9.44 + 0.17 g were reared at a density of 10 individuals per tank of 70 liters at a salinity of 20 ppt for six weeks. Urea was initially added at the concentrations of 0, 0.625, 1.25 and 2.50 ppm. It was found that using of urea at a concentration of 1.25 ppm was considered to be the best concentration according to the best growth rate of the shrimps and low nitrite concentration of the rearing water. The third experiment was to determine the optimal frequency of using urea in shrimp culture. The experiment consisted of three treatments: control (without urea add), single adding of urea at a concentration of 1.25 ppm (at the initial of experiment) and adding of urea at a concentration of 1.25 ppm weekly. The experiment was carried out for six weeks. The results showed that the continuous adding of urea at a concentration of 1.25 ppm weekly enhanced shrimp and the population of ammonia oxidizing bacteria (AOB) growths with no significant effect on water qualities. The last experiment was to determine growth of shrimp reared in tanks providing with soil substrate. There were two treatments and four replications including treatments of control (no urea added) and adding of urea at a concentration of 1.25 ppm weekly. The experiment was carried out for eight weeks. The results showed that the continuous adding of urea at a concentration of 1.25 ppm gave the better growth of shrimp under the condition of having soil substrate and without a significant effect on water qualities.
This study concluded that urea was a suitable source of nitrogen in order to promote growth of bacterial and phytoplankton populations and consequently enhancing growth of shrimp in the closed culture system. It suggested to add urea at a concentration of 1.25 ppm weekly into the closed culture system.
WALAILAK UNIVERSITY. CENTER FOR LIBRARY RESOURCES AND EDUCATIONAL MEDIA