Background. Luminous vibriosis caused by Vibrio harveyi, is a serious disease problem in shrimp aquaculture. The use of "green water", a technique that involves the stocking of tilapia, Oreochromis in the reservoir for the production of green microalgae, Chlorella is believed to help in the reduction of the incidence of luminous vibriosis. Though most shrimp farmers have been using Mozambique tilapia, Oreochromis mossambicus for the production of "green water", the mechanism involved in the efficiency of using this fish species in reducing luminous vibriosis remains unknown and is largely based on speculations. It was for this reason that the present experiment was conducted. The aim of the study was to determine the antibacterial activity of "green water" obtained from tank culture of Oreochromis mossambicus against Vibrio harveyi under laboratory conditions. Materials and methods. "Green water" collected from broodstock and juvenile tanks of Oreochromis mossambicus as well as water from a commercial shrimp pond, and filtered boiled seawater were inoculated with Vibrio harveyi. Luminous bacteria were counted in nutrient agar, while presumptive and green colony-forming vibrios were determined on thiosulfate-citrate bile salt (TCBS). The experiment was carried out for one week. Results. Luminous bacteria were not detected, and the presumptive and the green colony-forming vibrios were generally lowest in "green water" obtained from the broodstock tank. Luminous bacteria in "green water" obtained from juvenile tank were not detected starting day 5, indicating delayed inhibition. Conclusion. The present findings show that "green water" from broodstock tank of O. mossambicus has the ability to inhibit luminous vibriosis over a one-week period, and that broodstock are a better source of "green water" than juveniles in controlling luminous vibriosis.

green water, Vibrio harveyi, luminous vibriosis, shrimp, tilapia, fish