Intensive Tank Culture of Tilapia in the UVI Biofloc System

 James E. Rakocy, Donald S. Bailey, R. Charlie Shultz, Jason J. Danaher and Eric. S. Thoman

University of the Virgin Islands Agricultural Experiment Station

RR 2, Box 10,000

Kingshill, VI  00850, USA

ABSTRACT

A 200-m3 circular tank was evaluated in production trials stocked with sex-reversed Nile tilapia (Oreochromis niloticus) at 20 fish/m3 in Trial 1 and 25 fish/m3 in Trials 2 and 3. Water treatment methods consisted of aeration, water circulation (mixing), solids removal, nitrification and denitrification. The fish were fed ad libitum twice a day with a complete (32% protein), floating pellet. After 175,  201 and 182 days of growth, total production was 14.4, 13.7 and 15.3 kg/m3 in Trials 1, 2 and 3, respectively.  Ammonia and nitrite concentrations were generally acceptable for tilapia growth.  The nitrate-nitrogen concentration increased throughout the first two trials and reached 654 and 707 mg/L in Trials 1 and 2, respectively, which indicated a high rate of nitrification and the need for a denitrification treatment process.  Two external denitrification tanks were established and used in Trial 3, resulting in a lower peak nitrate-nitrogen concentration (341 mg/L).  Total suspended solids (TSS) increased throughout the first two trials and reached peaks values of 1,300 and 1,960 mg/L in Trials 1 and 2, respectively. The horizontal water velocity was too high for effective sedimentation of suspended solids for removal by a cone situated in the center of the tank.  The addition of an external clarifier to the system for the last 3 weeks of Trial 2 removed 360 kg of dry weight solids, resulting in the reduction of TSS levels from to 1,700 to 600 mg/L.  The reduction of TSS improved other water quality parameters and fish feeding response.  The external clarifier was used throughout Trial 3 and TSS reached a peak value of only 540 mg/L. For optimal performance the UVI biofloc system requires an external clarifier and denitrification unit.