The grant funding provided for the construction of fish tanks for the production of tilapia in bacterial-based culture systems. Yields of 13 – 15 kg per cubic meter were obtained. This is 20 times greater than yields typically produced in pond culture. The increase in yield is attributed to the maintenance of an active culture of suspended nitrifying bacteria in the fish tank that removes ammonia waste through oxidation to nitrite-nitrogen and nitrate-nitrogen. Solids are removed by settling. Electrical use is higher in this system than in pond culture but not prohibitive. Over 4,000 market size tilapia of 700 g each were obtained in the final trial.
One farmer constructed a fish tank near his vegetable garden site and will be able to use water from the fish tank for irrigation of the crops produced. Land ownership, zoning and environmental permitting prevented the construction of a second fish tank at another farm site.
Three tilapia production trials were conducted in the greenwater tank constructed on the UVI campus. In the first trial 4,000 tilapia fingerlings were stocked (20 per cubic meter) and harvest yields were 14.4 kg/cubic meter. Survival was low (78.9%) due to bird predation of fingerlings after stocking. Pelicans were observed landing on the water and catching fish, Night herons and snowy egrets were observed perching on the tank edge and also catching fish. A bird deterrent of a low voltage electric wire was installed.
In the second trial 5,000 fingerlings were stocked (25 per cubic meter). Suspended solids accumulated in the culture water and nitrate-nitrogen levels increased throughout the trial to levels that impeded tilapia growth. The central cone built in the tank bottom was ineffective in removing solid waste, possibably due to fish swimming into the cone and resuspending solids. Nitrate-nitrogen increased related to feed consumption and metabolic waste generated. For the final 3 weeks of the trial an external cone bottom clarifier was installed adjacent to the tank and culture water was pumped through it for solids removal. Suspended solids dropped from xx mg/l to yy mg/l over that period. Final yield for the tank was 13.7 kg/cubic meter. Survival was 81% and FCR was 1.8 kg feed per 1.0 kg gain.
In the third trial, 2 adjacent raceway tanks were constructed adjacent to the fish culture tank and were tested for their ability to remove nitrate-nitrogen through anearobic denitrification. Yield was 15.3 kg per cubic meter, survival was 86% and FCR was 1.84. Nitrate levels increased throughout the trial and reached a maximum of 341.27 mg/L.
A fish culture tank was constructed on a Virgin Island farm.