Operation of a water recirculating greenwater system for the semi-intensive culture of mixed-sex and all-male Nile tilapia, Oreochromis niloticus

Recirculating greenwater aquaculture technology is an appropriate method for producing commercial quantities of tilapia in locations that have water scarcity. Greenwater systems achieve three major goals; higher fish production, maximum water recycling (> 98%) and nutrient utilization. Some nutrients are recycled back in this system to sustain the development of photosynthetic algae and nitrifying bacteria creating a "suspended growth treatment process" that maintains adequate water quality. In this study, the greenwater system was implemented in triplicate, each replicate had a fiberglass fish culture tank and a cone bottomed clarifier for solid filtration. Water from the fish culture tanks is pumped to the cone clarifier through the airlift and the outflows of the cone clarifiers trickled back to the fish culture tanks at a rate of 7-8 L/minute. The study was designed to compare the performance of a greenwater system with mixed-sex, as well as, all-male tilapia. Culture tanks were stocked at a density of 40 fish/m³ with mixed-sex Nile tilapia, Oreochromis niloticus, fingerlings and fed ad libitum with floating pellets containing 34% protein. The water temperature was maintained at 28±1°C, intensive aeration was provided and sludge was collected twice daily from the clarifiers. Fish were harvested after six months and culture tanks were restocked for another six months with hand-sexed all-male Nile tilapia fingerlings at a density of 40 fish/m³. Values of critical chemical parameters in the water were found to be well within the acceptable limits based on weekly water sampling. The mean final weight (±SD), final biomass, growth rate, SGR, FCR, and percent survival for the mixed-sex tilapia was found to be 287.09 (94.63) g/fish, 11.25 kg/m³, 1.43 g/fish/day, 1.27%, 1.77%, and 97.92%, respectively. Values for the mean final weight (±SD), final biomass, growth rate, SGR, FCR, and percent survival for all-male tilapia populations were 359.67 (80.69) g/fish, 14.63 kg/m³, 1.75 g/fish/day, 1.29%, 1.64%, and to 98.05%, respectively. Contrasting to the semi-intensive aquaculture in Saudi Arabia where 20-50% of the total water is exchanged daily to produce 8-15 kg fish/m³ of water, 12-15 kg of fish/m³ was produced with addition of only 1% of the total water daily. Only 180 liters of water was consumed to produce one kg of fish in the system.