Environmental and energy requirements for different production biomass of Nile tilapia (Oreochromis niloticus) in recirculating aquaculture systems (RAS) in Kenya

Recirculating Aquaculture Systems (RAS) offers a better option to increase aquaculture production with limited land and water resources while minimizing water pollution. The biggest challenges in RAS is to maintain favorable water quality for the fish to thrive. The practice of RAS in Kenya is minimal with the improper matching of RAS components and production densities for the existing few and in most cases, leading to a system failure. This study aimed at evaluating environmental and energy requirements for different production biomass of Nile tilapia in a RAS established in a controlled environment. Both production densities and water flow rates were varied while at the same time, the water quality parameters (dissolved oxygen, ammonia, pH, electrical conductivity and temperature) were monitored. The energy consumed for pumping and aeration was also monitored. Tilapia stocking biomass varied from 2.3 kg/m3 to 10 kg/m3 while flow rate was varied from 2.0 L/min and increased at intervals of 1.0 L/min to the maximum attainable flow rate of 10.0 L/min. Crushed pumice rock packed in a 1000 L tank and equipped with a bell siphon was used as the biofilter. Ammonia removal reduced with increasing flow rate with removal rates of 75% at 2.0–3.0 L/min to 9% at 8.0–10.0 L/min. The water pH increased with increasing flow rate with R2 ranging from 0.4 to 0.9. Electrical conductivity increased with flow rate from 112 mg/L to as high as 209 mg/L. Dissolved oxygen increased with flow rate and ranged from 1.0 mg/L to 7.0 mg/L. Energy consumption increased with flow rate and raged from 0.4 kW h at 2.3 kg/m3 stocking biomass to 2.4 kW h at 10 kg/m3 stocking biomass. High stocking densities and flow rates resulted to lower purification efficiencies and higher power consumptions than low stocking densities and flow rates. In order to maintain good RAS water quality, increased production and profits among farmers using RAS in Kenya, there is need to give proper advice on the right combination of stocking biomass, power, and water flow rate. In addition, similar studies should be carried out for other common fish species such as the African catfish