Can water phosphorus level in recirculating aquaculture systems (RAS) compensate for low dietary phosphorus level in nile tilapia Oreochromis niloticus?

Mineral requirements of fish can be met by minerals originating from the diets as well as from water. The importance of the latter depends on the concentration (e.g., fresh versus marine water). Fresh water fish cultured in recirculating aquaculture systems (RAS) with a low water refreshment rate can be exposed to relatively high water mineral concentrations. Little information is present, if water born minerals in freshwater can partially cover the mineral requirements of fish. Therefore, the current study assessed the impact of water phosphorous concentration in relation to dietary phosphorous content on phosphorous balance, performance and energy balances in Nile tilapia. A 33-day study was conducted with 16 tanks (i.e., experimental unit; 27 fish/tank; initial BW, ~28 g) randomly assigned according to a 2 x 2 factorial design. Factor 1 was the water phosphorous concentration (WP): in one RAS system the WP level was kept below 1mg PO4-P/L while in a second identical RAS system, the WP level was kept at approximately 25mg PO4-P/L by a daily addition of phosphate salt. The second experimental factor was the dietary phosphorous concentration (DP): being either below (5.5 g/kg) or above (13.5 g/kg) the P requirement for Nile tilapia. Both diets were identical in ingredient composition except regarding the amount of monocalciumphosphate added. Fish were fed restrictively (approximately 80% of satiation) and isoenergetic one of the two diets. Performance, body composition and nutrient digestibility were measured. From these data, phosphorous, nitrogen and energy balances were calculated. Except for feed conversion ratio, all measured parameters were unaffected by the interaction effect of WP and DP (P>0.05). The results showed a positive effect of a phosphorus sufficient diet and a high P level in water on growth performance. Despite the equal amount of feed given, growth at the low versus high P diet was reduced by 28% (P