Investigations were undertaken related to inlet configuration and flow rate on efficiency of solids removal, maintenance of water quality, and production and welfare indicators in farmed Rainbow trout (Oncorhynchus mykiss) in Recirculating Aquaculture Systems (RAS). Nozzle-bar inlet placement benefits waste removal performance and best homogenises water quality. Juvenile trout were not negatively affected by higher flowrates regarding either growth performance or welfare, but this is an interesting field of further investigation. Higher flowrates are considered to have better system efficiency in RAS in terms of waste removal and tank reoxygenation rate, further mitigating O2 stress of fish during growth.
Two inlet configurations (nozzle bar and elbow pipe), and three flowrates (600L/h, 800L/h (normal for trout juveniles), and 1000L/h). In unstocked tanks, feed pellets were distributed as per feeding practice and movement recorded on overhead video cameras. Primary and secondary flow, pellet distribution in the tank, settling location at the tank bottom and number of pellets entering the central drain were assessed. Higher flowrate displayed stronger primary flow in both inlet configurations. Primary flow in the elbow configuration decreased with increasing depth regardless of flow rate. Higher flowrate created stronger secondary flow patterns and the nozzle bar configuration produced higher secondary flow than the elbow pipe, thus, pellets settled earlier than in elbow configured tanks with a nozzle inlet bar. Use of the nozzle bar inlet configuration with higher flowrate showed the most optimal option for waste removal of uneaten feed particles.
Efficiency of the six treatments for reoxygenation of the system was tested in unstocked tanks by using an oxygen scavenger (Na2SO3) to deplete the isolated tanks oxygen level to ~0% and then restoring water flow to assess time needed for each configuration to restore to 100% saturation. At lower flow rates (600L/h) there was a lower reoxygenation rate using the nozzle bar compared with the elbow pipe, but at higher flow rates the two configurations did not vary and at 1000L/h could achieve reoxygenation rates of 1.9-3%/min regardless of configuration.
In replicate tanks stocked with 9±0.35g rainbow trout at optimal production conditions (16.5?, 1-2ppt salinity, > 6.5 mg/l DO, automated O2 injection) nozzle bar configuration at flow rates (600L/h, 800L/h, 1000L/h), variation in DO and CO2 level were recorded during 1 hour over feeding (2.5-3% body weight, performed by hand 3 times per day at 9.00, 12.00 and 15.00). DO consumption of each treatment during feeding increased as the fish grew and stocking density increased, but no difference was measured within the flow rate treatments in any of the sampling weeks. CO2 generation changed as fish size, and thus stocking density, increased but no difference was measured among the flow rate treatments in any of the sampling weeks. Growth performance was assessed for fish in all treatments throughout the experimental period including final weight, FCR, SGR, visual indicators of welfare, as well as condition factor and visceral fat content. Flow rate was not found to have any effect on these factors.