Water velocity in commercial RAS culture tanks for Atlantic salmon smolt production

Authors: GORLE, JOAGAN - Nofima; TERJESEN, BENDIK - Nofima; MOTA, VASCO - Nofima; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2018
Publication Date: 3/26/2018
Citation: Gorle, J.M., Terjesen, B.F., Mota, V.C., Summerfelt, S.T. 2018. Water velocity in commercial RAS culture tanks for Atlantic salmon smolt production. Aquacultural Engineering. 81:89-100. https://doi.org/10.1016/j.aquaeng.2018.03.001.
DOI: https://doi.org/10.1016/j.aquaeng.2018.03.001

Interpretive Summary: The objective of this study was to empirically describe how water rotational velocities and dissolved oxygen are distributed across large commercial salmon smolt tanks at two commercial RAS facilities for salmon smolt production. Optimizing the culture tank environment is essential for achieving fish culture success, because much of a fish farm’s fixed capital costs and variable costs are invested in the culture tank. Fish were found to be a major source of turbulence, where fish increased mixing and reduced overall water rotational velocities by approximately 25 percent compared to the same tanks operated without fish. When fish were present, the water rotational velocity profile measured across the tanks were quite similar in all tanks sampled and were close to what are considered to be the optimal swimming speed for Atlantic salmon smolts. In addition, estimates of the oxygen respiration rate in the tank appeared to double as the suspended solids concentration measured in the tank more than doubled. Findings from this investigation will allow salmon farmers and aquacultural engineers to take steps to improve the culture tank environment, which can, in the long-term, improve fish performance, welfare and health.

Technical Abstract: An optimal flow domain in culture tanks is vital for fish growth and welfare. This paper presents empirical data on rotational velocity and water quality in circular and octagonal tanks at two large commercial smolt production sites, with an approximate production rate of 1000 and 1300 ton smolt annually. When fish were present, fish density in the two circular tanks under study at Site 1 were 35 and 48' kg per m3, and that in four octagonal tanks at Site 2 were 54, 74, 58 and 64' kg per m3, respectively. The objective of the study is twofold. First, the effect of biomass on the velocity distribution is examined, which was accomplished by repeating the measurements in empty tanks under same flow conditions. Second, the effect of operating conditions on the water quality is studied by collecting and analyzing the water samples at the tank’s inlet and outlet. All tanks exhibited a relatively uniform water velocity field in the vertical water column at each radial location sampled. When fish were present, maximum (40 'cm/s) and minimum (25–26' cm/s) water rotational velocities were quite similar in all tanks sampled, and close to optimum swimming speeds recommend for Atlantic salmon-smolt, i.e., 1–1.5 body lengths per second. The fish were found to decrease water velocity by 25% compared to the tank operated without fish. Flow pattern was largely affected by the presence of fish when compared to the empty tanks. Inference reveals that the fish swimming in the tanks is a major source of turbulence, and nonlinearity. Facility operators and culture tank designers were able to optimize flow inlet conditions to achieve appropriate tank rotational velocities despite a wide range of culture tank sizes, HRT’s, and outlet structure locations. In addition, the dissolved oxygen profile was also collected along the diametrical plane through the octagonal tank’s center, which exhibits a close correlation between the velocity and oxygen measurements. All tanks were operated under rather intensive conditions with an oxygen demand across the tank (inlet minus outlet) of 7.4 to 10.4 ppm. Estimates of the oxygen respiration rate in the tank appears to double as the TSS concentration measured in the tank increases from 3.0' ppm (0.3 'kg O2 per kg feed) up to 10–12' ppm (0.7 kg O2 per kg feed). Improving suspended solids control in such systems may thus dramatically reduce the oxygen consumption and CO2 production.