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/yr, respectively. When fish were present, fish density in the two circular tanks under study at Site 1 were 35 and 48 kg/m3, and that in four octagonal tanks at Site 2 were 54, 74, 58 and 64 kg/m3, respectively. The objective of the study was twofold. First, the effect of biomass on the velocity distribution was 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 was studied by collecting and analysing 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, recommended 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, 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 centre, 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 7.4–10.4 mg/L. Estimates of the oxygen respiration rate in the tank appears to double as the TSS concentration measured in the tank increases from 3.0 mg/L (0.3 kg O2/kg feed) up to 10–12 mg/L (0.7 kg O2/kg feed). Improving suspended solids control in such systems may thus dramatically reduce the oxygen consumption and CO2 production.
Highlights
The aquaculture industry shows a strong and growing interest in recirculation aquaculture systems (RAS; Verdegem et al, 2006; Martins et al, 2010; Dalsgaard et al, 2013; Summerfelt et al, 2016)
This paper presents the empirical investigation performed at two commercial scale RAS facilities for salmon smolt production
An empirical study to characterize water rotational velocity and water quality was completed in three large culture tanks at two commercial salmon smolt and post-smolt sites
Summary
The aquaculture industry shows a strong and growing interest in recirculation aquaculture systems (RAS; Verdegem et al, 2006; Martins et al, 2010; Dalsgaard et al, 2013; Summerfelt et al, 2016). Such facilities have only minimal direct hydraulic connection with the environment, and RAS can capture over 98% of the fish waste solids (Chen et al, 2002; Summerfelt and Vinci, 2008). Optimizing the culture tank environment is essential for achieving fish culture success, because much of a fish farm’s fixed capital costs – such as culture vessels, building foot-print, fish feeders, oxygen probes, flow or level switch – and variable costs – such as fish, fish feed, labour, disease treatments, water quality monitoring expendables, water pumping, and water treatment requirements – are invested in the culture tank (Timmons et al, 1998; Davidson and Summerfelt, 2004)
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