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ABSTRACT
The effects of juvenile Atlantic salmon (Salmo salar) on flow and turbulence in a circular tank were investigated. Three fish sizes were studied (37.5 g, 82.5 g and 218 g) with between 268,050 and 318,000 fish in a 15 m diameter by 4 m deep tank (mean stocking densities of 15.3 kg m–3, 35.6 kg m–3, and 79.4 kg m–3). Flow in the enclosed tank was driven by the inflow from the water supply system and a degassing system. Velocities were measured using acoustic Doppler velocimetry with and without fish present. Dissolved oxygen was also measured, and the turbulent transport of dissolved oxygen calculated from eddy correlation. The average water velocity was reduced by 15% at low and medium stocking densities, and 57% at high stocking density. Turbulent kinetic energy, turbulence intensity, and turbulence dissipation rates were higher with fish than without. Fish altered the distributions of mean velocity, turbulence and oxygen, and increased the turbulent transport of oxygen. Vertical distributions of turbulence were consistent with echo-sounder derived fish distributions.
Acknowledgements
The authors thank Marine Harvest AS and the staff of their facility in Slørdal, Norway for permitting access and providing assistance during the experiments. Thanks also to four reviewers and the Associate Editor for their comments and suggestions. This work was funded by the Norwegian Research Council through the research project Exposed Salmon Farming in High Current and Waves.
Notation
| A | = | = wetted surface area of tank (m2) |
| Af | = | = representative area of fish (m2) |
| Cd | = | = wall friction coefficient (–) |
| Cf | = | = drag coefficient of fish (–) |
| Ct | = | = tank resistance coefficient (–) |
| Ff | = | = drag force from fish per unit volume (N m–3) |
| Fi | = | = inlet impulse (N) |
| f | = | = frequency (s–1) |
| = | = drag force on a single fish (N) | |
| H | = | = depth of tank at sidewall (m) |
| h | = | = local depth of tank (m) |
| I | = | = turbulence intensity (–) |
| K | = | = thermal diffusivity of oxygen (m2 s–1) |
| k | = | = wave number (m–1) |
| M | = | = average fish mass (kg) |
| N | = | = total number of fish in the tank (–) |
| n | = | = fish concentration (number m–3) |
| n0 | = | = average fish concentration (number m–3) |
| O | = | = oxygen concentration (g m–3) |
| = | = turbulent fluctuation of oxygen concentration (g m–3) | |
| Qin | = | = volumetric inflow (m3 s–1) |
| Qdg | = | = volumetric flow from degasser (m3 s–1) |
| R | = | = tank radius (m) |
| R1 | = | = distance from centre of tank to inlet (m) |
| R2 | = | = distance from centre of tank to degasser return (m) |
| r | = | = radial distance from tank centre (m) |
| S | = | = velocity shear (s–1) |
| Su(f) | = | = frequency spectral density for the tangential velocity component (m2 s–1) |
| Sw(f) | = | = frequency spectral density for the vertical velocity component (m2 s–1) |
| Su(k) | = | = wave number spectral density (m3 s–2) |
| Swall | = | = intensity of wall reflection (dB) |
| S0 | = | = echosounder intensity (dB) |
| TKE | = | = turbulent kinetic energy (m2 s–2) |
| Tb | = | = torque from bottom friction (N m) |
| Ti | = | = torque resulting from inflow (N m) |
| Tw | = | = torque from sidewall friction (N m) |
| U | = | = local mean tangential velocity component (m s–1) |
| = | = instantaneous velocity fluctuations (m s–1) | |
| Uw | = | = velocity at sidewall (m s–1) |
| V | = | = local mean radial velocity, positive outwards (m s–1) |
| Vin | = | = inlet water velocity (m s–1) |
| Vavg | = | = average velocity in tank (m s–1) |
| Vdg | = | = degasser return velocity (m s–1) |
| V1 | = | = velocity in tank at inlet (m s–1) |
| V2 | = | = velocity in tank at degasser (m s–1) |
| W | = | = local mean vertical velocity component, positive upwards (m s–1) |
| = | = turbulent dissipation rate (m2 s–3) | |
| εf | = | = rate that work is done against flow due to fish drag (m2 s–3) |
| η | = | = Kolmogorov scale (m) |
| ρ | = | = water density (kg m–3) |
| ν | = | = kinematic viscosity (m2 s–1) |
| μt | = | = turbulent eddy viscosity (m2 s–1) |
| = | = volume of tank (m3) |