Abstract
The characteristic current flow field around a 55 m deep full-scale stocked conical Atlantic salmon sea-cage equipped with a 10 m permeable skirt was studied experimentally using acoustic Doppler velocimeters and profilers. The weakest current speed was inside the cage at 6 m depth and the highest reduction downstream was recorded behind the shielded volume. Downstream of the cage the reduction in speed became little to non-existing at 22 m depth, probably due to the decreasing diameter of the cage with depth. To reduction in current speed through the cage was compared with estimated reduction from theoretical expressions. The results compared reasonably well downstream of the shielded cage, while the reduction inside the cage was higher than the estimates. The difference in current flow field behind a conical cage compared with a cylindrical cage may have implications for the dispersal of waste, feed pellets and microorganisms from the cage influencing the benthic impact of the farm.
Highlights
The flow field characteristics around and through a sea cage govern the distribution of feed, waste and dissolved oxygen in the cage, and the sedimentation process that occurs under and behind the cage
Velocity spikes were not replaced, the Acoustic Doppler Velocimeters (ADV) data were averaged over 1 min and if more than 50% of the data in a minute had been removed, the entire minute was excluded from further analyses
This is clear for the current that was heading Southwards, which has an increase in normalized current speed from 9 m depth and deeper. This pattern was not as evident in the downstream current heading Northwards, but the highest reduction rates were still within the top 10 m. These results indicate that the permeable skirt enhanced the reduction of current flow downstream of the cage
Summary
The flow field characteristics around and through a sea cage govern the distribution of feed, waste and dissolved oxygen in the cage, and the sedimentation process that occurs under and behind the cage. The reduction in speed increases with solidity which can be due to biofouling (Bi et al, 2013; Gansel et al, 2015), biomass in cage (Klebert and Su, 2020) or increasing inclination angle between the net and vertical direction (Bi et al, 2013; Zhao et al, 2015) This in crease in inclination angle can be caused by the cage deformation, as when exposed to strong currents the cage wall upstream and down stream are deformed, and the bottom net is lifted upwards (Fredheim, 2005; Lader et al, 2008; Lien et al, 2014; Klebert et al, 2015)
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