Abstract
Shielding skirts are widely used on Atlantic Salmon sea-cages as a non-invasive preventive measure against salmon lice infestations. The skirts are however known to impact the current flow and thereby the environment within the cage. As the current is influenced by local factors such as topography, farm layout and stocking density of the cage, it is difficult to compare results from sites that apply skirts with those without. The same high-stocked cage was therefore studied with and without the skirt deployed, including the transition from shielded to unshielded, to investigate the influence the skirt had on the current flow within the cage and dissolved oxygen. When the skirt was deployed the velocity vector in the centre of the cage had a vertical component towards the surface and the reduction in current speed was higher. The dissolved oxygen level inside the cage improved within 30 minutes when the skirt was removed and there was no indication of the skirt influencing the vertical swimming behaviour of the salmon.
Highlights
In aquaculture sea-cages a sufficient water exchange is necessary to ensure a healthy environment by supplying dissolved oxygen (DO) and removing waste and nutrient-depleted water
As evidence indicates a higher lice density in the upper layers of the water column (Geitung et al, 2019; Heuch et al, 1995; Hevrøy et al, 2003; Huse and Holm, 1993; Oppedal et al, 2017), lice shielding skirts are designed to reroute this layer of the water column around the fish cage, and thereby keep the lice out by altering the current flow
The magnitude of current speed reduction as it passes through the cage is determined by the current flow pattern which is influenced by a number of factors such as farm layout (Rasmussen et al, 2015), local flow conditions at the site, local topography (Klebert et al, 2013), shielding skirts (Frank et al, 2015) and the cage structure (Kle bert et al, 2015)
Summary
In aquaculture sea-cages a sufficient water exchange is necessary to ensure a healthy environment by supplying dissolved oxygen (DO) and removing waste and nutrient-depleted water. For empty cages with a shielding skirt, CFD analysis indicate that part of the ocean current is forced underneath the skirt and into the sea cage producing a recirculation pattern, where it meets the skirt in the back and is pressed up and inwards towards the centre of the cage (Lien and Høy, 2011; Lien et al, 2015) This recirculation pattern is seen in full-scale cages with skirt when the cage is empty, but not when stocked (Klebert and Su, 2020). Most cages used in Norway are gravity nets, which deform as a function of the current speed (Lader et al, 2008) This deformation can alter the inclination angle and increase the reduction in flow velocity. The shielding skirt can deform, and in strong currents the skirt will increasingly be pushed back and up towards the surface, resulting in potentially less obstruction for the current and lice (Lien et al, 2014)
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