Effects of a Shielding Skirt for Prevention of Sea Lice on the Flow Past Stocked Salmon Fish Cages

Abstract

The effect of a shielding skirt, a tarpaulin mounted from the surface down to 5 m depth around a net cage, on the flow pattern at a commercially stocked salmon cage was inves tigated. Dye was used as a tracer for water movement and the dye spreading was moni tored using aerial images. Current meters were employed to investigate the flow close to the net inside and outside the cage. Tests were conducted with and without the shielding skirt. The focus was on the effectiveness of the shielding skirt to deflect water around the cage. This study shows that a shielding skirt can reduce horizontal flow components significantly inside a cage, which is related to a reduction of water exchange. The flow toward a cage is divided by a shielding skirt, i.e., some of the water is transported around the cage, while some is passing underneath the shielding skirt. Some water entering the fish cage from underneath the tarpaulin is transported toward the surface inside the cage. The use of a shielding skirt might not prevent interaction of the upper water layers inside and outside of a fish cage completely, but it has the potential to reduce the inflow o f surface water into the cage, if deployed properly.

Behaviour in the toolbox to outsmart parasites and improve fish welfare in aquaculture

“…Methods to prevent louse infestations that include additions or modifications to the sea cage (Fig. 2), such as plankton skirts (Lien et al 2014;Frank et al 2015), snorkels , fully submerged cages ), submerged feeding and lights (Frenzl et al 2014) or moving cages offshore (Fredheim & Langan 2009), have been thoroughly investigated in relation to efficacy and fish welfare issues and are being used throughout the industry in Norway. More recently, novel methods to reduce infestation are being employed but largely lack sufficient quantitative evidence of their efficacy.…”

Sea lice infestation levels decrease with deeper ‘snorkel’ barriers in Atlantic salmon sea‐cages

“…21,22 By exploiting this aspect of their behaviour, a possible solution is to prevent or limit the contact of sea-caged salmon with surface waters. Mismatching host-parasite encounters is the principle behind most depth-based lice-barrier technologies such as fully enclosed floating tanks where the water is pumped from the deep, 13 cages that incorporate plankton-shielding skirts, 23,24 or lice 'snorkel' barriers 11 that stop encounters in surface waters, and fully submerged cages. 25 Snorkel lice-barrier technology is a modification of a standard sea-cage where salmon are held deep via a net-roof but allowed surface access to refill their open swim bladders with air through a tube or snorkel.…”

Dynamics of dissolved oxygen inside salmon seacages with lice shielding skirts at two hydrographically different sites

“…Computational fluid dynamics (CFD) analyses of laminar current flow into a fish cage with a rigid shielding skirt show that part of the incoming current is pressed around the skirt, but a portion is pressed down along the skirt, underneath it and into the seacage (Lien & Høy 2011. The deflection of the current underneath the shielding skirt and into the fish cage was observed in full-scale dye experiments (Frank et al 2015). However, when repeating the dye experiment with similar ambient current conditions, the amount of dye entering the cage varied significantly (Frank et al 2015).…”

“…One reason for these variations could be differences in the water exchange due to local flow patterns. Farm layout (Rasmussen et al 2015), local topography (Klebert et al 2013), cage structure (Klebert et al 2015), bio-fouling (Gansel et al 2015), presence of fish (Klebert et al 2013, Gansel et al 2014, Klebert & Su 2020 and structures such as shielding skirts (Frank et al 2015) all influence how the ocean currents flow through and around fish cages. Computational fluid dynamics (CFD) analyses of laminar current flow into a fish cage with a rigid shielding skirt show that part of the incoming current is pressed around the skirt, but a portion is pressed down along the skirt, underneath it and into the seacage (Lien & Høy 2011.…”