Skirt around a salmon sea cage to reduce infestation of salmon lice resulted in low oxygen levels

Salmon fish farmers are seeking methods to reduce infestations of salmon lice. The highest concentrations of salmon lice copepodites are typically found in the upper meters of the water column and preventing this water from passing through the salmon cages has been proposed as a way to reduce salmon lice infestation. One simple implementation proposed by the industry is to place a permanent tarpaulin skirt around the upper part of the sea cages. Currently, there is little evidence that a skirt will block the entrance of surface water to the cages and thereby reduce infestation. However, this pilot study shows that putting a tarpaulin skirt around a full scale commercial sea cage may seriously decrease the oxygen saturation levels available for the fish inside the skirt. Further investigations into the effectiveness of skirts as a means of reducing the exposure of farmed salmon to sea lice must therefore take precautional measures.

► We set out to investigate the use of skirts to limit fish exposure to sea lice.
► The skirt, together with the fish, caused the oxygen levels to decrease below 50%.
► Future trials must therefore take precautional measures.