Current induced net deformations in full-scale sea-cages for Atlantic salmon (Salmo salar)

Net deformations of sea-cages in two full-scale commercial Atlantic salmon (Salmo salar) farms (Varaldsøy, Norway, Hestur, Faroe Islands) were determined in relation to incoming currents of varying velocities. Ambient currents were measured with acoustic current meters, and net deformation behaviour and cage volume reductions were found by using depth sensors (pressure sensors) placed on strategic locations at the net cage. Cages deformed in current flow largely through lifting of the bottom netting and deformation of the front and back walls. Currents and therefore net volume reductions differed between the two farms: at Varaldsøy, the most critical current/deformation combination was 0.13 m s−1 and an estimated 20% cage volume reduction, while at Hestur, current speeds of 0.35 m s−1 caused a corresponding 40% cage volume reduction. Substantial net deformation and cage volume reductions may have significant implications for both fish production and welfare. Development of a real time net volume indicator could assist farmers in maintaining adequate cage volumes for optimal production and serve as an indicator of the optimal timing for net replacement if biofouling levels contribute significantly to deformation.