SURFACE-ACTIVITY OF ATLANTIC SALMON (SALMO-SALAR L) IN NET PENS

The leaping and rolling activities of Atlantic salmon in net pens are behavioural features that may give important indications of the status of the fish. These surface activities were studied in fish in net pens in relation to environmental factors and operational procedures. The leap of salmon in net pens differs from the oriented leaps salmon make when ascending a river. The fish usually land on their side, and in about 6% of the leaps the fish hit the pen wall. Leaping activity was low in winter. Leaping increased with increasing louse infestation, and there was little leaping after de-lousing. However, during chemical de-lousing treatment, leaping activity was high. When rolling, the fish slowly breaks the water surface. Rolling activity varied between days, but was relatively constant throughout the year. Rolling activity increased after different types of disturbances, and may be related to compensation of swimbladder gas loss during a stress situation. High rolling activity is thus an indication of recent acute stress, whereas high leaping activity indicates heavy louse infestation or present acute stress.