Use of artificial light to control swimming depth and fish density of Atlantic salmon (Salmo salar) in production cages

The swimming behaviour of caged Atlantic salmon (Salmo salar) is influenced by both natural and artificial light conditions. This study reports the effects of light regimes and the positioning of lamps on swimming depth and fish density in groups of 170-230,000 Atlantic salmon (0.2-0.7 kg) held in 25m-deep production cages (17,500 m). Echo integration was used to estimate swimming depth and fish density as kilograms per cubic meter (observed fish density) and number of fish per cubic meter (packing density). In February, April and June, salmon groups swam consistently deeper and at lower density, both day and night, in cages illuminated by lamps at a depth of 15 in than in cages with lamps at 3 m. The packing density within treatments showed little variation across seasons, suggesting that interindividual distance was not size related. When subjected to short-term changes in light regimes, in January and March, the physostome salmon demonstrated a high degree of flexibility in vertical positioning. When underwater lamps were lowered from I to 15 m and subsequently raised again during a period of 48 h, swimming depth correlated with lamp depth. Both natural photoperiod and 24 L:0 D with lamps mounted above the surface resulted in the fish crowding near the surface at night. The results suggest that artificial light intensity gradients may be used to reduce exposure to suboptimal water layers and crowding of fish. (C) 2004 Elsevier B.V. All rights reserved.