Aquaculture and Fisheries Management 1994, 25, 439-451 Influence of hunger level and food availability on the spatial distribution of Atlantic salmon, Salmo salar L., in sea cages J.-E. JVELL Institute of Marine Research, Department of Marine Resources, Fish Capture Division, Bergen, Norway A. F E R N O Department of Fisheries and Marine Biology, University of Bergen, Bergen High Technology Center, Bergen, Norway D. FUREVIK Institute of Marine Research, Department of Marine Resources, Fish Capture Division, Bergen, Norway 1. HUSE Institute of Marine Research, Austevoll Aquaculture Research Station, Storeb0, Norway Abstract. The spatial distribution of Atlantic salmon, Salmo salar L., in sea cages was observed by echo integration, which measured fish density at seven lm depth intervals. At start of feeding the fish swam towards the surface and into the centre of the cage with a subsequent movement downwards and towards the periphery as hunger was reduced. These changes were more rapid at high than at low feeding intensities. The downward migration during feeding was primarily observed as a reduction in hsh density in the 0-1 mdepth layer and an increase in the 2-3mdepth layer. For a given feeding intensity, the change in fish density in these depth layers was largely explained by time from the start of feeding and initial hunger level, measured as food intake to satiation. The feeding regime also influenced vertical distribution outside feeding periods. Fish that were fed at low intensity swam generally closer to the surface, and this tendency was even stronger when a restricted food ration kept hunger level constantly high. While fish density pwaked at a single depth interval at full ration, a bimodal vertical distribution was observed at restricted ration, suggesting that different subgroups of fish had different feeding motivations. The results suggest that the vertical distribution of salmon in sea cages is based on a trade-off between attraction to food and avoidance of the surface. This tendency has been exploited in a new method of demand feeding in commercial farming of Atlantic salmon. Introduction As food amounts to about 40% of the costs of production of salmon, Salmo salar L., farmed in sea cages in Norway (Anon. 1991), profitability is highly dependent on effective food utilization. This requires precise regulation of food rations in order to avoid growth restrictions or waste of food (Brett 1979). Reports of food waste beneath sea cage systems indicate, however, that food ration regulation is a problem in salmon farming (see Gowen & Bradbury 1987, for review). To solve this problem a better understanding of the complex interplay of internal and external factors controlling food intake in fish is required. An important distinction can be made between structural factors, such as temperature and fish size, and motivational factors such as hunger level and metabolic debt (Colgan 1989). While Correspondence: Dr Jon-Erik Juell, Institute of Marine Research, Department of Marine Resources, Fish Capture Division, P.O. Box 1870, N-5024 Bergen, Norway. 439