Influence of hunger level and food availability on the spatial distribution of Atlantic salmon, Salmo salar L., in sea cages

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 m depth layer and an increase in the 2-3 m depth 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.

The behaviour of Atlantic salmon in relation to efficient cage-rearing

“…The visualrange during direct observations, however, may be severely limited by light conditions, turbidity and waves, but may be significantly improved by the use of light-sensitive underwater cameras. Underwater cameras, often connected to videotape recorders with timer and time-lapse functions, have been used by several authors to study temporal variations in swimming and feeding behaviour (Bjordal et al, 1986;Fern6 et al, 1988;Juell, 1988;Kils, 1989;Kadri et al, 1991;Blyth et al, 1993;Smith et al, 1993;Juell et al, 1994a).…”

Environmental drivers of Atlantic salmon behaviour in sea-cages: A review

“…Hungry fish remain at the surface in the feeding area after the feeding 256 period and fish fed at high intensities move towards the surface more 257 rapidly than at low intensities (Juell et al, 1994). The response rates of several pelagic fish species relies on visual contact (Glass et al, 1986).…”

Sea caged Atlantic salmon display size-dependent swimming depth

“…With regards to swimming depth, subgroups within the same sea cage are commonly observed, and may be explained by different hunger levels or unsynchronized feeding rhythms (Juell and Westerberg 1993;Juell et al 1994;Juell 1995;Fernö et al 1995). A positive correlation between hunger level and surface attraction has been showed at group level for caged salmon (Juell et al 1994), and could be a likely cause of small and thus potentially hungrier fish to maintain position close to surface.…”

“…If not already present in the surface feeding area, caged salmon normally ascend towards the surface during feeding periods (Juell et al 1994;Ang and Petrell 1998). Hence, one could expect a more homogenous size composition at 3 m during the hours of daylight.…”