Modifying Atlantic salmon behaviour with light or feed stimuli may improve parasite control techniques

The aquaculture of Atlantic salmon Salmo salar faces severe health, environmental and economic concerns caused by the parasitic sea lice Lepeophtheirus salmonis and Caligus spp. An experimental delousing method exists whereby the surface jumping behaviours of salmon are combined with a floating, oil-infused chemical therapeutant, resulting in the fish dousing them- selves passively. We tested whether a light stimulus or feed event, or a combination of both, during submergence (denial of surface access for the fish) increased the surface-oriented behaviours of salmon. Groups of 10 salmon were submerged in a sea-cage for 19 h and exposed to a light stim- ulus, feed event, or both. Control treatments involved submergence only. For a 2 h period after surface access was reinstated, light and feed treatments induced a higher proportion of individu- als to exhibit surface behaviours. On average, 84 and 82% of salmon in the light and feed treat- ments jumped, respectively, which was 1.6 times higher than fish in the control group (50%). Salmon exposed to light or feed jumped an average of 1.7 and 1.5 times after exposure to light or feed treatments, respectively, compared to 0.92 jumps fish -1 for the control. The combined light and feed treatment did not produce a synergistic effect. The average time until first jump was 31 to 50 min, with no difference in times among treatments. The elevated surface-oriented behav- iours effectively crowded the majority of the fish in the surface waters within a short period of time. This increased surface activity may be used for a range of salmon farming applications, including improving the efficacy of sea-lice treatment techniques.