Low intensity light of different colours modifies Atlantic salmon depth use

Abstract Underwater anti-maturation-lights have recently been exploited to position sea-caged Atlantic salmon ( Salmo salar L.) deeper at night in an effort to reduce infections by salmon lice ( Lepeophtheirus salmonis ) in surface layers. However, anti-maturation-light use is impermanent because lighting during decreasing day-lengths stimulates sexual maturation which is detrimental for fish welfare, growth and meat quality. The effects from lights on maturation are related to both light intensity and light spectrum. Here, we explored caged salmon depth use in response to lights of four low intensities (0.01, 0.10, 1.0 and 10.0 μE as measured 1 m from the lamps) and seven different colours (broadband white LED lamp and narrow spectrum violet, blue, green, yellow, red and deep red LED lamps). Triplicate sea cages (12 × 12 m and 11 m deep) holding approximately 5000 fish of 1.5 kg were exposed to each light positioned at 10 m depth for one night. Echo sounders registered fish vertical positioning on nights of light treatments and no light (control nights) before and after each light exposure. Results showed that submerged lights generally caused fish to maintain their day-time swimming depth near 10 m (light depth) during the night, as opposed to the typical migration of salmon to upper cage depths at dusk observed on control nights. Quantities of fish staying deep decreased with lowered light intensity, but even 0.1 μE had effects. All light colours, except deep red, significantly affected swimming depth, with a trend of increased effect at lower wavelength colours. Temperature stratification strengthened light effects when warmer water was near the lamps and weakened effects in the case of warmer water near the surface. This study opens up the potential of using low intensity lights at decreasing day-lengths that may not affect sexual maturation and remain suitable for guiding salmon away from surface waters rich in lice infective stages.