The use of cage lighting to reduce plasma melatonin in Atlantic salmon (Salmo salar) and its effects on the inhibition of grilsing

The effects of additional night time illumination on circulating plasma melatonin levels and maturation were studied in Atlantic salmon maintained in sea cages under commercial conditions. Salmon subjected to additional night-time lighting from November to July had significantly lower dark phase melatonin levels (303.1 ± 6.3 pg/ml) compared to control fish (600.0 ± 53.0 pg/ml) maintained under ambient photoperiod. More importantly, as far as the commercial farmer is concerned, only 6.1% of the group exposed to additional lighting matured compared to 61.5% of the fish in the control group. It is suggested that the clear effect of the additional lights on reducing grilsing in Atlantic salmon is mediated through a reduction in the amplitude of plasma melatonin below a putative threshold level during the subjective dark phase.