Effects of light regime on diurnal plasma melatonin levels and vertical distribution in farmed Atlantic cod (Gadus morhua L.)

Two studies were performed to study the effect of different light intensities on melatonin secretory profiles in Atlantic cod ( Gadus morhua L.). In an indoor tank study, plasma melatonin profiles were measured over a 24-hour cycle in juvenile Atlantic cod reared under a 12 h light:12 h dark photoperiod, having either a high or low daytime light intensity (15.5 or 0.16 μE·cm − 2 ·s − 1 ). In a second study, plasma melatonin levels were measured in Atlantic cod kept in traps at three depths (4, 9 and 14 m) in a large sea cage following exposure to the following five experimental light regimes: (1) natural light (NL), (2) NL + artificial constant light (LL) at 2, 7 and 12 m depth, (3) NL + LL at 2 or 12 m, (4) NL + LL at 2 m and (5) NL + LL at 2 m together with additional surface shading. The vertical distribution of free swimming Atlantic cod in the same sea cage was monitored using hydroacoustics. In the tank experiment, diurnal plasma melatonin profiles were similar under low and high light intensity, remaining low during the light period, then increasing rapidly within 15–30 min of lights off, before peaking around midnight. In the sea cage experiment, fish under NL showed a similar diurnal rhythm in plasma melatonin levels, with high melatonin levels during the dark period, although at sunrise melatonin levels started to fall earlier in fish kept at 4 m depth compared to fish kept at 9 and 14 m. Overall, melatonin profiles were most effectively reduced, in terms of relative (day/night) plasma levels, in those fish sampled at the deepest depth (14 m). Relative plasma melatonin levels were correlated to the distance from the artificial light source and the intensity of the ambient light. Dispersed light (lamps at 2, 7 and 12 m) was the most effective light regime for inhibiting the nocturnal rise in plasma melatonin ratios in fish sampled at all three depths. Finally, exposure to the different light regimes did not influence the vertical distribution of Atlantic cod within the sea cage, which consistently remained close to the bottom of the cage.