The effect of continuous light at low temperatures on growth in Atlantic salmon reared in commercial size sea pens

The aim of this study was to investigate the effect of continuous light of different duration, applied from late autumn to spring in the second year of the production cycle, on the production performance of Atlantic salmon in Northern Norway. The underlying hypothesis is that the introduction of continuous light (LL) superimposed on the natural light before December (the preferred continuous light regime in Northern Norway) could enhance growth and inhibit maturation in the subsequent year. To test this, two large, commercial scale experiments were performed [Experiment 1 in 2014 at 69.47°N, 18.26°E, and Experiment 2 in 2015 at 69.80°N, 19.42°E] where salmon of initial size of 1-1.5 kg were subjected to LL at different time points during the period between 11 November and 13 December, and reared under LL until 31 March the following year. In Experiment 1 the water temperature at 6 m depth ranged between 6.7 °C in November and 3.6 °C in March and in Experiment 2 the water temperature at 6 m depth ranged between 8.3 °C in November and 3.6 °C in March and 6.8 °C in May 2016. Before and after the period with LL, all fish were reared under natural light. Growth was improved by 13-20% in the early exposed groups (15 Nov and 11 Nov) compared to the late exposed groups (13 Dec.). No maturation was seen in the experimental groups at slaughter (Exp. 1: July-September 2015, Exp. 2: May 2016). Vertebra deformities did not differ between the early and late exposed groups suggesting that continuous light promotes growth at lower temperatures, while supporting normal vertebra development. Only minor differences in flesh texture (measured as differences in cathepsin L + B activity) were found in both experiments. It is concluded that a considerable growth benefit may be achieved by exposing Atlantic salmon to continuous light from early November in their first year in seawater, i.e. one month earlier than presently used by the salmon farming industry in Northern Norway.