Short‐term exposure to continuous light delays sexual maturation and increases growth of Atlantic cod in sea pens

Atlantic cod Gadus morhua L. (initial weight 130 g, final weight 2800 g) was exposed to continuous light at different stages during the production cycle, and the effect on growth performance, age at first maturation, plasma sex steroid levels and flesh quality were investigated. The entire photoperiod experiment was divided into five phases where the fish in each phase were exposed to either natural photoperiod (N, 65°28′) or continuous light (C). Thus, the following five photoperiod combinations were tested: (a) control group (NNNNN), (b) group 2A (NCNNN), (c) group 2B (NNCNN), (d) group 2C (NNNCN) and (e) continuous light group (CCCCC). Short‐term exposure to continuous light in the first (group 2A) and second (group 2B) growth phases stimulated growth, as the final mean weights of groups 2A and 2B were 11% and 19% higher than those of the control group (NNNNN). Growth in the control group and group 2C was arrested from January to May in the second year. Continuous light (CCCCC) stimulated growth at the beginning of the trial, but the effect diminished at the end. Reduced male plasma 11‐ketotestosterone (11‐KT) and female estradiol 17β (E2) levels were found in groups 2A and 2B and continuous light group, as compared with the control group and group 2C at 26 months post hatch, indicating a higher age at first maturation in these groups. The 11‐KT and E2 levels increased in May–June in group 2A indicating that first maturation was postponed by 6 months in this group, whereas first maturation was postponed by 1 year in group 2B and in the continuous light group. Photoperiod regime only had minor effects on flesh quality traits of the fish.