The effect of combining shading and continuous lighting on the suppression of sexual maturation in outdoor-reared Atlantic cod, Gadus morhua

Sexual maturation of Atlantic cod, Gadus morhua, is a major problem during commercial on growing as fish divert energy away from growth into gonadal development. While the photoperiod regimes that inhibit maturation are well described, when manipulations are applied in a commercial cage setting, using standard lighting technology, maturation is not completely inhibited. This has led to the hypothesis that the enhanced light sensitivity of cod allows it to perceive ambient illumination over the artificial lighting. A 13 month trial was conducted to determine the effectiveness of net shading ambient photoperiod in addition to constant lighting to suppress maturation of cod in outdoor conditions. By reducing the relative difference between day and night light intensities, it was hypothesised that maturation in cod could be inhibited as fish could not perceive, and thus use the ‘ambient daylength' signal to entrain their reproductive cycle. Two outdoor tanks were covered in either a low density (70% reduction in ambient illumination) or a high density (90% reduction in ambient illumination) shade netting and then illuminated continuously by cathode lighting. These were compared to two indoor tanks in which ambient light was excluded and instead they were illuminated by similar lighting running either under a simulated natural photoperiod or continuous illumination. The work demonstrated that the shade netting could improve the relative performance of the artificial lighting outdoors from less than 2% in a non-shaded system to 6.6% (low shade treatment) and 31.3% (high shade treatment) of the day light intensity. Importantly, both shading treatments were effective at suppressing sexual development as confirmed through histology and reduced GSI (female mean ≤ 2.18%, male mean ≤ 1.39%), oocyte diameter (mean less than 400 μm) and sex steroid profiles (female 17β-estradiol mean ≤ 0.24 ng.ml- 1, male 11-ketotestosterone mean ≤ 3.35 ng.ml- 1) as well as enhanced growth. These results are a promising demonstration of the potential value of shading systems to enhance the efficacy of photoperiod control.