Continuous high light intensity can induce retinal degeneration in Atlantic salmon, Atlantic cod and European sea bass

Retinal photodamage has previously been studied in teleost fish but very few have been performed on aquaculture species. To study retinal damage, Atlantic cod (Gadus morhua), Atlantic salmon (Salmo salar) and European sea bass (Dicentrarchus labrax) were previously acclimated to a control 12L:12D photoperiod with standard experimental low light intensity (0.1 W/m2, equivalent to 3.2 × 1013 photons/s/cm2) for at least 4 weeks and then kept under constant darkness (DD) for 3 days. Thereafter, fish were exposed to continuous high intensity light (51–380 W/m2, equivalent to 1.63 × 1016–1.22 × 1017 photons/s/cm2) for 3, 7, 15 or 25 days before returning to a control 12L:12D photoperiod (same intensity than during acclimation period) to study retinal regeneration over a period of 30 days. Retinal damage was exclusively assessed through the analysis of morphometric parameters. Results showed the presence of light-induced damage in the three species examined, as well as recovery once the control photocycle was restored. Cod was the most light-sensitive species as demonstrated by early signs of retinal damage (from three days of exposure) and reduced photoreceptor layer thickness (PRos/is) (43.1% relative to basal value in comparison to 51.6% and 73.3% respectively in salmon and sea bass). However, once the light–dark cycle was resumed the retina recovered in the three species studied (after 15 days in cod and 30 days in salmon and sea bass). Exposure to continuous high intensity light also resulted in significantly increased plasma cortisol levels in cod at LL15 (13.4 ± 2.0 ng/ml) and sea bass at LL3 (120.6 ± 12.2 ng/ml) and LD15 (54.2 ± 7.1 ng/ml). These results have important welfare implications with regards to the use of artificial light in culture and should be considered when designing lighting protocols in the aquaculture industry.