| Summary: | In Atlantic salmon (Salmo salar L.), photoperiod is well known to synchronise and influence the timing of important biological and physiological processes, like development of sea water tolerance and sexual maturation. However, there are few studies that are studying the influence of photoperiod on embryonic and post-hatch development in this specie. Therefore, in this study, early ontogenetic changes and photoperiodic effects on gene expression of several digestive enzymes and growth related factors were investigated. A total of 10 target genes were studied: pepsinogen (pep), trypsinogen (trp-1a), bile salt-activated lipase (bal), ghrelin (ghrl), growth hormone 1 (GH1), growth hormone 2 (GH2), growth hormone receptor 1 (ghr1), growth hormone receptor 2 (ghr2), cluster of differentiation 36 (cd36) and ATP binding cassette g5 (abcg5). Eggs and larvae from Atlantic salmon were exposed to three different light regimes, continuous light (LL 24hL:0hD), continuous dark (DD 0hL:24hD) and a compressed simulated natural photoperiod (LD). Eggs and larvae were collected at 8 different time points during development, from 77 day degrees (dd) to 1145 dd. Most of the genes related to development of the digestive tract were expressed at the first time point, and all were expressed before hatching (490 dd). Photoperiod was shown to influence the temporal expression patterns of genes related to the digestive system and the somatotropic axis, such as bal, pep, trp, GH1, GH2, ghr1 and ghr2. Fish in LD treated group had improved growth after start feeding and was significantly heavier than those in the LL and DD groups at 1145 dd. It is concluded that photoperiod during early ontogeny of Atlantic salmon influence temporal changes in expression of genes important for digestive capacity and growth regulation. These changes may explain better growth in fish exposed to a natural, but compressed, LD cycle prior to start feeding.
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