Effects of a temperature rise on Atlantic salmon, Salmo salar, melatonin and thyroid hormones during smoltification

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the fish life cycle, including gills’ Na+/K+-ATPase activity (NKA) and plasma hormonal levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ +4°C) combinations, on gills’ NKA, plasma free triiodothyronine (T3), thyroxine (T4) and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gills’ NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormones levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on S. salar endocrine system, and suggest that climate change might compromise salmon’s time “deciphering” and processes such as smoltification, downstream migration and seawater residence.