Osmotic imbalance as a factor for winter-induced skin health challenges in Atlantic salmon (Salmo salar)

Winter wounds are an increasing welfare and economic challenge in Norwegian Atlantic salmon farming. Low seawater temperatures may suppress drinking and water uptake, potentially disturbing osmotic balance and weakening skin integrity. This study examined how temperature affects drinking activity, osmoregulation, and osmolyte regulation in Atlantic salmon.
Seawater-acclimated smolts (~200 g) were reared for six weeks at 5, 8, 12, or 16 °C (32 ppt; four replicate tanks per treatment, 20 fish per tank). Growth, water flux, drinking rate, welfare indicators, and osmolyte profiles were assessed using UPLC, Conway analysis, and NMR. Correlation analysis and principal component analysis (PCA) were used to evaluate relationships among temperature, osmolality, and free amino acid (FAA) profiles.
Temperature significantly affected growth, increasing from 43.8 g at 5 °C to 183.8 g at 16 °C. Welfare was optimal at 8–12 °C, while fish at 5 °C showed poorer fin condition and reduced drinking rate. Osmolality increased positively with temperature. Muscle TMAO remained stable (~2 mg N 100 g⁻¹) across temperatures but was undetected in skin. Most FAAs accumulated at higher levels at 5 °C in both tissues, whereas hydroxyproline, glutamine, and arginine increased at 16 °C in muscle.
Spearman correlation analysis showed predominantly negative associations between skin FAAs and osmolality, particularly for glutamine, serine, glycine, glutamate, and hydroxyproline, while temperature was positively associated with osmolality. Muscle displayed fewer and weaker correlations. PCA identified a strong coordinated FAA response in skin (PC1 = 43.7% variance), whereas muscle showed a weaker and more heterogeneous pattern (PC1 = 26.9%).
These findings suggest that low temperature suppresses drinking and gastrointestinal water flux, increasing osmotic challenge in Atlantic salmon. Fish appeared to compensate through tissue-specific FAA accumulation consistent with osmotic adjustment. Ongoing metabolite and transporter expression analyses will further clarify temperature-related osmotic regulation.