Dietary electrolyte balance of Atlantic salmon (Salmo salar) freshwater feeds: Impact on osmoregulation, mineral metabolism and performance in seawater

Dietary electrolyte balance is the equilibrium of monovalent cations and anions that influence the acid-base balance of the feed (dEB = Na + K - Cl, mEq kg(-1)). Dietary electrolytes/minerals can influence the physiological changes during smoltification in Atlantic salmon. In this context, we aimed to study if the dEB of the freshwater feeds can be used to pre-adapt the hypoosmotic functionality and the associated effects on mineral metabolism. The dEB of commercial freshwater Atlantic salmon feeds in Norway varied from -9 to 400 mEq kg(-1) feed. Three experimental feeds were formulated to study incremental levels of dEB reflecting the low (L-dEB, -50 to 0), median (M-dEB, 200-250) and high (H-dEB, 350-400). Triplicate groups of Atlantic salmon parr (36 g) were fed one of the three feeds for 8 weeks in freshwater at 12 degrees C. The fish were transferred to full strength seawater in indoor tanks and fed a commercial diet for 6 weeks. Growth was not differentially affected by dEB levels, neither in the freshwater phase nor in the seawater. Plasma electrolytes (Na+ and Cl-) and gill mRNA expression of sodium potassium ATPase (NKA alb, seawater isoform) were significantly lower in L-dEB fed fish. In the intestine, carbonate precipitates 24 h after seawater transfer was higher in fish fed both L-dEB and H-dEB feeds compared to the M-dEB fed fish. Whole body and plasma mineral levels were significantly affected by dEB levels in freshwater feeds. Interestingly, the carryover effect of dEB in freshwater feeds was significant after 6 weeks in seawater for plasma and whole-body Zn status, with the H-dEB fed fish showing significantly increased body Zn status compared to L-dEB and M-dEB fed fish. The study revealed that mineral metabolism and intestinal response to seawater transfer can be pre-adapted by modulating the electrolyte and/or mineral balance in freshwater feeds in Atlantic salmon. Further, dEB did not affect long term development of cataract or vertebral deformities.

Automated summary

The study showed that modulating the electrolyte and/or mineral balance in freshwater feeds can pre-adapt mineral metabolism and intestinal response to seawater transfer in Atlantic salmon. Additionally, the dietary electrolyte balance did not affect the long term development of cataract or vertebral deformities.