Late transfer to seawater improves intestinal development of osmoregulatory capacity in Atlantic salmon smolts

The development and function of intestinal fluid transport capacity in Atlantic salmon, determine seawater (SW) readiness, but poorly understood in modern aquaculture practices. This study aimed to investigate the development of intestinal fluid transport capacity during smoltification and the effect of short-term fasting and a high-lipid diet. Two identical experiments were conducted using two feed types during smoltification: (1) control and (2) high-lipid diet with extra 5% fat. Smoltification was induced by a 6-week short-day (12L:12D) photoperiod, followed by continuous light (LD 24:0) at 12 °C freshwater (FW). Fish were transferred to SW at 144, 300, and 480 degree-days (dd), and each group was further divided into fed and fasted (4 days pre-transfer). In fish fed the control diet, intestinal tissue mass, Na⁺,K⁺,-ATPase (NKA) activity, and fluid transport capacity increased with transfer time in both FW and SW, indicating that 480 dd group had the most developed hypo-osmoregulatory capacity. In all groups, fasting reduced intestinal tissue mass for at least 6 weeks in SW. Fish fed the high-lipid diet exhibited higher NKA and fluid transport capacity in FW for both 300 dd and 480 dd groups. After 6 weeks in SW, the 480 dd group showed higher intestinal mass, NKA activity, and fluid uptake than the other groups and was not affected by fasting. Overall, late SW transfer promoted intestinal growth and increased transport capacity. Fasting negatively affected intestinal fluid uptake during the early SW phase, whereas a high-lipid diet accelerated intestinal development and mitigated the negative effects of fasting.