Development of osmo- and appetite regulation during transfer of smolt from freshwater to increasing salinities

Abstract: Atlantic salmon (Salmo salar) is common in Scandinavian aquaculture and must be transferred from freshwater to seawater to grow properly. Despite being assessed as ready smolts, meaning they have gone through the physiological changes to prepare for SW transfer, the transfer result in a mortality rate of around 13-19%, and significant appetite- and growth reductions. The aim of this study was to get a timeline of appetite- and osmoregulatory development as salmon are transferred from freshwater to higher salinities, to detect potential mismatches between osmoregulatory tissues, and alterations in central appetite control that can explain the reduction in appetite and growth. Na+/K+-ATPase (NKA) activity using a kinetic enzyme assay, and gene expression of NKA -isoforms (1a, 1b, 1c and 3a) were measured in the proximal and distal intestine, gills, and kidney using qPCR. Appetite regulating factors (npy, agrp, cart, cck-l, pomc-a and pomc-b) were measured (qPCR) in the hypothalamus, from six time points: freshwater (0 ppt), brackish water (12 ppt) and 3, 6, 12 and 30 days after seawater transfer (30 ppt). The NKA activity and expected isoform expression were timely developed in the gill and intestine. The development of NKA activity and isoform expression in kidney have previously been scarcely studied. The isoform nka⍺1c showed the most abundant expression in the kidney, peaking after three days in seawater, indicating a role of this isoform in renal hypo-osmotic regulation. The expression of npy, cart and cck-l increased temporarily upon transfer, followed by a decline after 30 days in seawater. The agrp expression was low in all timepoints until an increase after 30 days in seawater. The results indicate no mismatches in NKA activity or isoform development in the analyzed tissues. The changes in central appetite control suggest a temporary decrease in appetite upon seawater transfer increasing again after 30 days.