Salinity induced changes in the progression of water, ion and nutrient fluxes along the gastrointestinal tract of Atlantic salmon smolt (Salmo salar)

Summary

Water ingestion in fish increases with both water salinity and feeding. However, it is unclear whether, during feeding, water ingestion is intended to aid chyme liquefaction in the stomach or to maintain the osmotic homeostasis within the body of the fish. We investigated the effects of increasing water salinity (0, 10, 20, 35 ppt) on the progression of water, ion and nutrient fluxes in the gastrointestinal tract of Atlantic salmon smolt (Salmo salar) fed a commercial-like diet. Furthermore, the effect of water salinity on blood pH, plasma osmolality and ions was investigated. The experiment lasted for 8 weeks. Chyme was collected from 4 gastrointestinal tract (GIT) segments (stomach, proximal, middle and distal intestine) and analysed for dry matter, pH, osmolality, crude protein and mineral content. Water and electrolyte fluxes, kinetic of digestion and faecal digestibility were measured using yttrium oxide (Y₂O₃) as an inert marker. We found that between 0 and 35 ppt chyme dry matter decreased by 1.6% and 4.8% in the stomach and proximal intestine, respectively. Chyme pH was not affected by water salinity in the stomach, but it increased linearly (p < 0.001) with salinity in all intestinal segments. Chyme osmolality increased linearly (p < 0.001) with salinity in the stomach and it decreased in all intestinal segments. Water fluxes were similar among salinities in the stomach, but they increased nearly fivefold (6.2 versus 27.3 ml g⁻¹ ingested DM) in the proximal intestine between 0 ppt and 35 ppt. An efflux of monovalent ions (Na⁺ and K⁺) increased linearly (p < 0.001) with salinity in the proximal intestine. An efflux of divalent ions (Ca²⁺ and Mg²⁺) increased curvilinearly (p < 0.001) with salinity in the middle intestine. Plasma osmolality and ion levels increased with salinity. Crude protein digestibility and protease activity decreased significantly with water salinity in the intestine. Our study highlights that when Atlantic salmon moves from freshwater to higher water salinity environments, drinking of saltwater does not interfere with hydration of feed in the stomach, but instead bypasses to the proximal intestine to aid in osmoregulatory water uptake. Therefore, we suggest that water ingestion in seawater fish is intended for osmoregulation rather than to aid digestion by liquefying chyme in the stomach.