Dietary electrolyte imbalance alters drinking rate and gastrointestinal tract water fluxes of Atlantic salmon (Salmo salar) smolt in seawater

Summary

It was hypothesized that dietary electrolyte balance (dEB) would influence the dynamics of water, ions, and nutrient fluxes in the gastrointestinal tract (GIT) of Atlantic salmon (Salmo salar) smolts differently depending on water salinity. To date, a comparative study on how dEB alters these dynamics in freshwater- and seawater-adapted fish is lacking. The test diets were low versus high dEB (−100 versus 500 mEq kg⁻¹ DM⁻¹) and the test water salinities were 0 versus 30 ppt. Furthermore, the effect of the interaction between dEB and salinity on blood pH and osmolality were investigated. The experiment lasted for 6.5 weeks. Chyme was collected from four GIT segments (stomach, proximal, middle, and distal intestine) and analysed for dry matter (DM), pH, osmolality, crude protein, and ion (Ca²⁺, Mg²⁺, Na⁺, and K⁺) content. Water, ion, and nutrient fluxes were measured using yttrium oxide (Y₂O₃) as an inert marker. It was found that there was a diet effect on chyme pH in the stomach, being lower in fish fed the low dEB diet than the high dEB diet. Furthermore, the diet altered ion and nutrient fluxes in the stomach. Water salinity had the largest effect on chyme pH in all the GIT segments and on chyme osmolality in the stomach, which significantly increased in seawater conditions. The interaction between dEB and salinity had an effect on chyme DM, water and ion fluxes in the stomach, proximal and middle intestine. Our results showed that, depending on water salinity, dEB altered water fluxes differently. In freshwater-adapted fish, water influx to the stomach was higher in fish fed the high dEB diet than the low dEB diet, but the difference was neglectable. In contrast, in seawater-adapted fish, water influx into the stomach and proximal intestine was higher in fish fed the low dEB than the high dEB diet, and the amplitude was much larger. Additionally, in seawater conditions, drinking rate was 50% higher in fish fed the low dEB diet (3.07 ml kg⁻¹ h⁻¹) than the high dEB diet (2.04 ml kg⁻¹ h⁻¹). As a result, it was concluded that, in seawater conditions, a diet with a low dEB has a higher flux of water in the stomach and proximal intestine of Atlantic salmon smolts as well as enhanced drinking rate.