Effect of dietary sodium chloride levels on growth and tolerance to sea water of rainbow trout

Two feeding experiments were conducted to investigate the effect of dietary sodium chloride (NaCl) levels (0, 2.5, 5.0, 7.5, 10.0, 15.0%) on the growth of young and juvenile rainbow treout Oncorhynchus mykiss. After the feeding experiment, fish were acclimated to sea water (33 parts per thousand) to check the sea water tolerance. Young rainbow trout (BW, 59 g) fed diets containing 10 and 15% NaCl for 4 weeks showed lower weight gain than the trout fed diets containing less than 5% NaC1. However, no mortality was observed in any group of fish after 7 days of sea water acclimation. On the other hand, juvenile trout (BW, 1.7 g) fed the 2.5% NaC1 diet showed significantly better (p<0.05) weight gain and protein efficiency ratio than those fed the 0, 5.0, 7.5, and 10.0% NaCl diets during the 7 week feeding trial. Moreover, the mortality of fish fed the 2.5% NaCl diet during the sea water tolerance test was the lowest. Therefore, the influence of NaCl levels in diet on growth and tolerance to sea water depends on the fish size. It is shown that the excess amount of NaC1 level in diet is more than 7.5% (1.4 g/kg fish/day) and 11.2% (1.4 g/kg fish/day) for juvenile and young rainbow trout, respectively. The excess and minimum amounts of NaC1 in diet for juveniles should be considered.