The Effect of Nutrient Dilution with Sodium Bentonite in Practical Diets for Rainbow Trout

Abstract Rainbow trout (Salmo gairdneri) with a mean initial weight of 18.8 g (0.041 lb) were fed one of two experimental diets (29.2% protein or 38.5% protein), or the 38.5% protein diet plus 5%, 10%, or 15% sodium bentonite, for 196 days. The addition of sodium bentonite to the 38.5% protein diet reduced weight gain and increased the unit cost of fish produced. Metabolizable energy content of the diet was a primary factor in determining growth rate. As the percent of dietary lipid increased, the percent of fat in fish carcasses increased and the percent of moisture decreased. Kidney tubules from 5 of 10 fish fed the diet with 15% sodium bentonite showed extreme epithelial swelling and dialation along with cytoplasmic degeneration and renal casts apparent in some tubule lumens.