This research sought to develop low-protein, high-energy (less-polluting) diets for rainbow trout
to properly utilize protein and reduce total nitrogen excretion. Duplicate groups of rainbow trout
were fed one of four experimental diets with a protein to energy (P:E) ratio of 25, 24, 22 or 19
mg/kJ and a lipid content of 12, 13, 17 or 26%, respectively. The diets were given to fish with a
mean initial weight of 181 g for 70 days. At the conclusion of the trial, mean weights ranged
from 353.17 g (94% weight gain) to 394.75 g (118% weight gain) with 100% survival in all treatments.
Weight gain, feed efficiency and protein retention increased as the dietary lipid content
increased and as the P:E ratio decreased. Fish fed the 22 mg protein/kJ energy diet (17% lipid,
44% protein, 20.34 kJ/g gross energy) performed best. This indicates that the protein content in
practical trout feeds can be reduced from the currently used 47% to around 44% without reducing
the growth rate and feed efficiency, if high quality protein is used and the gross energy is
increased by lipid to about 20.34 kJ/g diet. The low-protein, high-energy diets reduced the total
nitrogen excretion from the rainbow trout by 27%.