A six-week feeding trial was conducted to evaluate the effects of protein free (PF) and essential amino acid deficient (EAAD) diets on the growth performance and digestive enzyme activities of silvery-black porgy (Sparidentex hasta) juveniles. Three experimental diets were formulated: a control diet in which 60% of dietary nitrogen was provided by intact protein (fish meal) and 40% by crystalline AA [(blends of essential amino acids (EAA) and none essential amino acids (NEAA)]; an essential amino acid deficient diet in which 60% of dietary N was provided by intact protein, whereas the rest was provided by NEAA; and a protein free (PF) diet, based on carbohydrate sources. Survival rates in fish fed the EAAD and PF diets were lower than in fish fed the control diet. Weight gain in fish fed with EAAD was 5.6 g lower than fish fed with the control diet. Furthermore, fish fed the PF diet lost weight and their final body weight was 12.2 g lower than the control group. Fish fed the PF diet had the highest whole body moisture, but the lowest whole body protein, lipid, threonine, aspartic acid, glutamic acid and serine levels. Digestive pancreatic enzyme activities including trypsin, lipase, α-amylase and carboxypeptidase A significantly decreased in fish fed EAAD and PF diets. The information obtained from this study testing two extreme diets (EAAD and PF) may serve for better understanding the impact of protein and EAA nutritional imbalances in fish performance.

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