Phenylalanine (Phe) , an indispensable aromatic am ino acid (AAA) is required for normal growth and metabolic processes, and it is the sole precursor of tyrosine . Sinc e phenylalanine can be converted to tyrosine in an irreversible reaction, its insufficiency in diets will reduce growth performance and physiological functions. Hence, the study determined the dietary phenylalanine requirement of African catfish with a known or fixed level of tyrosine.
A basal diet was prepared to contain 0.74g of phenylalanine. Then 4 levels of Phe were added to the diet to achieve diets with (0.74, 0.94, 1.14, 1.34 and 1.60g) Phe as diets 1-5, with a fixed quantity of (0.47g) tyrosine . The diets were fed to C. gariepinus for 70 days to determine the Phe requirement of the fish. Results (Table 1) show that Phe increased the growth performance up to the fish fed diet with 1.14g/100g of Phe, and then declined from the fish fed diet containing 1.34g of Phe, indicating 1.14g as the requirement for the fish.
However, 6th degree polynomial regression analysis revealed 1.1 g as the requirement. Therefore, the Phe requirement of African catfish (6.12g) lies between 1.1 and 1.14g/100g diet. Also, carcass cholesterol decreased significantly with increasing levels of Phe, while carcass protein deposition and red blood cell counts increased (P<0.05) with increasing levels of Phe in comparism with fish fed the control diet. Lastly, Phe marginally improved the white blood cell count, indicating better immunity in those fish fed diets with Phe. .