Studies On Nutritional Physiology Of Threonine And Phenylalanine For The Large Yellow Croaker, Pseudosciaena Crocea R. And Japanese Seabass, Lateolabrax Japonicus

Feeding trials were conducted to investigate the nutritional physiology of threonine and phenylalanine for the large yellow croaker, Pseudosciaena crocea R. and Japanese seabass, Lateolabrax japonicus. The dietary requirements of threonine (Thr) and phenylalanine (Phe) for juvenile large yellow croaker and Japanese seabass were conducted in seawater floating net cages (1.0×1.0×1.5 m or 1.5×1.5×2.0 m) for 56 days. Results of the present study are presented as follows:1. In large yellow croaker, no significant differences in survival were found among dietary treatment. Dietary threonine significantly affected the growth responses and feed utilization of large yellow croaker. Weight gain (WG), specific growth ratio (SGR), protein efficiency ratio (PER) and nitrogen retention (NR) were the lowest in fish fed the diet with 0.71% Thr (Diet 1) and increased with increasing threonine up to 1.75% Thr (Diet 4) (P<0.05), thereafter, declined (P<0.05). On the basis of SGR and NR, the optimum dietary threonine requirements of juvenile large yellow croaker were estimated by using second-order polynomial regression analysis to be 1.90% of diet (4.22% of dietary protein) and 2.06% of diet (4.58% of dietary protein), respectively. These results indicate that threonine is essential for growth of large yellow croaker. Large yellow croaker is able to utilize crystalline forms of threonine, but beyond the optimum requirement level for threonine, there was a marked decline in growth response and feed utilization. No significant differences in body composition were observed among dietary treatments (P>0.05). Dietary threonine levels significantly affected the proportions of threonine in fish muscle (P<0.05).2. About survival, no significant differences were observed among dietary treatments in Japanese seabass. WG, SGR, FCE, PER and NR were the lowest in fish fed the diet with 0.75% Thr (Diet 1) (P<0.05),and increased with increasing threonine up to 1.67% Thr (Diet 4) (P<0.05), and thereafter, declined (P<0.05). The results indicate that for growth of Japanese seabass, threonine is essential and Japanese seabass is able to utilize crystalline forms of threonine, but there was a marked decline in growth response and feed utilization beyond the optimum requirement level for threonine. No significant differences in body composition were found (P>0.05). On the basis of SGR and NR, the optimum dietary threonine requirements of juvenile Japanese seabass were estimated using second-order polynomial regression analysis to be 1.78% of diet (4.24% of dietary protein) and 1.87% of diet (4.45% of dietary protein), respectively. The proportions of threonine in fish muscle were significantly affected by dietary threonine levels (P<0.05).3. In large yellow croaker, no significant differences in survival were found among dietary treatment. The lowest WG, SGR, PER and NR were obtained in fish fed diet without phenylalanine supplementation (0.63% Phe, Diet 1). The growth response and feed utilization increased with increasing phenylalanine up to 1.47% Phe (Diet 4) (P<0.05), thereafter, declined. On the basis of SGR and NR, the optimum dietary phenylalanine requirements of juvenile large yellow croaker were estimated by using second-order polynomial regression analysis to be 1.62% of diet (3.68% of dietary protein) and 1.56% of diet (3.55% of dietary protein), respectively. These results indicate that phenylalanine is essential for growth of large yellow croaker. Large yellow croaker is able to utilize crystalline forms of phenylalanine, but beyond the optimum requirement level for phenylalanine, there was a marked decline in growth response and feed utilization. No significant differences in body composition were observed among dietary treatments (P<0.05). The proportions of phenylalanine in fish muscle were significantly affected by dietary phenylalanine levels. (P<0.05).4. About survival, no significant differences were observed among dietary treatments in Japanese seabass. WG, SGR, FCE, PER and NR were the lowest in fish fed the diet with 0.61% Phe (Diet 1) (P<0.05),and increased with increasing phenylalanine up to 1.36% Phe (Diet 4) (P<0.05), and thereafter, declined (P<0.05). The results indicate that for growth of Japanese seabass, phenylalanine is essential and Japanese seabass is able to utilize crystalline forms of phenylalanine, but there was a marked decline in growth response and feed utilization beyond the optimum requirement level for phenylalanine. No significant differences in body composition were found (P>0.05). On the basis of SGR and NR, the optimum dietary phenylalanine requirements of juvenile Japanese seabass were estimated using second-order polynomial regression analysis to be 1.30% of diet (3.10% of dietary protein) and 1.26% of diet (3.00% of dietary protein), respectively. The proportions of phenylalanine in fish muscle were significantly affected by dietary phenylalanine levels (P<0.05).