Dietary Tryptophan Requirement of Juvenile Hybrid Grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂).

A 10-week feeding study was conducted to examine the effects of dietary tryptophan (Trp) levels on the growth performance and protein metabolism of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) with the aim of determining the optimal Trp requirement. Six isonitrogenous and isolipidic experimental diets were formulated, containing varying levels of Trp (0.26%, 0.32%, 0.42%, 0.46%, 0.58%, and 0.62% of diet). Each diet was administered to 90 hybrid groupers with an initial average weight of 10.52 ± 0.02 g. Dietary levels of 0.32-0.62% Trp generally promoted growth performance and feed utilization in hybrid grouper, with the 0.46% Trp level demonstrating the highest weight gain and protein efficiency ratio. Furthermore, fish fed with 0.46% Trp exhibited higher activities of intestinal lipase and amylase compared to other groups. Furthermore, the activities of Trp metabolism-related enzymes in the liver were markedly elevated by this Trp level, including indoleamine 2,3-dioxygenase, 5-hydroxytryptophan decarboxylase, and tryptophan-2,3-dioxygenase. Additionally, a dietary level of 0.46% Trp resulted in a substantial increase in the activities of protein metabolism-related enzymes such as adenosine monophosphate deaminase and glutamate dehydrogenase in the liver. Dietary levels of 0.32-0.46% Trp activated the hepatic mammalian target of rapamycin pathway, while a dietary 0.46% Trp level inhibited the hepatic amino acid response signaling pathway. In summary, suitable dietary Trp levels enhanced intestinal digestive enzyme activities, facilitated both protein synthesis and catabolism, and ultimately improved growth performance in hybrid grouper. Analysis using a second-order regression curve of weight gain and protein efficiency ratio indicated that the optimal dietary Trp requirement for hybrid grouper ranged from 0.41-0.46% of diet (0.82-0.92% of dietary protein).