QUANTIFYING THE DIETARY TRYPTOPHAN REQUIREMENT OF JUVENILE RED DRUM Sciaenops ocellatus

The red drum, Sciaenops ocellatus, is cultured in the United States for both stock enhancement and food production.  Ongoing studies in this laboratory are quantifying optimum dietary levels of indispensable and dispensable amino acids of red drum in order to compile a complete set of nutritional requirements.  The present study was conducted to determine the optimum dietary tryptophan requirement of red drum.  It was a follow up of an initial trial which evaluated a larger range of dietary tryptophan concentrations (0.15%- 0.75%).  The previous trial concluded that the optimum dietary tryptophan concentration was approximately 0.25-0.35% of diet.  The present trial included a narrower range of dietary tryptophan concentrations to refine the requirement based on the preliminary trial.  Groups of thirteen red drum fingerlings, of initial average weight of 2.0 -2.7 g each were placed into each of 15, 38-L aquaria containing brackish water (7 ppt) and operated as a recirculating system.  Five semi-purified diets composed of red drum muscle and crystalline amino acids were supplemented with incremental levels of tryptophan.  Each diet was given to triplicate groups of fish which were weighed every week to observe health and growth as influenced by dietary tryptophan.    At the end of the 49-day feeding period, the optimal tryptophan requirement of red drum was estimated based on weight gain, feed efficiency and protein conversion efficiency.  Red drum fed the basal diet without adequate tryptophan exhibited significantly (P< 0.05) reduced weight gain, feed efficiency and survival compared to fish fed the other diets.  However, no other specific signs of deficiency were observed. All of the diets supplemented with tryptophan from 0.25 to 0.35% supported similar weight gain (Fig. 1).  Based on these data fitted by a quadratic regression model, the minimum dietary tryptophan requirement of red drum was determined to be 0.248 ± 0.02% of dry diet.  This information can be used in formulating diets for red drum to ensure adequate levels of tryptophan are provided.