Dietary Threonine Requirement of Juvenile Red Drum Sciaenops ocellatus

Threonine is an indispensable amino acid required by all animals for normal growth and metabolic functions. An experiment was conducted in a brackish water (5 ± 1 ppt) recirculating system to quantify the minimum dietary threonine requirement of juvenile red drum Sciaenops ocellatus . The experimental diets contained 350/0 crude protein from red drum muscle and crystalline amino acids and 3.2 kcal available energy/g diet. Incremental levels of L-threonine were added to the diets and fed to juvenile red drum initially averaging 2.8 g/fish for 8 wk. Weight gain and feed efficiency of fish fed the various diets were significantly ( P < 0.05) affected and increased linearly as dietary threonine increased until plateauing around 0.8% of dry diet. Protein efficiency ratio and protein conversion efficiency values for fish fed the different diets also were significantly affected by threonine level and indicated requirement values of 0.8–0.9% of dry diet. Free threonine in plasma also significantly responded to increasing dietary threonine but indicated a slightly higher requirement value of approximately 1.0% of dry diet. Based on these data, the minimum threonine requirement of juvenile red drum was determined to be approximately 0.8% of dry diet (2.28% of dietary protein). This requirement level is similar to values reported for some other fish species. Based on this information diets may be formulated more precisely for aquacultural production of red drum.