DIETARY LEUCINE AND ISOLEUCINE REQUIREMENTS OF RED DRUM Sciaenops ocellatus AND THE EFFECTS OF EXCESS LEUCINE ON BRANCHED-CHAIN AMINO ACIDS UTILIZATION

Sergio Castillo* and Delbert M. Gatlin III
 
Department of Wildlife and Fisheries Sciences
Texas A&M University, College Station, TX 77843
sergio_sca@tamu.edu

A current priority in aquaculture is the potential replacement of fishmeal with alternative feedstuffs to ensure sustainability. However, most alternative protein sources are deficient in ≥1 indispensable amino acids (IAAs). Thus, there is a critical need to establish refined estimates of IAA requirements of fish. Although the red drum's dietary requirements for some IAAs have been determined, the need still exists to quantify all IAA requirements for this species. Therefore, the objective of this study was to quantify the dietary leucine (Leu) and isoleucine (Ile) requirements of juvenile red drum Sciaenops ocellatus and elucidate a possible antagonistic interaction of leucine with the other branched-chain AAs (BCAAs), which include valine (Val) and isoleucine (Ile).

Two separate feeding trials were conducted where a basal diet containing 35% crude protein [CP] and 3.2 kcal/g diet was prepared by combining lyophilized red drum muscle (RDM) as an intact protein (10.5% of dietary protein) and supplemented with crystalline L-AAs to simulate the pattern found in 35% CP from RDM. Leu and Ile contributed by RDM in the basal diet was analyzed at 0.89% and 0.50% of dry diet, respectively. In the first feeding trial, six experimental diets were supplemented with L-Leu in increments of 0.40% (0.90, 1.30, 1.70, 2.10, 2.50 and 2.90% dry diet) and kept isonitrogenous by adjusting the inclusion level of an aspartate/glutamate premix. In the second feeding trial, six experimental diets were supplemented with L-Ile in increments of 0.30% (0.50, 0.80, 1.10, 1.40, 1.70 and 2.00% dry diet); furthermore, a leucine-deficient diet (0.80%) and a diet with excess Leu (6.80%) were prepared, where the levels of Val and Ile were kept close to the estimated dietary requirements of red drum, 1.20% and 1.00% respectively. In both feeding trials, red drum juveniles were stocked in 38-L glass aquaria (15 fish/aquarium), and diets were fed to fish in triplicate aquaria at a rate approaching apparent satiation, twice daily, for 7 weeks.

Final values of weight gain and feed efficiency significantly improved as Leu level increased from 0.90% to 1.70% of diet; at higher inclusion levels of Leu there were no significant differences among treatments in any of the evaluated responses. Analysis of the weight gain data using a quadratic broken-line regression model estimated the Leu requirement of juvenile red drum to be 1.57% ± 0.17% of dry diet (Fig 1). The Ile feeding trial is currently ongoing; however, the dietary Ile requirement appears to be approximately, 1.05% of dry diet. The finalized Ile requirement, together with the effect of excess leucine on BCAAs' utilization will be included in the final presentation.