Two feeding trials were conducted to evaluate possible dietary arginine-lysine antagonisms in the red drum (Sciaenops ocellatus). In the first feeding trial, seven dietary treatments with arginine levels of 1.0, 1.25, 1.5 (previously established requirement), 1.75, 2.0, 2.5 and 3.0% of dry diet with lysine at the previously established requirement of 1.6%, and three diets with arginine at 1.75% and lysine at 1.75, 2.05 and 2.65% of dry diet were evaluated. In the second trial, similar treatments were used except the highest level of arginine (2.8% of dry diet) was combined with a higher level of lysine (2.5% of dry diet) to evaluate low and high levels of arginine and lysine in a factorial arrangement. Each feeding trial was conducted with juvenile red drum in 38-L aquaria connected as a recirculating system with the first trial continuing for 9 weeks and the second trial for 8 weeks. In the first feeding trial, fish were stocked as groups of 12 fish initially averaging 1.57 g/fish and in the second trial groups consisted of 15 fish initially averaging 2.51 g/fish. The feeding rate was initially set at 6% of body weight per day and adjusted equally among dietary treatments as the fish grew to maintain a level close to satiation without overfeeding. Results from both feeding trials indicated that red drum fed the lowest dietary arginine level had the lowest weight gain. Other parameters such as feed efficiency, protein retention and plasma amino acid concentrations also showed that high levels of dietary arginine did not significantly increase fish performance or interfere with lysine utilization. In terms of dietary lysine, increasing concentrations above the previously determined requirement level tended to increase growth performance of the fish, although a significant difference was only found in the first trial. Therefore, based on the results of both feeding trials, no antagonism between dietary arginine and lysine was evident in red drum.
