Dietary L-Ornithine L-Aspartate Alleviates Ammonia Load and Improves Hepatic Health in Largemouth Bass (Micropterus salmoides)

The intensive culture of largemouth bass (Micropterus salmoides) under high-protein feeding conditions is frequently associated with ammonia nitrogen stress, which adversely affects hepatic health. L-Ornithine L-aspartate (LOLA) has been proposed as a regulator of nitrogen metabolism. However, its effects in aquafeeds remain unclear. Five experimental diets (LOLA0, LOLA3, LOLA6, LOLA9, LOLA12) containing graded levels of LOLA (0%, 0.03%, 0.06%, 0.09%, 0.12%) were formulated, along with a diet supplemented with 0.09% L-ornithine + L-aspartate (OA9). Each diet was fed to fish reared in triplicate cages (30 fish per cage, initial body weight: 7.38 ± 0.02 g) for 10 weeks. No significant effects were observed on growth performance (weight gain, specific growth rate) or feed utilization (feed conversion rate, protein efficiency ratio) among the different LOLA treatments (p > 0.05). Nevertheless, second-order polynomial regression analysis revealed that dietary inclusion of 0.05–0.08% LOLA significantly reduced plasma ammonia and urea nitrogen levels, decreased hepatic aspartate aminotransferase and glutamate dehydrogenase activities, as well as plasma adenosine monophosphate deaminase activity, activated the hepatic TOR-S6K1-related markers, and reduced mortality under ammonia nitrogen stress (p < 0.05). Furthermore, supplementation with 0.07–0.08% LOLA significantly enhanced glutathione peroxidase and superoxide dismutase activities in both serum and liver, as well as serum levels of lysozyme and immunoglobulin M. Additionally, compared with the OA9 group, the LOLA9 group showed more pronounced changes in TOR-S6K1-related markers, ammonia load, antioxidant and immune parameters. Overall, while no significant growth enhancement was observed, these biochemical and physiological modifications suggest that LOLA supplementation is associated with alterations in nitrogen metabolism, antioxidant, and immune indices in largemouth bass, but further studies are needed to confirm the functional significance of these changes.