This study aimed to elucidate the effects of selenium-loaded chitosan nanoparticles used as a dietary supplement on Nile tilapia (Oreochromis niloticus) antioxidant and growth responses. First, chitosan-based nanoparticles containing selenium (Se) were synthesized using the ionotropic gelation method and their physicochemical characteristics, controlled release profile, and antioxidant activity properties were investigated. Thereafter, the effects on glutathione peroxidase and antioxidant activities (by radical scavenging activity), growth, and whole-body composition of Nile tilapia were evaluated when they were fed with Se-loaded chitosan nanoparticles and compared with other selenium dietary supplements. Se-loaded chitosan nanoparticles showed high entrapment efficiency (87%), spherical shape, smooth surface, and broad size distribution. The controlled release of Se consisted of an initial burst followed by a gradual release over 48 h. Se-loaded nanoparticles presented significantly higher antioxidant activity compared to free Se. A 60-day feeding trial was conducted to compare the effects of supplementing different dietary Se sources, including selenomethionine (as organic source), sodium selenite (as inorganic source), and Se-loaded chitosan nanoparticles (Se-Nano and Se-Nano x1.5) on antioxidant and growth responses of Nile tilapia. A basal diet without Se supplementation was used as the control. The dietary supplementations with different Se sources (free and encapsulated selenium) lead to significant improvements in final weight and feed efficiency of Nile tilapia fingerlings. However, dietary treatments did not affect whole-body protein and lipid content. Diets containing Se-Nano and Se-Nano x1.5 were more effective than sodium selenite and selenomethionine in preventing oxidative stress and improving antioxidant activity in Nile tilapia. Overall, Se-loaded nanoparticles presented a great potential as an efficient source for delivering dietary Se to Nile tilapia, directly affecting the growth performance, feed efficiency, oxidative stress, and antioxidant activity of this species.

The high cost and limited availability of fish oil makes plant‐derived lipids attractive for aquafeed manufacturing, but replacing fish oil with these lipids can result in long‐chain polyunsaturated fatty acid (LC‐PUFA) deficiencies. Fatty acid metabolism, specifically the efficiency of LC‐PUFA utilization, may be influenced by the dietary saturated fatty acid (SFA) content versus that of C₁₈ polyunsaturated fatty acids (PUFAs). We assessed the growth and tissue composition of Cobia Rachycentron canadum (55.3 ± 0.2 g initial weight [mean ± SE]; 10 fish/tank, 3 tanks/diet) fed diets (∼49% protein, ∼10% lipid) containing fish oil; 22:6(n‐3)–amended standard, partially hydrogenated, or fully hydrogenated soybean oil; and these same soybean oils supplemented with soybean lecithin for 8 weeks. Although survival (range = 97–100%), final weight (160–189 g), and feed conversion ratio (1.40–1.52) were unaffected by diet, differences were observed in weight gain (185–241%), specific growth rate (1.87–2.19% body weight/d), and feed intake (2.94–3.44% body weight/d). Significant effects of soybean oil type on final weight, weight gain, feed conversion ratio, specific growth rate, and feed intake were noted, with standard soybean oil generally outperforming the other soybean lipids when oil types were pooled across phospholipid supplementation treatments, whereas phospholipid supplementation had no significant effect on any of the performance measures. Differences in dietary fatty acid profile yielded differences in tissue composition. Feeding standard soybean oil resulted in the most greatly modified profiles, whereas the profiles of fish fed fully hydrogenated, completely saturated soybean oil were most similar to those of the fish oil–fed fish. The magnitude of profile change was greatest in the liver and fillet tissues and smallest in the eye and brain tissues. Although further research is necessary to demonstrate whether SFA‐rich lipids can effectively reduce the LC‐PUFA requirements of Cobia, it is clear that SFA‐rich oils offer a strategic advantage in minimizing the effects of fish oil replacement on tissue fatty acid profile.