Do Optimum Dietary Protein and Early Mild Stress Events Prepare Oscar (astronotus Ocellatus) for a Stressful Future?

This study aimed to investigate the effects of dietary protein (380 vs 510 g/kg) and duration of early mild stress (0, 2 and 4 weeks) on growth, haematology, serum biochemistry, immunological response, antioxidant system, liver enzymes, and stress responsiveness of oscar (Astronotus ocellatus; 4.7 +/- 0.6 g). After the nine-week feeding trial, all fish were exposed to final acute AC stress (acute confinement stress) and sampled for serum and haematological parameters. Fish fed dietary 510 g/kg protein grew faster (gain 54.21 g vs 42.98 g) and had a higher condition factor (2.22 vs 1.81) than those fed diets containing 380 g/kg protein after ten weeks feeding ad libitum. After AC stress, oscars subjected to four-time stress (4EMS) had a higher survival rate than the 0EMS group (77.08% vs 64.59%). Additionally, white blood cells, blood performance, total protein, high-density lipoproteins, immunoglobulin M, complement component 4, complement component 3, superoxide dismutase, and glutathione peroxidase were significantly greater in the two-time stress (2EMS) group than the 0EMS treatments. Although fish welfare must be considered, mild stress during the trial can reduce glucocorticoid feedback sensitivity in fish, resulting in greater survival after the acute stress reaction at the end of the study.