Collaborating effects of rearing density and oregano oil supplementation on growth, behavioral and stress response of Nile tilapia (Oreochromis niloticus)

To investigate the impacts of rearing density and oregano essential oil (OEO) supplementation on growth, behavioral and stress response of Nile tilapia, a factorial design (3 × 3 × 2) was assembled using 18 (100 L) glass aquaria where each aquarium had a rearing density and an OEO supplemental level. Six aquaria had a density of 10 fish aquarium−1 (low rearing density; LRD), another six had a density of 20 fish aquarium−1 (medium rearing density; MRD) and the remaining six aquaria had a density of 40 fish aquarium−1 (high rearing density; HRD). For each rearing density, fish were fed three diets with variable OEO levels (0.0, 1.0, and 2 ml kg−1 diet) and were assayed in two replicas for 10 weeks. OEO improved the growth performance of fish maintained at LRD and MRD. Tilapia exhibited behavioral changes in all densities with a reduced aggression in the HRD group. The OEO reduced the oxidative stress and the aggressiveness of juvenile tilapia as deduced from stress and welfare indicators' levels (cortisol antioxidant enzymes). In addition, a density-stress related effect as well as a nonspecific immune stimulant effect of OEO was detected from the increased antioxidant and NO activity in this study.