Dominance hierarchies within different size groupings of Nile tilapia (Oreochromis niloticus) and effects on growth and physiological responses

Hierarchies are prevalent in social animals and display of aggression by dominant individuals often results in appetite and growth suppressions in lower-ranked fish. This study investigated the effects of dominance hierarchies on growth and some physiological responses in Nile tilapia (mass range: 120–300 g). Fish were grouped based on body mass into three classifications of one dominant and three subordinates (1D:3S), two dominants and two subordinates (2D:2S) and four equal-sized (4ES) individuals, and each grouping was triplicated. Rates of aggressive interactions under conditions of food deprivation, hypoxia and increased temperature stressors were also assessed. The 4ES treatment was characterised by more frequent aggressive interactions, compared with the other groupings. The number of lost scales recovered from the different groups after 24 h of group formations were significantly higher in the 1D:3S group. Food deprivation resulted in progressively increasing rates of aggressive interactions up to 48 h, but decreased at 72 h in all treatments. Feed intake for the 4ES group was significantly higher for than the other groups, because access to feed was largely unrestricted for all individuals. The lower-ranked individuals in the 1D:3S and 2D:2S groups had lower growth rates, compared with the dominants. Subordinate fish under treatment 1D:3S and dominant individuals in 2D:2S had elevated mean red blood cells, haemoglobin, haematocrit and white blood cell counts relative to other fish. Although aggression counts were highest in the 4ES group, the results of this study provide evidence ensuring homogeneous weights improve feed intake and growth in Nile tilapia.