Repeatability of Boldness and Aggression in the Nile Tilapia (oreochromis Niloticus)

This study examined the repeatability of boldness and aggression in Nile Tilapia (Oreochromis niloticus), with particular focus on sex differences and the potential heritability of these behavioural traits. Boldness was defined as the tendency to engage in risk-taking behaviour, while aggression referred to hostile interactions such as ramming or nipping. Using open-field and mirror tests, 80 fish with a mean weight and standard length of 46.8g and 10.9cm (40 males and 40 females), respectively, were individually observed across two testing sessions to determine behavioural consistency. Results revealed that boldness exhibited moderate to high repeatability, especially in females (r = 0.73), indicating a stable and potentially heritable trait. Males showed moderate repeatability (r = 0.35), and both sexes combined yielded an intermediate value (r = 0.51). In contrast, aggression showed low repeatability in both sexes (r < 0.15), suggesting it is more context-dependent and less influenced by stable individual differences. Females were significantly bolder than males, although aggression levels did not significantly differ between sexes. These findings highlight boldness, especially in females, as a consistent behavioural trait that may be useful in selective breeding programs aimed at improving welfare and productivity in aquaculture. Conversely, the low repeatability of aggression indicates the need for repeated, context-specific assessments if aggression is to be used as a selection criterion. The study contributes to the growing field of fish behavioural ecology by demonstrating sex-specific behavioural stability and its potential evolutionary and applied significance.