Husbandry management for Nile tilapia usually selects similar sized animals. Size similarity is one factor increasing agonistic behavior and might affect the social instability (as a cichlid, Nile tilapia exhibits social hierarchy). Increase in such aggressive interaction might also increase the stress husbandry, due the high probability of injury and energetic expenditure from social interaction. Although the effect of symmetry on the contests and social stress are well studied for salmonids and some cichlids species (Nile tilapia included), there is a lack of knowledge about genetically modified fish, as Thai Nile tilapia. Thus, the aim of this study is to test the effect of social instability on the levels of aggressiveness and social stress in Nile tilapia. Levels of aggressiveness and stress will be tested in two treatments with five reversed males on each: 1. with social instability - group of matched-size males, and 2. with social stability - group of no matched-size males. Groups will be kept by seven days and agonistic interaction will be recorded every day (20 min video recording). Social status will be achieved by Dominance Index and it will be correlated with cortisol levels and ventilatory frequency (social stress predictors). Agressiveness (frequency of agonistic acts) and cortisol levels will be analyzed by two-way ANOVA, and correlation will use Pearson's test. Social stability will be analyzed by Spearman's correlations, according to previous tests for Oreochromis niloticus and O. mossambicus. Fish welfare is considered on several ways, and all information about animal behavior in cultured fish is important to plan techniques for husbandry and management. Thus, this study will provide information that could help some new insights for husbandry and culture of the Thai Nile tilapia.
News published in Agência FAPESP Newsletter about the scholarship: