Disruptive effects of stress contagion on zebrafish (Danio rerio) shoaling.

Social behaviour involves constant interactions that can modulate emotional states. In fish, emotional contagion - encompassing the spread of both negative and positive affective conditions - has been documented but remains underexplored in group contexts. Here, we investigated how the proportion of stressed individuals within a zebrafish (Danio rerio) group affects collective behaviour in response to a threat. We induced stress in subsets of fish and formed groups of eight with varying ratios of stressed (S) to non-stressed (NS) individuals (0S8NS to 7S1NS; n = 8). Group behaviour was assessed immediately after group formation and again 24 h later following exposure to alarm substance (AS). Our results showed that group cohesion is higher when more nonstressed animals compose it, although locomotor parameters were not changed. After AS exposure, groups with more than 25% of stressed individuals exhibited significant behavioural alterations compared to controls, with increased immobility and altered shoal cohesion under AS. These changes suggest a threshold for stress contagion within the group, where a critical proportion of stressed members amplifies group-level stress responses. Interestingly, groups with similar internal states - whether predominantly stressed or not - were more cohesive during AS exposure, highlighting the importance of social synchrony in stress modulation. This work underscores that stress contagion in zebrafish depends on both the number of stressed individuals and the perceived threat, supporting the idea that social and emotional cues influence collective behaviour in fish. [ABSTRACT FROM AUTHOR]