Is social buffering consistent across the time of the day? A study in a diurnal (Nile tilapia Oreochromis niloticus) and a nocturnal fish species (tench Tinca tinca)

In social animals, interactions with conspecifics provide several benefits that ultimately enhance welfare and fitness, including the attenuation of individuals’ responses to environmental challenges often referred to as social buffering. This effect has been reported across several fish taxa. However, evidence suggests that social buffering is remarkably variable in fish, depending on intrinsic factors of species and environmental conditions. In this research, we explored whether social buffering effects vary throughout the time of the day in two fish species with opposed activity patterns: the diurnal Nile tilapia ( Oreochromis niloticus) and the nocturnal tench ( Tinca tinca) . To this goal, we collected behavioural indicators of welfare in fish subjected to two different social conditions (i.e., isolation or groups of five fish) for 24 h every 4 h. These indicators were based on well-reported behaviours associated with stress in fish, such as bottom-dwelling, freezing, activity and erratic movements. Our results revealed social buffering effects in both species in almost all analysed indicators, showing increased welfare in the presence of conspecifics. Critically, while the social buffering effect was constant throughout the day in tench, it varied between day and night in tilapia. Furthermore, the isolation condition disrupted daily rhythmicity only in tilapia, but not in tench, highlighting interspecific variation in the influence of conspecifics on behavioural rhythms. Overall, our findings supported the presence of social companions to improve fish welfare in captivity and underscored the importance of considering species-specific behavioural rhythms for tailoring management practices that minimise stress. • Social behaviour varies differently across the day in nocturnal and diurnal fish. • Social buffering was constant in tench but differed between day and night in tilapia. • Species-specific behavioural rhythms should be considered to improve fish welfare.