My stress, our stress: Blunted cortisol response to stress in isolated housed zebrafish

•Long-term isolated zebrafish presented a reduced cortisol response to stress.•Stress response of isolated fish depends solely on their own stressor perception.•The introduction of a stressed shoal in a resident non-stressed shoal induces stress in all fish.•Stress response of grouped fish was augmented by chemical cues from the other members of the shoal.

Here, we show that individually housed zebrafish presented a reduced cortisol response to an acute stressor (persecution with a pen net for 120 s) compared to zebrafish housed in groups of 10. We hypothesized that the cortisol response to stress was reduced in individually housed zebrafish because they depend solely on their own perceptions of the stressor, whereas among grouped zebrafish, the stress response might be augmented by chemical and/or behavioral cues from the other members of the shoal. This hypothesis was based on previous described chemical communication of stress in fish as well on individual variation in stressor perception and potential individual differences in fish personality.