Random timing of aversive events extends stress responses in fish

Responses to potential stressors are shaped by their predictability. While predictable events may reduce negative effects, the impact of temporal uncertainty remains unclear. This study examined how different degrees of predictability influence stress responses. Nile tilapia ( Oreochromis niloticus ) were exposed to a confinement stressor under four conditions: predictable (fixed timing), random (variable timing), unpredictable (single unexpected event), and unhandled control. Ventilatory frequency (VF) was used as a non-invasive indicator of physiological activation and measured before and after exposure. All confinement treatments increased VF immediately after exposure. However, response patterns differed. In the predictable condition, VF increased but was not sustained. In contrast, both random and unpredictable conditions produced elevated and prolonged responses, with the strongest effect under complete unpredictability. Partial predictability (random condition), where the stressor was anticipated but not temporally defined, was sufficient to maintain activation. These findings show that temporal uncertainty plays a key role in sustaining stress responses. The results support the concept of emotional plasticity, in which physiological responses are dynamically modulated by environmental predictability, with implications for animal welfare and stress regulation.