Effects of Chronic Low-Temperature Exposure on the Behaviour of Adult Zebrafish (Danio rerio).

Temperature is a major environmental factor influencing fish behaviour and physiological function. In the present study, adult zebrafish were exposed to 18 °C for 21 days and compared with controls maintained at 26 °C to evaluate the behavioural effects of chronic low-temperature exposure. Behaviour was assessed using four individual-based paradigms-the novel tank diving test, light-dark preference test, social preference test, and mirror biting test-covering locomotion, exploration, social behaviour and anxiety-related responses. Chronic exposure to 18 °C was associated with a behavioural profile characterised by increased immobility, reduced exploratory activity, enhanced bottom-dwelling, and reduced active engagement with the mirror stimulus, whereas social preference towards conspecifics was largely preserved. Comparison with behavioural responses previously reported after 4 days of exposure to 18 °C suggests that several behavioural differences are still detectable after 21 days, indicating that the behavioural profile of fish maintained at 18 °C remained distinct from that of controls within the time window examined. These findings should be interpreted in light of the absence of independent tank-level replication. Previous studies conducted under comparable thermal conditions have reported molecular changes in the brain and in other neural-derived structures, such as the eye. Although these molecular findings were not generated in the present experiment and cannot be directly linked to individual behavioural endpoints, they provide a broader context for interpreting the behavioural effects of prolonged low-temperature exposure. Overall, the present findings show that chronic exposure to 18 °C is associated with persistent differences in locomotor, exploratory, and stimulus-directed behaviours, while social preference is comparatively preserved.