Behaviour and spatial distribution of rainbow trout (Oncorhynchus mykiss) in experimental tanks: Influence of water flow and husbandry procedures

Rainbow trout ( Oncorhynchus mykiss ) is increasingly employed as an experimental model, yet knowledge of its behaviour in laboratory settings is limited. This study examined short-term (24 h), mid-term (1 week), and circadian changes in behavioural and spatial patterns of individually-housed trout under two experimental conditions differing in the extent of underwater current: control vs water flow. Moreover, the effects of feeding and tank cleaning were evaluated to identify potential behavioural indicators of welfare of the species in this context. Short-term effects revealed that flow rate impacted on swimming behaviour, with trout showing less species-specific behavior in the absence of water flow. Over time, trout exhibited a marked reduction of behavioral expression, interpretable as behavioural stabilization but likely reflecting suboptimal welfare. Trout behaviors were affected by the day time being less active in mornings. Event-related findings indicated an increased in active behaviors and a decrease in maintenance behaviours after both husbandry procedures, with that of cleaning being more pronounced and lasting remarkably longer, suggesting that the two reactions were fundamentally different. This study highlights the need to critically evaluate behavioural expression, and the need to provide species-specific husbandry conditions. We also provide foundational insights into rainbow trout behaviour valuable for refining research practices and improving welfare assessment of this experimental fish. • Tank cleaning and feeding elicit different behavioral responses in trouts. • Reduction in behavioural expression was observed during the first 24 hrs. • Further reduction over one week may indicate suboptimal welfare. • Circadian rhythmicity was observed with lower activity in the morning. • Water flow supports expression of species-specific behaviour.