Impact of physical and sensory enrichment on the physiological status and disease resistance of triploid rainbow trout (Oncorhynchus mykiss)

Environmental enrichment is increasingly recognised as a practical approach to improve the welfare of farmed fish. In rainbow trout ( Oncorhynchus mykiss ), a species of high economic and scientific relevance, few studies have simultaneously evaluated multiple dimensions of welfare under different enrichment strategies. Here, we investigated the effects of structural and sensory enrichments on growth, physiology, behaviour, neurochemistry, and disease resistance. Female triploid rainbow trout were reared for eight weeks under eight conditions: neutral control, positive stress control (no enrichment with additional handling stress), structural enrichments (rubber hose, rubber wall, floating grass mat, flexible laminaria), and sensory enrichments (continuous gas bubbles, blue light filter). Growth performance, blood markers of stress (cortisol, glucose, lactate), behavioural indicators, enrichment use, and brain monoamine turnover (5-HIAA/5-HT for serotonin; HVA/L-DOPA for dopamine) were assessed at three time points in eight weeks. To evaluate robustness, fish were subsequently challenged with Aeromonas salmonicida subsp. salmonicida . No significant differences in growth were detected across conditions, showed the highest weight gains, raising the hypothesis that they could provide benefits over longer rearing cycles, a question that will require dedicated long-term trials. Behavioural analyses indicated high use of structural enrichments and reduced stress-related behaviours under the blue filter and rubber hose conditions. Stress physiology revealed condition-specific reductions in lactate or cortisol, but no consistent enrichment effect across markers. Neurochemical analyses demonstrated condition-dependent modulation of serotonergic and dopaminergic turnover, with laminaria increasing both systems, blue filter reducing dopaminergic activity, and bubbles strongly decreasing serotonergic turnover. Infection outcomes showed slower disease progression in rubber hose, laminaria, blue filter, and rubber wall conditions, with rubber hose fish exhibiting the slowest mortality kinetics. Overall, our integrative approach highlights that environmental enrichment modulates multiple welfare dimensions in rainbow trout, with laminaria, blue filter, grass mat, and rubber hose emerging as the most promising strategies. These findings support enrichment as a valuable tool to promote fish welfare and resilience, while underlining the need for long-term and context-specific validation before implementation in aquaculture.