Environmental enrichment as an immunostimulant for rainbow trout aquaculture.

Environmental enrichment (EE) offers a non-invasive strategy to enhance fish welfare, yet its immunological benefits in fish remain incompletely explored and rarely applied in aquaculture practice. This study assessed whether EE exposure alone can modulate immune responses in rainbow trout (Oncorhynchus mykiss). EE was implemented as simple physical structures added to otherwise barren tanks, either as horizontally placed gravel substrate (H) simulating a riverbed or vertically suspended rubber cords (V) simulating plant like structures. Tanks without enrichment served as barren controls (C). In experiment 1 (Exp1), trout reared with H, or V enrichment were compared to C for behavioural, neuroendocrine, and immune parameters. The H setup increased social interactions and showed signs of inflammatory activation in the head kidney leukocyte (HKL) functional assays. The V setup led to reduced monocyte counts and slightly higher spontaneous respiratory burst activity (SRBA), although the latter was not statistically significant (p > 0.05). In the subsequent experiment using a 2 × 2 factorial design combining enrichment and high (× 2) stocking density (Exp2), trout in the V setup maintained significantly higher immune activity (evident through elevated SRBA, cellular peroxidase content in the HKL, and serum lysozyme activity) under × 2 stocking density conditions (p < 0.05). In both experiments, the coefficient of variation (CV) for serum cortisol was higher in the V setup, including at × 2 stocking density, but not significantly different from the C setup (p > 0.05). Behavioural modulation was indirectly evidenced by a negative correlation between cortisol levels and body size, suggesting hierarchy formation. Overall, EE influenced neuroendocrine, behavioural, and immune parameters in trout, with V emerging as a practical immunostimulatory measure suitable for juveniles (< 1 year; 5-50 g).