The Immunomodulatory Effects of Physical Enrichment in Rainbow Trout ( <i>Oncorhynchus mykiss</i> )

Environmental enrichment refers to the addition of stimuli or modifications to captive environments that promote the expression of natural behaviors and enhance animal welfare. While enrichment has the potential to enhance welfare, its effects on fish immune function remain unclear. This study examined whether enrichment modulates the immune system beyond welfare benefits. Growth (length, weight, and Fulton’s body condition factor [ K ]), immune parameters (leukocyte counts and respiratory burst activity), and serum markers (cortisol, myeloperoxidase, antiprotease, and lysozyme) were assessed in rainbow trout ( Oncorhynchus mykiss , n = 240). Trout (20 per aquarium, initial weight: 9.84 ± 0.77 g) were reared for 48 days in four conditions: horizontal (H, gravel), vertical (V, rubber cords), combined (VH), or a control (C, no enrichment) in 60 L glass aquaria (40 cm × 40 cm × 40 cm). Immune cell counts, including monocytes and lymphocytes, were significantly lower in enrichment setups compared to the C setup ( p &lt; 0.05), whereas spontaneous respiratory burst activity, although higher in enrichment setups, did not differ significantly. Growth and organ weights were unaffected by enrichment, except for a lower hepatosomatic index (HSI) in VH. No differences in serum markers were detected between setups. In conclusion, the V setup acts as an immunomodulator by reducing immune cell counts in rainbow trout but not affecting its respiratory burst activity. Disease resistance experiments are necessary to determine whether these immune changes affect susceptibility to infections, which is critical for assessing practical implications of V setup in aquaculture and research.