Does environmental enrichment mitigate parasite infection by reducing stress in Masu Salmon Oncorhynchus masou ?

ABSTRACT Objective Captive animals are exposed to various stressors originating from their artificial environment. Although stress can initially be an adaptive physiological response, persistent stress generally decreases immune function and makes animals more susceptible to pathogens. Environmental enrichment is a strategy that has been widely used to mitigate excessive stress and enhance animal welfare, but the effectiveness of individual enrichment components remains controversial. Hence, this study aimed to reveal the effect of shelter enrichment on stress levels and parasite infections in Masu Salmon Oncorhynchus masou. By using a single component, the study will help to provide a better understanding of the underlying mechanism of the enrichment effect. Methods We explored whether shelter-enriched conditions relieved stress in Masu Salmon by evaluating plasma cortisol levels. Fish provided with shelters were expected to experience less aggression from conspecifics and less stress from human disturbance. Moreover, we conducted a parasite challenge test using Ichthyobodo sp. to determine whether the presence of shelters inhibited infection by mitigating stress levels. Results Fish in enriched tanks had significantly lower basal cortisol levels than those in control tanks, suggesting that they were less affected by stressors. Although the difference was not significant, fish in enriched tanks also tended to experience less infection by the parasites than fish in control tanks. Conclusions Given these results, simple shelter enrichment appears to be an effective method for mitigating stress among captive fish and reducing stress intensity or frequency. In addition, shelter enrichment could be used to inhibit infectious diseases, although further work is needed to prove the effect. These findings confirm that shelter enrichment has measurable effects on the factors related to fish welfare, which should encourage the adoption of enrichment in fish farms.