Positive welfare effects of physical enrichments from the nature-, functions- and feeling- based approaches in farmed rainbow trout (Oncorhynchus mykiss)

The present study aimed to investigate whether environmental enrichment had significant effects on rainbow trout's welfare through nature-, functions- and feeling-based approaches. Fish group behavior was analyzed during the rearing phase, as well as growth performance and health status. We assessed individual's emotional reactivity through a novel-tank test by measuring fear-related behaviors and stress-related physiological responses. Fish boldness and neophobia were then evaluated towards a novel object. We showed that more complex environments decreased aggression levels and improved growth, without impacting fish immune status. Enriched fish were also found less fearful when isolated in a novel tank and bolder when facing a novel object. We concluded that complexifying the environment through the addition of physical structures which stimulate and encourage fish to explore promotes rainbow trout's welfare in farming conditions, according to the three different welfare approaches.