Does environmental enrichment promote recovery from stress in rainbow trout?

The EU Directive on animal experimentation suggests that all protected animals should have enrichment to improve welfare yet relatively little research has been conducted on the impact of enrichment in fish. Studies employing enrichment in zebrafish have been contradictory and all fish species should be provided with species-specific enrichments relevant to their ecology. Salmonids are important experimental models in studies within aquaculture, toxicology and natural ecosystems. This study therefore sought to establish whether an enriched environment in an experimental aquarium may promote improved welfare in rainbow trout (Oncorhynchus mykiss) by enhancing their recovery from invasive procedures. Trout were held individually in either barren (no tank ornamentation) or enriched (gravel, plants and an area of cover) conditions. Recovery rates after a noxious stimulus and a standard stressor were investigated by monitoring behaviour, opercular beat rate and plasma cortisol concentrations. Fish were randomly assigned to one of four treatment groups: Control (undisturbed), Sham (handled but not manipulated), Stress (air emersion) and Pain (subcutaneous injection of acetic acid). The results suggest that for rainbow trout environmental enrichment appears to promote recovery and ameliorate adverse effects following a stressor. However, recovery rate did not differ between environments in the pain treatment groups. Thus environmental enrichment may not be an important factor when the fish is responding to a painful stimulus. These results have important implications for the husbandry and welfare of captive rainbow trout and possibly other salmonids and suggest that enriched environments may be preferable to barren environments in experimental studies.