Enriched rearing environment enhances fitness-related traits of salmonid fishes facing multiple biological interactions

To improve stocking success in threatened populations, captive-reared animals are often familiarized to natural environment. However, whether such actions improve ability to cope with multiple biological interactions, such as competition, predation and parasitism that impose contradicting pressures on decision-making, is not well understood. Here, we explored short-term (4 weeks) and long-term (10 months) effects of enriched rearing on fitness-related traits of Atlantic salmon (Salmo salar). Salmon with different backgrounds of enrichment and parasite infection (eye fluke Diplostomum pseudospathaceum) were released to semi-natural ponds and monitored for activity, growth and predation susceptibility. Fish from enriched rearing showed lower short-term activity and higher short-term growth, suggesting that they coped better with novel conditions. However, predation susceptibility, and longer-term growth and survival, were unaffected by rearing treatment. Importantly, parasitism did not remove the positive effect of enrichment on growth, although the infection decreased both short-term and long-term growth and survival. These results suggest that enriched rearing can enhance fitness-related traits of stocked fish particularly during the critical early days, which can have important implications for stock enhancement activities.