Fasting and its implications for fish welfare in Atlantic salmon aquaculture

Periods of fasting occur for a multitude of reasons in Atlantic salmon aquaculture. Feed withdrawal is widely used prior to transport, parasite treatments, preslaughter and for depuration purposes in recirculating aquaculture systems. Voluntary fasting is a coping response when fish have poor health or are exposed to poor farm environments. Owing to increased attention to animal welfare in aquaculture, concerns have been raised regarding ethical issues when farmed fish are subjected to fasting. However, thorough science-based recommendations for fasting and feed-withdrawal regimes have been lacking. The purpose of this review is to provide a synthesis of the various causes for fasting in Atlantic salmon aquaculture and evaluate their associated welfare implications so that guidelines for appropriate practices can be formulated. To interpret impacts, we describe biological responses and tolerance limits to fasting in Atlantic salmon and consider adaptations in the wild. Fry and parr are highly sensitive to feed withdrawal. However, post-smolts and adults are well-adapted to endure prolonged fasting without experiencing compromised functionality or health. Here, short periods of feed withdrawal prior to operations should therefore not constitute significant welfare concerns. Serious concerns are instead associated with voluntary fasting that may continue for weeks. We emphasize that environmental extremes that exceed appetite impairing thresholds must be avoided. Additionally, farmed fish should not be subjected to practices that lead to chronic stress that induce cessation of appetite. Diseases or parasites that impair appetite should also be mitigated. Fasting is here a symptom rather than a cause for poor welfare.