Feed withdrawal is a widespread practice in Atlantic salmon aquaculture to empty the gut prior to major farming operations. Moreover, emerging production practises such as RAS and offshore farm sites also occasionally subject fish to prolonged fasting periods while suboptimal farm environments and diseases can induce voluntary fasting. However, such extended fasting periods may conflict with ethical and legal obligations to farm animals. Presently, science-based recommendations on responsible fasting times that consider fish welfare are lacking for Atlantic salmon and other aquaculture species. We have investigated physiological, environmental, behavioural, growth, and welfare related effects of prolonged fasting periods in a series of four experimental studies on Atlantic salmon post-smolts. The purpose of this work was do define welfare guidelines for allowable fasting periods in aquaculture by identifying thresholds for significant impairments in performance traits. We conclude that Atlantic salmon are well adapted to cope with prolonged fasting periods without suffering poor welfare or physiological impairments. When poor welfare is observed in fish that have been fasting for long periods, the underlying cause will likely instead by suboptimal environmental conditions or diseases, not fasting on its own. Formulating welfare guidelines for allowable fasting periods may therefore ultimately be redundant since the required time to initiate severe starvation takes much longer than any realistically encountered fasting period in modern aquaculture practices.