Atlantic salmon spend their freshwater phase in recirculating aquaculture systems before sea transfer, with welfare challenges during the parr-to-smolt transition and vaccination procedures.
Atlantic salmon in freshwater production systems undergo the parr-to-smolt transformation during which their physiology transforms to enable seawater survival. Poor management of this transition through incorrect timing of seawater transfer causes osmoregulatory failure and prolonged suffering. Vaccination by intraperitoneal injection causes acute pain and may lead to chronic adhesion formation between organs. Continuous light regimes used to control maturation timing can disrupt normal behaviour and rest patterns. The freshwater phase welfare, while less researched than sea cage welfare, significantly affects the welfare trajectory of fish throughout their production cycle.