Smoltification — the physiological transformation that prepares Atlantic salmon for saltwater life — is a critical welfare-sensitive period in both wild and farmed salmon populations.
Smolt welfare is a specialised welfare domain requiring understanding of salmon physiology. The smoltification process itself produces behavioural and physiological changes — including increased swimming activity, rheotactic reversal, and schooling behaviour — that indicate a state of heightened physiological readiness rather than stress when conditions are appropriate. Welfare problems arise when this transition occurs in suboptimal conditions: premature seawater transfer causes chronic osmotic stress and immune suppression; delayed smoltification in RAS systems from inappropriate photoperiod management prevents the physiological preparation for saltwater. In wild smolts, downstream migration is predation-intensive, but the primary human-caused welfare harm is dam passage injury and delay. Fish passage infrastructure must be designed for rapid, unimpeded smolt passage with minimal turbine contact. Monitoring smolt quality — through physiological and behavioural indicators of readiness — is both a welfare and productivity intervention, as high-quality smolts survive seawater entry better and grow more efficiently.