Smolt transfer from freshwater hatcheries to marine sea pens is a critical welfare juncture in Atlantic salmon aquaculture. The physiological changes of smoltification must align with transfer timing to minimise welfare compromise.
Smolt transfer represents a fundamental physiological challenge for salmon as they transition between freshwater and marine environments. The timing of transfer relative to the smoltification window is the primary welfare determinant: fish transferred too early or after the physiological window has closed face osmoregulatory failure and death. Well-boat transfer with seawater acclimatisation allows gradual osmotic adjustment and is welfare-preferable to direct freshwater-to-seawater transfer. The transfer itself involves crowding, handling, and pumping that cause acute stress responses. Post-transfer welfare management in the first weeks in sea pens is critical: sufficient oxygen, appropriate feeding, and monitoring for disease that exploits post-transfer immune vulnerability all affect outcomes. Industry investment in photoperiod manipulation and hormone monitoring to optimise smolt transfer timing has significantly improved welfare outcomes over historical practices.