Comparing the stress physiology of hard- and soft-released juvenile Atlantic Salmon after transportation for reintroduction

Abstract Objective We examined whether an extended acclimatization period prior to release (soft release) can allow transported fish to recover from the physiological stress associated with transport compared with conventional release methods, which provide fish with no acclimatization period prior to release (hard release). Methods We monitored an Atlantic Salmon Salmo salar stocking team during a standard reintroduction operation and compared their conventional hard-release method (i.e., immediate release after transport with no acclimatization period) to a soft-release method (i.e., 2 and 4 days in-river acclimatization prior to release). Following a 2.5-h transport event, hard-release fish were immediately blood-sampled for their physiological stress response (cortisol, glucose, and lactate). Soft-release fish were blood-sampled for their physiological stress response following 2 or 4 days of in-river acclimatization. Result While hard- and soft-release fish demonstrated significantly higher cortisol, glucose, and lactate concentrations compared with control fish, cortisol concentrations remained elevated for both the hard- and soft-release groups. However, glucose and lactate concentrations were significantly lower in soft-release fish compared with hard-release fish. Conclusion Soft-release provides fish an extended acclimatization period that was found to impact transport-related physiological stress in fish. Our findings will inform management agencies and practitioners focused on improving the success of salmonid stocking and reintroduction programs.