Smoltification

Smoltification (parr–smolt transformation) is the coordinated set of physiological, morphological, and behavioural changes that prepare juvenile salmonids for life in saltwater. Changes include: silvering (iridophore deposition replaces parr marks), increased gill Na⁺/K⁺-ATPase activity (enabling seawater osmoregulation), increased seawater tolerance, downstream migratory behaviour (positive rheotaxis reversal), increased metabolic rate, and changes in growth hormone and thyroid hormone profiles. Smoltification is triggered by the interaction of photoperiod (increasing daylength) and temperature in spring. The "window" for successful seawater entry is narrow; fish that miss it (e.g. due to dam obstruction) desmoltify and revert to parr physiology.

Atlantic salmon smolts must complete seawater transfer within the smolt window (typically 3–8 weeks in spring) or lose seawater tolerance. This is a critical vulnerability in hatchery programmes.