Smolt

A smolt is a juvenile anadromous salmonid, principally Atlantic salmon (Salmo salar), that has undergone parr-smolt transformation: a hormonally driven physiological change that equips the fish to migrate from fresh water to sea water. Visible markers include silvery body coloration, black fin margins, and a slender body; internally, the gills remodel their chloride cells and the kidneys and intestine switch ion-transport direction so the fish can osmoregulate in full-strength sea water (30–35 ppt) instead of fresh water. Smoltification is triggered by lengthening photoperiod and is graded in commercial hatcheries using the gill Na+,K+-ATPase activity test and the seawater challenge test (measuring plasma chloride after 24 h in 35 ppt). Releasing under-sized or under-developed smolts to sea causes high post-transfer mortality and poor growth, a costly and irreversible error. For salmon farmers, smolt production is a precisely scheduled RAS-based phase where light, temperature, and feeding are manipulated, and smolt status determines the transfer date to sea cages — the single biggest calendar decision in the salmon production cycle.