The post-smolt phase — the first weeks after Atlantic salmon are transferred from freshwater hatcheries to marine sea cages — is one of the highest-risk periods for welfare in the entire salmon production cycle.
Post-smolt welfare is determined largely by the quality of the smolt produced and the conditions of sea transfer. Fish that are not fully smoltified — transferred too early or at the wrong season — suffer from osmoregulatory failure that is physiologically analogous to severe dehydration in terrestrial animals. Transfer operations involving crowding, pumping, and well boat transport create cumulative stress that may exceed individual fish's capacity to recover, contributing to delayed mortality in the weeks after transfer. Once in sea cages, post-smolts face a cascade of novel challenges: predator encounters (seals, seabirds), environmental variation, and the immediate pressure of sea lice colonisation. Welfare-positive operations prioritise robust smolt quality assessment before transfer, use optimal temperature windows, minimise handling stress during transfer, and deploy early sea lice monitoring and management to prevent infestations reaching welfare-impacting levels before the fish are large enough to tolerate standard treatments.