Characterization of the smoltification process and post-sea transfer stunts of Atlantic salmon (salmo salar) in full-scale intensive production

Stunting, characterized by impaired growth following seawater transfer, poses a significant challenge in the aquaculture of Atlantic salmon ( Salmo salar ). Stunts exhibit markedly reduced growth compared to normally developing conspecific individuals. This condition is associated with increased mortality and heightened susceptibility to infectious agents, compromising both fish welfare and profitability through decreased production efficiency. This study investigates the underlying mechanisms of stunting by analyzing key physiological markers of smoltification, including gene expression of gill-specific ion transporters and genes involved in endocrine regulation, as well as hormone profiles in salmon raised in a full-scale aquaculture facility. A longitudinal approach was employed, sampling from pre-smolt through early post-smolt phases to characterize the physiological adaptations critical for seawater tolerance. Gill gene expression analysis showed distinct patterns in the Na + /K + -ATPase (Nka) subunits. Elevated nkaα1a and lower nkaα3 expression in the stunted group suggested incomplete osmoregulatory adaptation. The expression patterns of pituitary tshβb , which are linked to photoperiodic regulation, demonstrated a potential desynchrony within the population, suggesting individual variation in photoperiod responsiveness before seawater transfer. Stunts had significantly lower plasma levels of thyroxine (T4), triiodothyronine (T3), and growth hormone (Gh), but cortisol levels were similar to normal salmon. These results highlight crucial endocrine and osmoregulatory mechanisms that differ from those in normally developing salmon and may be linked to the underlying cause of stunting, particularly when connected to compressed photoperiod smoltification regimes during intensive salmon farming production. • Comprehensive description of endocrine and hormonal changes from fresh water pre-smolt phase to post sea transfer smolts. • Comprehensive physiological characterization of stunted Atlantic salmon occurring in early marine stage of production. • Possible illumination of the possible importance of the nkaα3 isoform functions in osmoregulation adaptation to seawater. • New insights into tshβb expression during the smoltification process.