Variation in skin barrier function throughout smoltification in Atlantic salmon (

Introduction: Smoltification involves distinct morphological, physiological and behavioral changes in Atlantic salmon, yet skin barrier function has not been functionally assessed throughout this transition. Method: We measured electrophysiological parameters at three life stages - parr, smolt, and post-smolt - to assess changes in skin permeability and transport capacity. Results: Parr skin showed significantly lower transepithelial resistance (higher permeability) and more negative short-circuit current than smolts under identical Ringer`s conditions. Seawater exposure elevated transepithelial potential in post-smolt relative to smolt, while freshwater exposure elevated short-circuit current in smolt relative to parr. No significant differences in mRNA expression of selected tight junction genes were found between life stages. We detected no significant correlation between skin permeability and any of the selected genes. Discussion: The results suggests that other claudin isoforms or posttranscriptional mechanisms underlie the observed functional changes. From a husbandry perspective, elevated skin permeability following seawater exposure highlights the importance of prioritizing skin health during and after smoltification, as skin function may be compromised at this time.