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In preparation for migration from freshwater to marine habitats, Atlantic salmon undergo smoltification, a springtime transformation that includes the development of seawater tolerance. The ability of smolts to inhabit seawater is attained through the coordinated development of ion and water‐transporting processes in key organs such as the gill, kidney, and intestine. To promote solute‐linked water absorption, the intestinal epithelium of fishes acclimated to SW secretes HCO3− to form luminal Ca2+ and Mg2+ precipitates. As a source of HCO3− for apical anion exchange, Na+/HCO3− cotransporter 1 (Nbce1) facilitates the transport of plasma HCO3− into the interior of epithelial cells across the basolateral membrane. While Atlantic salmon (Salmo salar) express three nbce1 isoforms (nbce1.1, nbce1.2a, and nbce1.2b), there is no information on whether their expression corresponds with the development of intestinal fluid absorption that occurs during smoltification. We first determined that nbce1.1 and nbce1.2b gene transcripts were highly expressed in pyloric cacae and anterior intestine of smolts, whereas nbce1.2a showed low expression in the intestine but robust expression in gill. Next, we sampled salmon undergoing parr‐smolt transformation and observed elevated branchial Na+/K+‐ATPase activity in May characteristic of smoltification. Moreover, smolts exhibited an improved ability to maintain ionoregulatory balance when transferred to seawater in May versus March. We will report corresponding patterns of nbce1.1 and nbce1.2b expression in pyloric cacae and anterior intestine during smoltification and following seawater transfer to resolve whether their transcriptional patterns correspond with the acquisition of intestinal fluid absorptive processes. Support or Funding Information Supported by the National Science Foundation (IOS‐1755131).

The life history of anadromous Atlantic salmon (Salmo salar) entails an initial freshwater stage that precedes their migration to marine environments. The development of physiological systems that will ultimately support the survival of juvenile salmon upon their entry into marine habitats is termed parr‐smolt transformation. This springtime transformation (smoltification) includes the acquisition of seawater (SW) tolerance. Thus, the ability of smolts to successfully acclimate to SW is attained through the development of branchial pathways for ion‐secretion. In addition, the capacity to rapidly attenuate ion‐absorptive processes upon exposure to SW is a vital aspect of SW tolerance. In the current study, we investigated the branchial gene expression patterns of three Na+/HCO3−cotransporter 1 isoforms (nbce1.1, nbce1.2a, and nbce1.2b) during smoltification and following seawater challenges. Given the proposed roles of Nbce1s in the absorption of environmental Na+ by branchial ionocytes in other teleosts, we hypothesized that nbce1s would be downregulated in salmon undergoing smoltification and/or following SW exposure. In two separate years, we observed springtime increases in branchial Na+/K+‐ATPase activity, Na+/K+/2Cl−‐cotransporter 1 and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no seasonal changes in the expression of nbce1 isoforms. On the other hand, we observed diminished nbce1.2a expression within 2 days after the transfer of parr and smolts to SW that remained low throughout the experiment (>2 weeks). Collectively, our results suggest that while there are no clear seasonal nbce1 dynamics during smoltification, the modulation of nbce1.2a expression upon exposure to SW may underlie the attenuation of environmental Na+ uptake. Support or Funding Information Supported by the National Science Foundation (IOS‐1755131).