Development of seawater tolerance and subsequent downstream migration in wild and stocked young-of-the-year derived Atlantic salmon Salmo salar smolts

This study investigated the development of hypo-osmoregulatory capacity and timing of downstream migration in wild Atlantic salmon Salmo salar smolts from the River StjOrdalselva and stocked young-of-the-year (YOY), derived S. salar smolts from the tributary River Dalaa. Both wild and stocked S. salar smolts developed seawater (SW) tolerance in early May, persisting through June, measured as their ability to regulate plasma osmolality and chloride following 24h SW (salinity = 35) exposure. Although the majority of downstream migration among the stocked S. salar smolts occurred later than observed in their wild counterparts, the development of SW tolerance occurred concurrently. The wild S. salar from StjOrdalselva and stocked YOY smolts from the River Dalaa started to migrate on the same cumulative day-degrees (D degrees). The study revealed no downstream migration before development of SW tolerance. This emphasizes the importance of incorporating physiological status when studying environmental triggers for downstream migration of S. salar smolts. Overall, these findings suggest that the onset of smolt migration in stocked S. salar smolts was within the smolt window from an osmoregulatory point of view.