Effects of ocean acidification on salinity tolerance and seawater growth of Atlantic salmon Salmo salar smolts

Human activity has resulted in increasing atmospheric carbon dioxide (CO₂), which will result in reduced pH and higher levels of CO₂ in the ocean, a process known as ocean acidification. Understanding the effects of ocean acidification (OA) on fishes will be important to predicting and mitigating its consequences. Anadromous species such as salmonids may be especially at risk because of their rapid movements between fresh water and seawater, which could minimize their ability to acclimate. In the present study, we examine the effect of future OA on the salinity tolerance and early seawater growth of Atlantic salmon Salmo salar smolts. Exposure to 610 and 1010 μatm CO₂ did not alter salinity tolerance but did result in slightly lower plasma chloride levels in smolts exposed to seawater compared with controls (390 μatm). Gill Na⁺–K⁺‐ATPase activity, plasma cortisol, glucose and haematocrit after seawater exposure were not altered by elevated CO₂. Growth rate in the first 2 weeks of seawater exposure was greater at 1010 μatm CO₂than under control conditions. This study of the effects of OA on S. salar during the transition from fresh water to seawater indicates that elevated CO₂ is not likely to affect osmoregulation negatively and may improve early growth in seawater.