• chloride ion regulation;
  • Na+;
  • K+-ATPase;
  • sodium

Salinity tolerance in wild (Glendale) and hatchery (Quinsam) pink salmon Oncorhynchus gorbuscha (average mass 0·2 g) was assessed by measuring whole body [Na+] and [Cl] after 24 or 72 h exposures to fresh water (FW) and 33, 66 or 100% sea water (SW). Gill Na+, K+-ATPase activity was measured following exposure to FW and 100% SW and increased significantly in both populations after a 24 h exposure to 100% SW. Whole body [Na+] and whole body [Cl] increased significantly in both populations after 24 h in 33, 66 and 100% SW, where whole body [Cl] differed significantly between Quinsam and Glendale populations. Extending the seawater exposure to 72 h resulted in no further increases in whole body [Na+] and whole body [Cl] at any salinity, but there was more variability among the responses of the two populations. Per cent whole body water (c. 81%) was maintained in all groups of fish regardless of salinity exposure or population, indicating that the increase in whole body ion levels may have been related to maintaining water balance as no mortality was observed in this study. Thus, both wild and hatchery juvenile O. gorbuscha tolerated abrupt salinity changes, which triggered an increase in gill Na+, K+-ATPase within 24 h. These results are discussed in terms of the preparedness of emerging O. gorbuscha for the marine phase of their life cycle.