Fig. 4. (A) Gill Na⁺,K⁺-ATPase activity (µmol ADP mg^-1 protein h^-1), (B) plasma osmolality (mOsm), and (C) plasma [chloride] (mmol l^-1) levels for Chilko River sockeye salmon captured in seawater and freshwater during migration to the spawning grounds (distance, km). Sampling locations as identified in Fig. 1. The mouth of the Fraser River is defined as 0 km. Spawners were sampled in Horsefly River, a major spawning tributary in this system. The last two samples were collected on the spawning grounds (562 km; H, holding, S, spawners), but sexually mature fish are offset to prevent overlap of the data points. Sample size ranged from 30 (W, 2003) to 2 (HG, 2002). Values are means ± S.E.M. Symbols with a common letter do not differ significantly.