Fig. 2. (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 Harrison sockeye salmon captured in seawater and freshwater during migration to the spawning grounds (distance, km). The mouth of the Fraser River is defined as 0 km. Sampling locations as identified in Fig. 1. Spawners were sampled in Weaver Creek, a major spawning tributary in this system. The last two samples were collected on the spawning grounds (117 km; H, holding, S, spawners), but sexually mature fish are offset to prevent overlap of the data points. Sample size ranged from 54 (JS, 2003) to 3 (holding, 2003; W, 2002). Values are means ± S.E.M. Symbols with a common letter do not differ significantly.