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Two year-classes of juvenile spring chinook salmon Oncorhynchus tshawytscha from the Yakima River, Washington, were sampled from July (3-4 months postemergence) through May (yearling smolt out-migration). Physiological characters measured included liver glycogen, body lipid, gill Na ϩ -K ϩ ATPase, plasma thyroxine (T 4 ), and plasma insulin-like growth factor-I (IGF-I). Distinct physiological changes were found that corresponded to season. Summer and fall were characterized by relatively high body lipid and condition factor. Winter was characterized by decreases in body lipid, condition factor, and plasma hormones. An increase in condition factor and body lipid was found in February and March. Finally, April and May were characterized by dramatic changes characteristic of smolting, including increased gill Na ϩ -K ϩ ATPase activity, plasma T 4 , and IGF-I and decreased condition factor, body lipid, and liver glycogen. These results create a physiological template for juvenile spring chinook salmon in the drainage that provides a baseline for comparison with other years, populations, and life history types. In addition, this baseline provides a standard for controlled laboratory experiments and a target for fish culturists who rear juvenile spring chinook salmon for release from conservation hatcheries. The implications of these results for juvenile chinook salmon ecology and life history are discussed.At an organismal level, one must understand an

We examined the relation of size and growth rate to downstream migration in yearling spring chinook salmon Oncorhynchus tshawytscha. A group of juvenile chinook salmon was graded by size into small and large categories; half the fish in each category were reared at an elevated temperature beginning in mid‐February, resulting in four distinct treatment groups: large–warm, large–cool, small–warm, and small–cool. Fish from warmwater treatment groups displayed significantly higher growth rates through the spring than coolwater groups. Fish were released into a natural creek on two dates (25 March, release 1 and 12 April, release 2), and downstream movement was monitored. For each release, fish that migrated past a weir within the first 5 d postrelease had significantly higher spring growth rates than fish that did not migrate within that period. A similar comparison of release length to migration demonstrated significant differences only in release 2. Also for release 2, fish from the large–warm and small–warm treatment groups were recovered in higher proportions than fish from large–cool and small–cool groups. These results indicate that fish with relatively higher spring growth rates moved downstream sooner than fish with relatively lower growth rates. Furthermore, smolt size and migration were related with larger fish moving downstream sooner than smaller fish; however, this relation was weaker than that found between growth rate and migration.