Physiological status of naturally reared juvenile spring chinook salmon inthe Yakima River: Seasonal dynamics and changes associated with smolting

Two year-classes of juvenile spring chinook salmon Oncorhynchus tshawytscha from the Yakima River, Washington, were sampled from July (3-4 months post emergence) through May (yearling smolt out-migration). Physiological charac ters measured included liver glycogen, body lipid, gill Na+-K+ ATPase, plas ma thyroxine (T-4), and plasma insulin-like growth factor-I (IGF-I). Distin ct 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, an d plasma hormones. An increase in condition factor and body lipid was found in February and March. Finally, April and May were characterized by dramat ic changes characteristic of smelting, including increased gill Na+-K+ ATPa se activity, plasma T-4, and IGF-I and decreased condition factor, body lip id, and liver glycogen. These results create a physiological template for j uvenile spring chinook salmon in the drainage that provides a baseline for comparison with other years, populations, and life history types. In additi on, this baseline provides a standard for controlled laboratory experiments and a target for fish culturists who rear juvenile spring chinook salmon f or release from conservation hatcheries. The implications of these results for juvenile chinook salmon ecology and life history are discussed.