Migration Characteristics of Hatchery and Natural Spring Chinook Salmon Smolts from the Grande Ronde River Basin, Oregon, to Lower Granite Dam on the Snake River

Smolts of spring Chinook salmon Oncorhynchus tshawytscha experience substantial mortality while migrating through free-flowing reaches of the Snake River basin before reaching Lower Granite Dam, the first dam encountered in the Columbia-Snake river hydrosystem. We investigated the patterns of travel time and survival of hatchery and natural smolts fitted with passive integrated transponder (PIT) tags through specific reaches of the migration corridor during the 2000-2006 migration years for two populations originating in the Grande Ronde River basin (Lostine River and Catherine Creek). For both populations. median travel times for natural smolts were significantly longer in the upper reaches of the migration corridor but shorter in the lower reaches than for their hatchery counterparts. Also, among both hatchery and natural smolts, smaller individuals spent more time in the upper reaches, presumably feeding to attain a larger size before continuing their migration. Within populations, both hatchery and natural smolts showed similar patterns of survival through the reaches of the migration corridor above Lower Granite Dam. Size-selective mortality was evident for hatchery and natural smolts from both populations. especially in the upper reaches, larger individuals experiencing higher survival. The Catherine Creek population experienced the majority of natural and hatchery smolt mortality (32.8-65.8%) in a relatively short (91-km), low-gradient reach immediately below the summer rearing habitat. In contrast, the Lostine River natural and hatchery smolts experienced lower mortality (3.6-46.1%) in a 174-km reach below the summer rearing habitat. The results of this study demonstrate the dynamic nature Of Survival and migration rate among spring Chinook salmon smolts during their initial seaward migration from tributaries.