Growth and Long-Range Dispersal by Wild Subyearling Spring and Summer Chinook Salmon in the Snake River Basin

Wild juvenile spring and summer chinook salmon Oncorhynchus tshawytscha typically pass downstream in the Snake River as yearling smolts when migrating seaward. A small number of these fish disperse downstream from natal streams into the Snake River as subyearlings and then continue moving seaward. We estimated that the naturally produced subyearling spring and summer chinook salmon that dispersed into the Snake River in 1993, 1994, and 1997 originated from both wild (67%) and naturalized hatchery (33%) stocks. These fish were large in terms of mean fork length (range, 78–87 mm) during shoreline rearing. While moving seaward, they grew rapidly (range of means, 1.0–1.3 mm/d) to yearling smolt fork lengths (range of means, 122–128 mm). The estimated dispersal distances from natal streams to the first two dams encountered during seaward movement ranged from 172 to 810 km. Although we could not demonstrate seawater entry, we believe that the wild spring and summer chinook salmon had met the growth and size criteria for successful smoltification. We suggest that rapid growth to yearling smolt size reflects comparatively high growth opportunity in the Snake River and that this helps to explain why wild spring and summer chinook salmon that disperse into the Snake River migrate seaward as subyearlings while those that rear in presumably less productive natal streams migrate seaward as yearlings.