The Effects Two Pre-Smoltification Life History Expressions Have on Juvenile Chinook Salmon Growth, Migration Timing, and Fork Length in a Wilderness Environment

Declines in populations of Chinook salmon, Oncorhynchus tshawytscha, in the United States have spurred collaborative multi-state, multi-agency efforts to try to understand the dynamics of threatened populations in an attempt to manage and restore them. In this study we look at the physical outcomes of the two distinguishable life history expressions, early and late smoltification, for juvenile Chinook salmon in a designated wilderness area in Central Idaho. Our methods combine information from tracking individually tagged fish from natal rearing through detection sites distributed throughout the Columbia River basin with environmental (e.g. temperature) and ecological (e.g. density) information at juvenile rearing sites.  We found that juvenile Chinook that move out of natal reach rearing areas prior to their first winter were on average 23 mm larger during spring migration, have nearly double the winter growth rate, and enter the main stem Snake River an average of 9 days earlier during spring migration than individuals that resided in natal reach rearing areas. We are currently modeling the driving climatic, hydrologic, and ecological variables that might influence an individual’s overwinter decision in order to forecast the impacts that climatic and hydrologic shifts may have on survival and reproductive success.