The Effects of Variation in Rearing Conditions on Growth, Smolt Development, and Minijack Rate in Yearling Chinook Salmon: a Hatchery Scale Experiment

In this investigation a single genetic stock of Hood River, Oregon, Chinook Salmon Oncorhynchus tshawytscha was reared at three different hatchery facilities over three brood years (2008–2010) and monitored for size, growth rate, gill Na+,K+‐ATPase activity, condition factor, whole body energetics, and precocious male maturation (age‐2 minijack rate). This experimental design provided a unique opportunity to isolate environmental from genetic effects on salmonid life history. Differences in the seasonal thermal regimes and associated growth profiles among the three facilities resulted in modest differences in smolt development but significant variation in size at release (range = 18 g body weight, 118 mm FL to 31 g body weight, 142 mm FL) and minijack rates (range = 4.8–57.1%) among groups. Previous studies have found a positive relationship between body size at release and minijack rates. However, in this investigation the release group with the largest mean body size consistently had the lowest minijack rates. This unique result may be due to the more natural thermal regime and feeding profile experienced by fish at this facility compared with that of the other two facilities and highlights the importance and potential benefits of adhering to a more “wildlike” growth profile in hatchery supplementation programs. Received February 24, 2014; accepted May 29, 2014