Early Life History Variation in Hatchery- and Natural-Origin Spring Chinook Salmon in the Yakima River, Washington

In male Chinook Salmon Oncorhynchus tshawytscha , age of maturation is phenotypically plastic, occurring at age 1 (referred to as precocious parr or microjack), age 2 (minijack), age 3 (jack), age 4, or age 5. Microjacks and minijacks are thought to forego migration to the ocean as smolts, instead remaining in headwaters and employing a “sneaking” strategy to fertilize eggs. We compared the prevalence of minijacks (minijack rate) among hatchery- and natural-origin spring Chinook Salmon from the Yakima River, Washington, over seven brood years (2001–2007). We quantified minijack rates and sex ratios in the hatchery population prior to release and during out-migration at a trap located 230 km downstream. Within this time period, we also monitored minijack rates in a 3-year (brood years 2002–2004) growth study designed to reduce minijack production at the hatchery. Minijacks made up an average of 41% of the male population in the hatchery, but annual minijack rates varied in response to the growth rate or fish size at release. Average minijack rate was approximately 20% among out-migrating hatchery fish, about half the rate found prior to release. Among out-migrants, minijack rates of hatchery fish were approximately 10 times those of natural-origin fish, but sex ratios were significantly skewed toward females in both hatchery- and natural-origin groups. Data from this study and related studies suggest that the predominant age of early male maturation in the Yakima River and similar rivers is age 2 (minijack) in hatchery fish and age 1 (microjack) in natural-origin fish. Based on this and other studies, we now recognize three minijack life history types in spring Chinook Salmon: resident, fluvial, and anadromous, depending on the migration pattern exhibited in the spring and summer. Finally, we discuss the broader impacts that high minijack production may have on the establishment of size-at-release targets for salmon supplementation programs in the future. Received July 11, 2012; accepted November 13, 2012