Minijack prevalence in spring Chinook salmon of the Yakima River, Washington: Management implications for determining smolt-to-adult return rates

In male Chinook salmon age of maturation is phenotypically plastic occurring at age-1 (precocious parr), -2 (minijack), -3 (jack), -4 or 5 years. Precocious parr and minijacks are thought to forgo migration to the ocean as smolts in favor of remaining in the headwaters to employ a "sneaking strategy" to fertilize eggs during mating events. We monitored minijack prevalence in hatchery and wild spring Chinook salmon from the Yakima River, Washington (tributary of the Columbia River). From 2003 to 2009 we quantified minijack rates and gender ratios in the hatchery population just prior to release and then during outmigration at a smolt trap 230 km downstream. Major conclusions: 1) approximately 40% of all male fish released are minijacks. 2) A significant number of hatchery minijacks migrate downstream. 3) Among migrants, minijack rates of hatchery fish are approximately 10 fold that of wild fish. 4) Downstream gender ratios favor females for both hatchery and wild fish suggesting that for both groups a significant proportion of males remain upstream or juvenile outmigration survival favors females. 5) Smolt-to-adult return rates (SAR's) can be impacted by this variation in age of maturity since these fish are not smolts being released, but precociously maturing adults.