Variation in Minijack Rate among Hatchery Populations of Columbia River Basin Chinook Salmon

Abstract

AbstractIn Columbia River spring and summer Chinook Salmon Oncorhynchus tshawytscha, age of male maturation ranges from age 1 (microjack), 2 (minijack), 3 (jack), to 4 or 5 (adult) years. The presence of minijacks has been noted in several experimental studies and documented for a few hatchery programs; but, a comprehensive survey of their occurrence in hatchery production programs has never been conducted. We measured the proportion of minijacks among males released from several spring‐ and summer‐run Chinook Salmon hatchery programs throughout the Columbia River basin among brood years 1999–2010. The hatcheries surveyed included both segregated (uses only hatchery‐origin spawners in broodstock) and integrated (includes some degree of natural‐origin spawners in broodstock) programs. Minijacks were found in all programs monitored, and rates varied approximately 10‐fold across release groups, ranging from 7.9% to 71.4% of males in spring Chinook Salmon programs and from 4.1% to 40.1% of males in summer Chinook Salmon programs. Cumulative growth (i.e., size at release) was found to be positively correlated with minijack rate, but for only the integrated Chinook Salmon programs. Domestication selection may have occurred in segregated spring Chinook Salmon programs, increasing the threshold size for maturation and lowering minijack rates. Elevated minijack rates in Chinook Salmon hatchery programs result in a direct reduction in both the number of male smolts released and potential adult males available for harvest and spawning.