Minijacks: Should These Small Fish be a BIG Concern for Chinook Salmon Supplementation Programs?

Male Chinook salmon that precociously mature after just two years of age are often referred to as “minijacks” and are one year younger than more commonly recognized “jacks”.  While minijacks are two or more orders of magnitude smaller than anadromous adults, they’re an important component of a diverse life-history portfolio for wild salmonids.  Over the past decade we have quantified the proportion of minijacks released from several Chinook salmon hatcheries throughout the Columbia River Basin from both segregated (only hatchery-origin broodstock) and integrated (some natural-origin broodstock) programs and rates varied 10-fold from 7.9 - 71.4% and 4.1 - 40.1% of the males in spring and summer stocks, respectively. Smolt size at release and minijack rate were positively correlated in integrated, but not segregated programs. Rates were significantly higher in integrated compared to the segregated programs despite the fact that most of the integrated programs released smaller fish suggesting that domestication selection for age at maturation has occurred in segregated programs.  Consequences of high minijack production including ecological impacts to wild stocks, loss of adult production, domestication selection and altering the accuracy of smolt-to-adult return (SAR) estimates among other factors will be discussed in the context of salmon supplementation and management.