Grande Ronde and Imnaha River Basins Minijack Chinook Salmon

Recent literature suggests that hatchery-origin minijack (mature age 2 males) spring Chinook salmon (Oncorhynchus tshawytscha) are abundant in the Columbia River hydrosystem.  Hatchery rearing practices are believed to contribute to the minijack life history strategy.  We examined PIT-tagged hatchery smolt releases from the 2003-2010 migration years in Catherine Creek, and the Lostine and Imnaha rivers to determine minijack rates in these three hatchery programs.  The first smolt releases into the Imnaha River occurred in 1984, and in the Lostine River and Catherine Creek in 1997 and 1998, respectively.  Spring Chinook minijacks were defined as PIT-tagged smolts detected moving upstream after 1 June of the same release year at adult fish ladders at Bonneville, McNary, Ice Harbor and/or Lower Granite dams.  Minijack Chinook were present in all three populations. The estimated number of minijacks in a given brood ranged from 0 to 0.47% of the release group.  The mean percentage of minijacks from the 2003-2010 migration years was 0.06, 0.07, and 0.14% from Catherine Creek, the Lostine River, and the Imnaha River, respectively.  We identified thirteen minijacks detected in the estuary trawl that spent a mean of 71 days below Bonneville Dam before migrating back upstream. Both barged and run-of-the river fish were represented by minijacks, with the greatest proportion represented by run-of-the river smolts.  For each of the three stocks, the proportions of PIT-tagged minijacks observed were significantly correlated with Ranking Ocean Conditions.  The estimated number of minijack detections was a significant predictor of the estimated number of age 3 jack salmon returns in the subsequent migration year.  From the 2002-2005 brood years, we estimate that minijacks comprised up to 27% of the total male returns from a given brood year. In these hatchery programs, there are few observations of minijacks at adult weirs in NE Oregon streams, as the minijacks are able to swim through the weir pickets.  Currently, PIT-tag detections at Columbia and Snake river dams are the only method available for tracking this life history. Collectively, these findings suggest that minijacks represent a small but meaningful portion of smolt releases, and provide a novel opportunity to predict jack returns to the Columbia River Basin.  Additionally, factors outside of hatchery rearing process also appear to influence the minijack life history strategy.