Upstream migration of minijack (age-2) Chinook salmon in the Columbia River: Behavior, abundance, distribution, and origin

It has recently been demonstrated that a large percentage of yearling Chinook salmon Oncorhynchus tshawytscha smolts released from a production-scale hatchery on the Yakima River display an underappreciated life history strategy. Instead of migrating to the ocean for long-term rearing and growth, males may instead undertake a short-term migration downstream, turn around, and migrate back upstream where they reproductively mature at age 2. Following local hatchery traditions, these fish are referred to as minijacks. We used information produced from (1) large-scale passive integrated transponder (PIT)-tagging programs at hatcheries in the Columbia and Snake River basins and (2) the installation of PIT tag detectors in the fish passage ladders built into dams on these rivers to investigate the occurrence, behavior, distribution, and origin of minijacks. A minijack was operationally defined as a fish detected in a ladder after 1 June in the same year it was released. Minijacks were found in every hatchery release group of more than 7,500 PIT-tagged Chinook salmon smolts reported (2002, 13 groups; 2003, 15 groups). Minijack population migration upstream past Bonneville Dam peaked in late June and July. This lagged migrations of age-4 adults (from the same populations) by approximately 2 months. However, minijacks also displayed population-specific variation in timing of upstream migrations past Bonneville Dam; minijacks from spring Chinook salmon populations appeared before fall Chinook salmon populations. Detection rates of minijacks at all dams ranged from 0.02% to 0.33% of the total smolts released for different hatchery groups. Expansion of this rate, based on the relation between minijack estimates at Cle Elum Hatchery (before release) and detections at Bonneville Dam (after release), suggests that minijack abundance in the Columbia and Snake River basins exceed hundreds of thousands of fish.