Migration of precocious male hatchery chinook salmon in the Umatilla River, Oregon

Between 1993 and 2000, precocious yearling males of hatchery-produced fall and spring chinook salmon Oncorhynchus tshawytscha composed 3.6-82.1% of chinook salmon runs to the Umatilla River, Oregon. These yearling males are smaller than typical jack salmon, which spend a full winter in the ocean, and are commonly referred to as "mini jacks." Minijack fall chinook salmon are characterized by enlarged testes and an increased gonadosomatic index. Our goal was to determine if minijacks migrated to saltwater between the time they are released from the hatchery and the time they return to the Umatilla River, a period of 4-6 months. During 1999-2000, we collected otoliths from an adult male fall chinook salmon, 12 spring chinook salmon minijacks, and 10 fall chinook salmon minijacks. We measured strontium:calcium (Sr:Ca) ratios from the age-1 annulus to the edge of the otolith to determine whether these fish had migrated to the ocean. The Sr:Ca ratios increased from low values near the age-1 annulus, similar to ratios expected from freshwaters, to higher values near the edge of the otolith. The Sr:Ca ratios increased to levels similar to ratios expected in saltwater, indicating that these fish had migrated to saltwater before returning to the Umatilla River. Analysis of published water chemistry data from the Columbia and Snake rivers and rearing experiments in the main-stem Columbia River confirmed that high Sr:Ca ratios measured in otoliths were not the result of high strontium levels encountered in the freshwater environment. Previously assumed to remain within freshwater and near the point of release, our results suggest these minijack salmon migrated at least 800 km and past three hydroelectric dams to reach saltwater and return to the Umatilla River.