Age at Maturity, Fork Length, and Sex Ratio of Upper Willamette River Hatchery Spring Chinook Salmon

We used data from 17 brood years of coded-wire-tagged hatchery spring Chinook Salmon Oncorhynchus tshawytscha from the upper Willamette River to test for changes in mean age at maturity, fork length, and sex ratio. We found only limited evidence for any trend in age at maturity or sex ratio. However, Chinook Salmon sampled from tangle nets, recreational fisheries, spawning grounds, and hatcheries all presented trends of decline in mean fork length. Rates of change in fork length ranged from 0 to 5mm per year in most sample collections, though fork length declined more rapidly for samples from tangle nets. We also observed a positive relationship between adult fork lengths and the median monthly Pacific Decadal Oscillation index in the year prior to juvenile liberation (the brood year). We suggest that future research should investigate the potential cause(s) for the decline in size of hatchery spring Chinook Salmon from the upper Willamette River, with attention to harvest, broad-scale environmental conditions, and hatchery spawning and rearing practices.