Rates of straying by hatchery-produced Pacific salmon (Oncorhynchus spp.) and steelhead (Oncorhynchus mykiss) differ among species, life history types, and populations

Here we ask whether straying differs among species, life history types, and populations of adult hatchery-produced Pacific salmon (Oncorhynchus spp.) and steelhead (Oncorhynchus mykiss) in the Columbia River basin. Previous estimates of straying have been confounded by various factors influencing the probability of individuals returning to non-natal sites (e.g., off-station releases), whereas analyses undertaken here of nearly a quarter million coded-wire tag recoveries control for these factors. Our results revealed large and generally consistent differences in the propensity to stray among species, life history types within species, and populations. Paired releases indicated that (i) Chinook salmon (Oncorhynchus tshawytscha) strayed more (mean population range 0.11%-34.6%) than coho salmon (Oncorhynchus kisutch) (0.08%-0.94%); (ii) ocean-type Chinook (5.2%-18.6%) strayed more than stream-type Chinook (0.11%-10%); and Chinook salmon (0.90%-54.9%) strayed more than steelhead (0.30%-2.3%). We conclude these patterns are largely the result of species-specific behavioral and endocrine factors during the juvenile life stages, but analyses also suggest that environmental factors can influence straying during the adult upstream migration.