Length and Condition of Wild Chinook Salmon Smolts Influence Age at Maturity

Anadromous Chinook Salmon Oncorhynchus tshawytscha commonly mature at 3-5 years of age. Age at maturity is an important measure of population diversity, and has ecological importance. In the John Day River, a northeast Oregon tributary of the Columbia River, we tagged 24,240 wild Chinook Salmon smolts from 10 successive cohorts with passive integrated transponder tags and subsequently monitored their age at maturation. We used multinomial logistic regression and multimodel selection to assess the influence of biotic and abiotic factors on age at maturity. Smolt fork length, condition factor, and day of tagging all had significant associations with age at maturity. Conversely, we found no significant association between ocean conditions and age at maturity. The probability of age-3 maturation increased concomitant with increased smolt length. In contrast, the probability of maturation at age-5 was inversely related to smolt condition factor. Day of tagging had a significant effect, indicating that smolts tagged later in the spring had a higher probability of maturing at age 4 than age 3. Our data indicate that mean smolt size was greater when smolt abundance was lower, and larger smolts were ultimately associated with a younger age at maturity. For John Day Chinook Salmon, spawner escapements approaching or exceeding the stock-recruit replacement level appear to favor production of older Chinook Salmon.