Influence of Juvenile Size on the Age at Maturity of Individually Marked Wild Chinook Salmon

Abstract

The age at maturity for an organism represents a trade-off between survival and reproductive fitness and is controlled by both genetic factors and environmental conditions. In the case of Pacific salmon Oncorhynchus spp., most of our understanding of the importance of various drivers in determining age at maturity has relied on information from cohort analyses using average size and age conditions or from hatchery fish that might not reflect observed patterns in wild fish. I used 14 years of adult return data from individually passive integrated transponder (PIT)–tagged wild Chinook salmon O. tshawytscha from the Snake River to examine the influence of juvenile size on subsequent age at maturity. I also included information on the natal watershed as a surrogate for genetic influence. I found a significant, positive relationship between juvenile length and the proportion of adult salmon returning at age 4 versus age 5, but the effect of juvenile length varied among the three watersheds. This suggests that local environmental conditions supporting juvenile growth largely control the ultimate age composition for a given brood of this stock of salmon but that genetic differences also exist.