A century-long time series reveals large declines and greater synchrony in Nass River sockeye salmon size-at-age

Abstract

The ability to comprehend the nature of changes in body size is often limited by time series of relatively short duration. Using archival records of 118 573 individual measurements, we developed a 106-year time series of mean size-at-age, by sex, of Nass River sockeye salmon ( Oncorhynchus nerka). Size-at-age declined during the last century in several distinct stanzas. Until the 1930s, there was weak covariation in size-at-age among age classes of both sexes. Thereafter, all time series exhibited a coherent cyclical pattern, superimposed on an underlying decline, reaching smallest average size-at-age in 2019. Age classes sharing the same years of ocean growth had more similar patterns of variation than those sharing a common brood year, suggesting a dominant role of marine life history. Since 1914, mean size-at-age declined from 5% up to 13% depending on age class and sex, resulting in an estimated 7%–19% decline in fecundity, which is likely to reduce the productivity of these populations. In the absence of increased survival, management targets based on fixed adult escapements may result in overexploitation.