Weakening Portfolio Effect Strength in a Hatchery-Supplemented Chinook Salmon Population Complex

Recent research has revealed that diverse life histories contribute to asynchronous populations dynamics, which buffers a stock complex in a temporally heterogeneous environment (i.e., a “portfolio effect”). We previously revealed a weak, but remnant portfolio effect in California’s Central Valley salmon populations, despite considerable habitat loss and degradation.  Here we explore factors influencing variability in adult abundance on individual rivers over time.  We evaluate recent data on trends in hatchery practices that through their role in increasing the number of hatchery fish spawning in natural areas could act to homogenize among-river population dynamics.  Consistent with a weakening portfolio effect and concurrent with increasing off-site releases of hatchery fish, we found evidence for increasing synchrony among rivers and an increase in the variability of returns to individual rivers. System evenness remained fairly stable over this period, but the identity of the rivers contributing the most to total returns changed over time. Wavelet analysis revealed a change in the dominant timescale of variability, with moderate variability at a 3-4 year period early in the timeseries, while variability was higher and at a longer time period later in the timeseries. This increasing variability is concomitant with increasing hatchery production and increased off-site releases.