Influence Of Environmental Variability On Chinook Salmon And Coho Salmon Population Dynamics

Author : Lauren Ann Yamane
Publisher :
ISBN 13 : 9781369201086
Total Pages : pages
Book Rating : 4.2/5 (1 download)

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Download or read book Populations in Varying Environments written by Lauren Ann Yamane and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This research aimed to understand the mechanisms underlying variability in population abundances over time. Many studies have suggested that age-structured demographic rates and interactions together with density-dependent regulation are important in determining how populations fluctuate in response to changes in the environment. More recent research has determined that through a phenomenon called cohort resonance, the combination of age-structure, density-dependence, and environmental stochasticity can result in population sizes fluctuating over two characteristic time scales. These time scales include short time scales equal to the mean population spawning age (i.e., generational frequencies), and also long time scales. These specific time scales of variability (the population's sensitivities to changing environmental conditions) have been found in empirical time series of cod and tuna abundances. Salmon populations also have demographic characteristics that make them particularly susceptible to cohort resonance. However, the potential importance of cohort resonance for salmon populations rests on whether supplementation with hatchery fish reduces the strength of the cohort resonance effect, and to what extent it might dampen the increase in variability associated with increased harvest rates (Chapter 1). Of additional importance in evaluating the potential for cohort resonance to drive salmon population variability is how life history characteristics might strengthen or diminish the effect. I examined how the central spawning age of the population and the fraction of spawners at that central age affect the sensitivity of populations to both low frequencies (environmental variability over long time scales) and generational frequencies (Chapter 2). Finally, I analyzed how temporal variability in the time series of subpopulations (specifically asynchrony) within a broader aggregate stock can lead to statistical dampening of variability at the stock level through the portfolio effect (Chapter 3). I developed a new metric, which quantifies the variability reduction possible with increased diversity among subpopulation dynamics (the Diversity Deficit; DD). I provide an example of applying this metric to retrospectively identify gains and losses in statistical independence for the Sacramento River Fall-run Chinook (SRFC) salmon stock. I found that compared to increased fishing levels, hatchery fish had a relatively small effect on the overall population variability (Chapter 1). However, the addition of hatchery fish to a population could alter the time scales of variability in salmon population time series, specifically magnifying fluctuations at intermediate to longer time scales. Such periods of population variability are normally reduced with higher levels of fishing. In addition, supplementation can be expected to have less of an effect on reversing the increased variability in abundances occurring over generational time scales. However, it is important to note that these effects occurred when populations were supplemented for extremely long periods of time. Analyses of the effects of the spawning age distribution revealed that higher central spawning ages increased population sensitivity to low frequencies, but had little impact on the generational frequencies (Chapter 2). The opposite was true for the fraction of central age spawners. Increasing the fraction of central age spawners did little to excite low frequency variability, but it greatly increased the population's propensity to exhibit sensitivity to generational frequencies. Analyses for the SRFC stock revealed that there has been an increase in the DD over time (Chapter 3). Thus, stock variability could have been reduced to a greater extent if independence among subpopulation dynamics had been restored. However, the mechanism of lost independence among subpopulation abundances is largely attributed to a rise in correlations with a single subpopulation, early in the time series.