Variability and Asynchrony in Salmon Returns: Implications for Monitoring and Ecosystem Services
Davis, Brooke Margaret
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Pacific salmon are well-known for their unique life-history characteristics, complex population structures, and the wide range of ecosystem services they provide. Variability in life-history characteristics across and within species, along with their tendency to return to their natal sites, leads to phenotypically distinct populations that create portfolios of populations within watersheds. Pacific salmon are important for supporting valuable fisheries and for supporting key ecosystem processes in the marine and freshwater environment. Alaskan sockeye salmon populations display overall population stability despite large commercial harvests, a characteristic that has been attributed to their intact population complexity. Those fish that are not captured by the commercial fishery support key ecosystem processes in freshwater environments. This yearly, pulsed, resource subsidy provides a reliable source of food and nutrients to the watersheds where sockeye salmon spawn and die. These complex populations may pose challenges for management due to difficulties separating the contributions of individual populations or habitats to the overall population complex (or portfolio). In Chapter 1 we used abundance data for sockeye salmon (Oncorhynchus nerka) spawning in a set of eight streams in the Wood River watershed, southwest Alaska, to demonstrate how natural patterns of variability affect the ability of fixed assessment windows to characterize the contribution of an individual spawning population to the entire portfolio. Additionally, simulated data are used to explore how different levels of synchrony and autocorrelation affect the ability of monitoring schemes to estimate the contributions of individual populations to a portfolio. We find that the ability of fixed assessment windows to characterize a population’s contribution to a portfolio is distinctly limited; asynchronous or independent dynamics among populations in a portfolio, and the presence of autocorrelation that creates slow changes in productivity, weaken the ability to characterize a stream's potential contribution to a portfolio. These results suggest that the structure of complex portfolios, and the presence of directional changes in productivity within individual populations, need to be taken into account when carrying out environmental risk assessments that aim to measure the contribution of an individual population or piece of habitat to dynamics observed at broader spatial and temporal scales. The reliable yearly pulse of marine-derived nutrients, in the form of spawning salmon, provides inland freshwater habitats with food and nutrients in the form of live fish, their gametes, and their carcasses. The highest quality food is provided by live fish and their eggs, which are important food sources for resident fish, bears, and birds, are only available for a short period. While the effects of this specific resource pulse are widely appreciated, little attention has been paid to the role that timing plays in conferring benefits to consumers, and previous research has mainly focused on biomass as the main control on the magnitude of effects. In Chapter 2 we used multiple in-stream counts of adult sockeye salmon abundance within the spawning season, and tagging data to estimate in-stream life span, to estimate how the amount of time that consumers have access to live salmon as a food resource is related to the adult spawner density in an individual stream. Our results demonstrate that duration of salmon availability as a food source is non-linearly related to escapement; across 3 orders of magnitude of spawner abundance, salmon were available to predators from about 2 weeks to about 5 weeks. This saturating relationship indicates that higher escapement values may not translate to proportionally higher benefits for consumers when these benefits are available during a fleeting window of opportunity. This result demonstrates that ecosystem based fisheries management (EBFM) of anadromous salmon should assume that benefits inferred to consumers are inherently time-mediated, and the numerical benefits of increased salmon density will not be straight-forward to estimate. Conservation strategies to maintain a range of spawn timing across watersheds may be the most successful for maintaining the importance of salmon subsidies in watersheds.