Performance of salmon fishery portfolios across western North America

Abstract

Quantifying the variability in the delivery of ecosystem services across the landscape can be used to set appropriate management targets, evaluate resilience and target conservation efforts. Ecosystem functions and services may exhibit portfolio-type dynamics, whereby diversity within lower levels promotes stability at more aggregated levels. Portfolio theory provides a framework to characterize the relative performance among ecosystems and the processes that drive differences in performance. We assessed Pacific salmon Oncorhynchus spp. portfolio performance across their native latitudinal range focusing on the reliability of salmon returns as a metric with which to assess the function of salmon ecosystems and their services to humans. We used the Sharpe ratio (e.g. the size of the total salmon return to the portfolio relative to its variability (risk)) to evaluate the performance of Chinook and sockeye salmon portfolios across the west coast of North America. We evaluated the effects on portfolio performance from the variance of and covariance among salmon returns within each portfolio, and the association between portfolio performance and watershed attributes. We found a positive latitudinal trend in the risk-adjusted performance of Chinook and sockeye salmon portfolios that also correlated negatively with anthropogenic impact on watersheds (e.g. dams and land-use change). High-latitude Chinook salmon portfolios were on average 2·5 times more reliable, and their portfolio risk was mainly due to low variance in the individual assets. Sockeye salmon portfolios were also more reliable at higher latitudes, but sources of risk varied among the highest performing portfolios. Synthesis and applications. Portfolio theory provides a straightforward method for characterizing the resilience of salmon ecosystems and their services. Natural variability in portfolio performance among undeveloped watersheds provides a benchmark for restoration efforts. Locally and regionally, assessing the sources of portfolio risk can guide actions to maintain existing resilience (protect habitat and disturbance regimes that maintain response diversity; employ harvest strategies sensitive to different portfolio components) or improve restoration activities. Improving our understanding of portfolio reliability may allow for management of natural resources that is robust to ongoing environmental change. Portfolio theory provides a straightforward method for characterizing the resilience of salmon ecosystems and their services. Natural variability in portfolio performance among undeveloped watersheds provides a benchmark for restoration efforts. Locally and regionally, assessing the sources of portfolio risk can guide actions to maintain existing resilience (protect habitat and disturbance regimes that maintain response diversity; employ harvest strategies sensitive to different portfolio components) or improve restoration activities. Improving our understanding of portfolio reliability may allow for management of natural resources that ...

Changing central Pacific El Niños reduce stability of North American salmon survival rates

“…The increased covariability between survival rates of these two species reduces the contribution to combined population stability by a reduced diversity in responses to ocean conditions, similar to the effects of increasing coherence within salmon species observed in other instances (3)(4)(5). This is a specific type of biodiversity, in the sense that it reflects covariability among species rather than within populations of the same species (3)(4)(5)11).…”

“…This is a specific type of biodiversity, in the sense that it reflects covariability among species rather than within populations of the same species (3)(4)(5)11). It is, specifically, variability in population rates rather than abundance itself, and thus is an example of response diversity (41).…”

“…Recent studies show that increased covariability in abundance among separate populations of Chinook salmon has increased coherence of salmon stocks, thus increasing aggregate variability of the stocks through the portfolio effect (3)(4)(5). These effects of declining diversity among salmon subpopulations are an example of the general global concern for the loss of biodiversity in many forms (e.g., loss of genes, species, withinspecies variability) and its effects on ecosystem services (6)(7)(8)(9)(10).…”

Portfolio theory as a management tool to guide conservation and restoration of multi‐stock fish populations

“…Portfolio theory provides a powerful tool for management of multi-stock populations that occupy large, spatially heterogeneous systems and experience temporal variation in environmental conditions during the reproductive period (Winemiller and Rose 1992, Hilborn et al 2003, Moore et al 2010, Carlson and Satterthwaite 2011, Griffith et al 2014, Schindler et al 2015. By quantifying interactions among stocks, managers can recognize the value of individual stocks as opposed to the largest current producer(s) (Begg andMarteinsdottir 2002, Hilborn et al 2003).…”

Bidirectional connectivity in rivers and implications for watershed stability and management

“…Salmon population dynamics were homogenized in this scenario, decreasing the response diversity within the watershed. Homogenization of salmon population dynamics has been observed in heavily altered salmon watersheds (Moore et al 2010;Carlson and Satterthwaite 2011;Griffiths et al 2014), potentially driven by erosion of population diversity through hatchery propagation (Pearse et al 2010), anthropogenic homogenization of habitats, or a shift from local to shared environmental forcing. These changes were modeled by specifying the standard deviation of the error from independent (q) and shared (r) sources to be 10 and 150, respectively.…”