Population diversity and the portfolio effect in an exploited species

One of the most pervasive themes in ecology is that biological diversity stabilizes ecosystem processes and the services they provide to society, a concept that has become a common argument for biodiversity conservation. Species-rich communities are thought to produce more temporally stable ecosystem services because of the complementary or independent dynamics among species that perform similar ecosystem functions. Such variance dampening within communities is referred to as a portfolio effect and is analogous to the effects of asset diversity on the stability of financial portfolios. In ecology, these arguments have focused on the effects of species diversity on ecosystem stability but have not considered the importance of biologically relevant diversity within individual species. Current rates of population extirpation are probably at least three orders of magnitude higher than species extinction rates, so there is a pressing need to clarify how population and life history diversity affect the performance of individual species in providing important ecosystem services. Here we use five decades of data from sockeye salmon in Bristol Bay, Alaska, to provide the first quantification of portfolio effects that derive from population and life history diversity in an important and heavily exploited species. Variability in annual Bristol Bay salmon returns is 2.2 times lower than it would be if the system consisted of a single homogenous population rather than the several hundred discrete populations it currently consists of. Furthermore, if it were a single homogeneous population, such increased variability would lead to ten times more frequent fisheries closures. Portfolio effects are also evident in watershed food webs, where they stabilize and extend predator access to salmon resources. Our results demonstrate the critical importance of maintaining population diversity for stabilizing ecosystem services and securing the economies and livelihoods that depend on them. The reliability of ecosystem services will erode faster than indicated by species loss alone.

Schindler et al. 2010.  Population diversity and the portfolio effect in an exploited species. Nature.  465: 609-613. doi:10.1038/nature09060

Processing costs limit potential for managing predator-prey interactions in a commercial fishery.

In Chignik, Alaska, sockeye salmon support a valuable commercial fishery and, as juveniles, are preyed upon by coho salmon, a species not subject to a targeted harvest. Whether exploitation of coho salmon would enhance overall fishery value by releasing sockeye salmon from predation constraints is frequently discussed, but not understood. In this project we used simulation models to examine the ecological and economic conditions necessary for directly targeting coho salmon in a way that would benefit both fishers and seafood processors, two distinct but inter-dependent stakeholders in this ecosystem.  Model results indicate fishers are likely to experience increased value regardless of economic constraints, as long as coho salmon predation negatively affects sockeye salmon productivity. However, seafood processors are much more limited in the conditions which produce increased economic value, constrained by greater operation costs required to process harvested coho salmon. The unique economic constraints and opportunities of different stakeholders can present contrasting outlooks on the potential benefits of alternative harvest strategies, even if the alternative strategies are predicted to increase yield. The findings herein demonstrate the importance of considering multiple stakeholders when considering alternative management strategies. Depending on the level of risk stakeholders are willing to accept, an active adaptive management strategy reducing coho salmon escapement to low levels could provide valuable information about ecosystem structure as well as potentially providing the greatest economic benefit to the fishery.

Walsworth et al. 2017.  Constrained by markets: processing costs limit potential for managing predator-prey interactions in a commercial fishery. Journal of Applied Ecology. 54: 1946-1956.  doi: 10.1111/1365-2664.12900

Centennial-scale fluctuations and regional complexity characterize historical Pacific salmon population dynamics

Large fluctuations in abundance are a hallmark of fish stocks; fish abundance can fluctuate substantially over interannual to centennial time scales. Recent short-term variation in stock abundance can be characterized by fisheries catch records and scientific survey data.  But knowing how fish stocks have varied over long time scales can provide an important context for recently observed shifts in abundance, provide insight into the potential effects of climate change, and inform management frameworks. In this study, we describe and analyze time series of nitrogen (N) stable isotopes in sediments from 20 lakes as a proxy for the abundance of anadromous sockeye salmon to quantify how salmon stocks have varied in abundance during the past ~500 y. This analysis provides a comprehensive synthesis of the natural patterns of variability in sockeye salmon abundance prior to the onset of commercial fishing and encompasses a region that currently produces over 70% of global sockeye salmon harvest.

The picture that ultimately emerges from this synthesis is that low frequency fluctuations in abundance are a fundamental component of natural (unexploited) salmon populations and that these fluctuations are not necessarily synchronous across southwestern Alaska.  Although some stocks varied on interdecadal time scales (30- to 80-y cycles), centennial-scale variation, undetectable in modern-day catch records and survey data, has dominated salmon population dynamics over the past 500 years. Before 1900, variation in abundance was clearly not synchronous among stocks, and the only temporal signal common to lake sediment records from this region was the onset of commercial fishing in the late 1800s. Thus, historical changes in climate did not synchronize stock dynamics over centennial time scales, emphasizing that ecosystem complexity can produce a diversity of ecological responses to regional climate forcing. The current study places the interdecadal variation observed in fishery catches from the past century within a deeper temporal context and suggests that salmon stocks may remain in persistent high- or low-productivity regimes for not just decades, but for centuries. Our results also suggest that management models that assume time-invariant productivity or carrying capacity parameters may be poor representations of the biological reality in these systems.

Rogers et al. 2013.  Centennial-scale fluctuations and regional complexity characterize Pacific salmon populations dynamics over the last five centuries.  Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1212858110.