Tracking restoration of population diversity via the portfolio effect

In this week’s blog, Associate Editor, Andre Punt comments on the recent paper by Yamane et al. Tracking restoration of population diversity via the portfolio effect.

Many fisheries are managed to avoid populations dropping below threshold levels, and closing them to harvest when this happens. The implications of closures can be substantial for those who gain commercial or recreational benefits from harvest, as well as the communities that rely on income indirectly from commercial and recreational fisheries.

Fished resources may consist of an aggregate of multiple populations that mix and are caught together. While this can be a management challenge, especially when some of the populations are less productive than others, it is also a benefit in that multiple populations whose dynamics are somewhat independent may provide an insurance policy against some populations being at low abundance, through the so-called ‘portfolio effect’. The greater the portfolio effect, the more resilient a system will be and the greater its ability to provide ecosystem benefits. However, the strength of any potential portfolio effect needs to be quantified to help mangers, and this requires an understanding of diversity among populations.

Many definitions of diversity exist and have been applied to fish populations, in particular populations of Pacific salmon and herring. However, none are completely satisfactory. Lauren Yamane, a postdoctoral fellow at University of California Davis and colleagues proposed a new metric, the Diversity Deficit (or DD). This metric is the difference between the coefficient of variation of aggregate population abundance over a given period relative to the same coefficient of variation had all the populations been independent. This metric plotted over time to explore the extent to which diversity has changed, and proximate causes for such changes.

The team focused on changes in abundance for five of the tributaries the Sacramento River, including the American River. (Image: Teresa.r.ogrady)

The value of DD depends on both the variability within populations and the correlation in abundance between populations, with higher values of this correlation leading to higher values of DD, or lower diversity and hence any portfolio effect.

Yamane and colleagues apply the DD metric to understand changes in California’s largest salmon population, the Sacramento River-Fall-run Chinook (SRFC). This population is subject to natural and fishery-related impacts. Unprecedented low returns led to a closure of the fishery in 2008 and 2009, with major economic and social consequences.

The SRFC meta-population consists of many populations, but Yamane and colleagues focused on changes in abundance for five of the tributaries the Sacramento River, the mainstem Sacramento River, Battle Creek, Feather River, Yuba River, and the American River. DD for SRFC has been increasing over time for SRFC (indicating a loss in diversity), albeit with four major shifts. Removing one tributary at a time from the calculation of DD suggests that the first major increase in DD was related to the mainstem of the Sacramento River, either through a change in variance of abundance or in the correlation with other tributaries. Other changes can be attributed primarily to increased variability in abundance.

The new metric for quantifying diversity and hence the portfolio effect will be of direct help to managers of systems with multiple sub-populations, as it can be used to retrospectively understand changes in diversity and hence resilience. It could also help managers in a prospective sense by providing a way to quantify the management and conservation benefits of adopting management measures to increase diversity. While the initial application is to Pacific salmon in California, the DD metric could be applied to any fishery resources that behave as meta-populations, including salmon in other regions such as North Pacific where the world’s large wild salmon fisheries occur, herring and even cod in the North Atlantic, populations of which are still depleted and have not recovered at expected rates.

Read the full article, Tracking restoration of population diversity via the portfolio effect in Journal of Applied Ecology.


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