Managing California Central Valley Chinook salmon with the portfolio effect

Background/Question/Methods

A major focus of the diversity-stability debate is how or to what extent increased diversity at a lower level of ecological organization may reduce variability in abundance or productivity at a higher level of ecological organization.  The portfolio effect (PE) was proposed as a statistical explanation for observed reductions in variability with increased biodiversity. The concept has recently been embraced to understand differences in the variability and stability of salmon stocks, which are often composed of multiple populations.  Returns of the Sacramento River Fall-run Chinook (SRFC) salmon stock to spawning grounds were low following poor ocean conditions in 2005, leading to closure of the fishery in 2008 and 2009.  Compared to the Bristol Bay sockeye complex, research indicates that the SRFC shows a less robust PE and one that has diminished in strength over time.  Recent investigations also argue that loss of life history and genetic diversity among SRFC populations has caused greater population coherence and a diminished PE.  Importantly, however, PE calculations assume all populations are identical.  Here, we present analyses of the SRFC PE using a different, empirically-based metric and also examine the proposal that the SRFC closure could have been avoided if the PE were maximized.  

Results/Conclusions

Based on sliding window estimates of the coefficient of variation, analyses incorporating our new metric corroborate previous findings that the SRFC stock's portfolio effect is diminishing over time.  However, the increased correlations among neighboring populations, which are thought to contribute to a weakened portfolio effect, are in some cases heavily influenced by the large and simultaneous reduction in population abundances following the exceptionally poor ocean conditions of 2005. The probability of the 2008 and 2009 SRFC stock abundances, based on yearly abundances until 2005, are ~1%.  Only by completely maximizing the portfolio effect would the total abundance of the SRFC stock have been high enough to meet the minimum escapement of spawners needed to keep the fishery open.