Synchronization and portfolio performance of threatened salmon

Interpopulation variation in dynamics can buffer species against environmental change. We compared population synchrony in a group of threatened Chinook salmon in the highly impacted Snake River basin (Oregon, Washington, Idaho) to that in the sockeye salmon stock complex of less impact Bristol Bay (Alaska). Over the last 40 years, > 90% of populations in the Snake River basin became more synchronized with one another. However, over that period, sockeye populations from Alaska did not exhibit systemic changes in synchrony. Coincident with increasing Snake River population synchrony, there was an increase in hatchery propagation and the number of large dams, potentially homogenizing habitats and populations. A simulation using economic portfolio theory revealed that synchronization of Snake River salmon decreased risk-adjusted portfolio performance (the ratio of portfolio productivity to variance) and decreased benefits of population richness. Improving portfolio performance for exploited species, especially given future environmental change, requires protecting a diverse range of populations and the varied habitats upon which they depend.