Ecological applications of portfolio theory demonstrate the utility of this analytical framework for understanding how biodiversity underpins the stability of commercial and indigenous Pacific Salmon fisheries. Portfolio theory also has the potential to aid in recovery planning for threatened and endangered Pacific salmon but it has not been applied in this context. This study applies portfolio theory to inform recovery planning for the Oregon Coastal Coho Salmon (Oncorhynchus kisutch) Evolutionary Significant Unit (ESU). This ESU is an aggregation of 21 independent populations organized into five geographically and genetically coherent strata. Federal recovery goals aim to stabilize a minimum of 50 percent of independent populations within each stratum. Within this context of recovery planning, we apply portfolio theory to time series of spawner abundance to identify stability properties of populations within and among ESU strata. We also correlate population stability properties with watershed-scale habitat features to explore landscape controls on coho salmon population dynamics. The goal of this work is to identify populations and landscape settings most likely to contribute to ESU recovery.
Our work identified high variation among five ESU strata in the temporal stability of coho salmon populations. Variation among strata was associated with watershed-scale habitat features. In particular, the stratum with highest level of temporal population stability (lowest coefficient of variation in population abundance) is notable for the presence of coastal lakes that are known to relax winter habitat survival bottlenecks for juvenile coho salmon. In comparison, strata without prominent coastal lakes had 60 – 100% greater temporal variability in spawner abundance. We observed low temporal coherence in population abundance within other strata, suggesting considerable response diversity of populations in close proximity. Our results identify those strata that are currently closest to meeting recovery targets, as well as independent populations that contribute disproportionately to the temporal stability of abundance within their respective strata. Conservation planners may therefore wish to prioritize efforts that protect those populations that currently have the greatest population stability and emulate watershed processes associated with population stability in the restoration of remaining populations. These results suggest that portfolio theory may be a useful analytical framework for designing conservation strategies for population aggregations such as Pacific Salmon ESUs.