Salmon Watersheds Program

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Assessing the Vulnerability of Pacific Salmon to Freshwater Habitat Pressures

Pacific salmon have experienced widespread declines throughout their range. There have been numerous efforts to assess their vulnerability to various human and environmental pressures, and freshwater habitat destruction and degradation are oft-cited causes of declines. Despite seemingly obvious implications of habitat loss and degradation for the survival and productivity of fish populations, quantifying the relationship between habitat and population change at broad spatial scales can be described as a “wicked problem” due to the multiple, interacting pressures on freshwater habitats and the diversity of Pacific salmon life-histories and habitats. Nonetheless, decisions about recovery planning and land-use developments need to be made, requiring quantitative evaluation of how different threats impact population status.

With Nature Legacy Funding, PSF partnered with Fisheries and Oceans Canada (DFO) in this project to quantify the linkages between habitat and population status in order to facilitate targeted recovery actions for at-risk Pacific salmon populations. Over the past 5 years, DFO and PSF have collaborated to compile and synthesize information on population and habitat status for Pacific salmon in BC. This extensive dataset on population and habitat status, which forms the foundation of our Pacific Salmon Explorer, presents a unique opportunity to quantify the relationship between trends in salmon abundance and freshwater habitat pressures at a broad spatial scale.

Approach

We assessed the evidence for relationships between trends in spawner abundance of Pacific salmon populations and ten different freshwater habitat pressures within spawning watersheds (agriculture and urban development, riparian disturbance, linear development, forestry and non-forestry roads, stream crossings, forest disturbance, Equivalent Clearcut Area, and mountain pine beetle defoliation). This analysis drew on almost 4,000 individual Pacific salmon populations (i.e., unique combinations of species and streams) from across BC, and accounted for shared variation among populations due to life-history traits and biogeoclimatic regions (Marine Adaptive Zones and Freshwater Adaptive Zones; see figure).

We then quantified the vulnerability of salmon populations in different regions throughout the province based on the baseline trend in abundance (‘status’) plus the magnitude of freshwater habitat pressures (‘exposure’) and the strength of the population-habitat relationship estimated from the data (‘sensitivity’).

Figure 1. Marine Adaptive Zones (MAZs) and Freshwater Adaptive Zones (FAZs) are biogeoclimatic regions that were applied in the identification of Pacific salmon Conservation Units (Holtby and Ciruna 2007). In this project, we accounted for differences among MAZs (and species) in the baseline trend in spawner abundance, and differences among FAZs in the population-habitat relationship.

Findings

We detected a strong negative baseline trend in spawner abundance for all salmon species and Marine Adaptive Zones, with few exceptions. Evidence of relationships between freshwater habitat pressures and trends in spawner abundance was weak at the province-wide scale, highlighting the challenges of quantifying effects at this broad scale. Nonetheless, idiosyncratic responses of salmon populations to habitat pressures across watersheds and Freshwater Adaptive Zones emphasize that local adaptations and processes mediate this relationship. Future work should investigate the population-habitat relationships at intermediate (regional) scales to zero-in on the best spatial scale for describing vulnerabilities and implementing conservation and recovery actions.

The power to detect relationships between habitat status and salmon population trends would be increased by improving the collection, archiving, and accessibility of spatial datasets for both salmon populations and their habitats. The compilation, synthesis, and sharing of these data remains a priority of the Salmon Watersheds Program, and we look forward to revisiting this analysis with improvements to the quality and quantity of data on Pacific salmon populations and their habitats.