Quantifying lost habitat for Pacific salmon in the Lower Fraser

Up to 85 percent of historical salmon habitat lost in Lower Fraser region.

For perhaps the first time ever, researchers have mapped out the true extent of habitat loss for salmon in the Lower Fraser River, one of the most important spawning and rearing grounds for Pacific salmon in British Columbia.

Salmon have lost access to as much as 85 per cent of their historical floodplain habitat – the biologically rich wetlands next to a river or stream that typically harbour wildlife – due to dikes and similar infrastructure.

Lead author of the paper, Riley Finn, a research associate with Raincoast Conservation Foundation and the Conservation Decisions Lab in the faculty of forestry at UBC, found that, “only around 101 square kilometers out of an estimated 659 square kilometers of historical floodplains remain accessible to salmon.”

The research paper, “Quantifying lost and inaccessible habitat for Pacific salmon in Canada’s Lower Fraser River,” was peer reviewed and published in Ecosphere.

“Restoring connectivity to isolated habitats such as side channels and sloughs has the potential for large habitat gains which can help restore our Lower Fraser salmon populations. The degree of habitat loss which we revealed demonstrates the need to protect what little habitat still remains if we hope to have sustainable salmon populations in the future.”

Dave Scott, co-author and Raincoast’s Fraser Estuary Research and Restoration Coordinator

The research team also found that up to 64 percent of streams are now off-limits to salmon due to in-stream barriers like dams, floodgates and road culverts, blocking off important channels for migrating salmon, which spend part of their lives at sea but return to their natal streams to spawn and rear their young. There are currently more than 1,200 barriers preventing salmon from accessing approximately 2,224 kilometers of streams.


Finn, R. J. R., Chalifour, L., Gergel, S. E., Hinch, S. G., Scott, D. C., and Martin, T. G.. 2021. Quantifying lost and inaccessible habitat for Pacific salmon in Canada’s Lower Fraser River. Ecosphere 12 (7):e03646. 10.1002/ecs2.3646


Loss of connectivity caused by anthropogenic barriers is a key threat for migratory freshwater species. The anadromous life history of salmonids means that barriers on streams can decrease the amount of habitat available for spawning and rearing. To set appropriate targets for restoration, it is important to know how different populations have been impacted in terms of the location and extent of historically available habitat that has been lost or has become inaccessible. Using mapped and predicted barriers to fish passage in streams and diking infrastructure, the amount of both floodplain and linear stream habitat that remains accessible today was estimated for 14 populations of salmon in the Lower Fraser River, British Columbia, Canada’s most productive salmon river. To place these estimates within a historical context, the floodplain area was estimated using vegetation records from the 1850s, and lost streams were estimated using a digital elevation model-derived stream network. To bolster areas where little mapping has been done, current barrier data were used to predict locations likely to have barriers. Accessibility to floodplain was poor across the entire region with only 15% of the historical floodplain remaining accessible. Linear stream habitat ranged in accessibility from 28% to 99% across populations based on mapped barriers. Inclusion of predicted barriers revealed an additional 33 km of potentially inaccessible stream habitat and the modeled stream network located approximately 1700 km of stream length that has been completely lost. Comparing habitat accessibility and barrier density against the assessed status of populations revealed insights useful for understanding the impact of barriers on spawning and rearing and guiding the allocation of restoration effort. Applying methods for addressing missing data, such as lost streams and unmapped barriers, was essential for estimating the accessibility of habitat within a historical context. While much emphasis has been placed on the role of marine conditions in wild Pacific salmon recovery, the magnitude of habitat loss in the Fraser cannot be ignored and suggests it is a major driver of observed salmon declines.

Select figures

Figure 1

Figure 1, a map of the study area around the Lower Fraser River
Figure 1: Map of study area comprised of the Lillooet, Fraser Canyon, Chilliwack, Harrison, and Lower Fraser watershed groups showing location of mapped barriers. Sources of barrier data include the Fish Information Summary System (FISS), Provincial Stream Crossing Inventory System (PSCIS), and Watershed Watch Salmon Society (WWSS). Inset shows context of study in the broader Pacific Northwest Region.

Figure 3

Figure 3, a map of lost streams and habitat for Pacific Salmon.
Map of stream connectivity in the Lower Fraser according to currently mapped barriers to fish passage. Also included are lost streams identified from a digital elevation model-derived stream network, and a map of historic Sumas Lake derived from North and Teversham (1984). This map depicts the 16% gradient threshold for natural accessibility, and for the 5% threshold used to assess chum salmon habitat accessibility, see Appendix S3.

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Research scientist, Adam Warner conducting genetics research in our genetics lab.
Photo by Alex Harris / Raincoast Conservation Foundation.