Effect of culvert removal on abundance of Pacific salmon: A review

Abstract Many populations of Pacific salmon Oncorhynchus spp. along the West Coast of North America are at risk of extinction, with habitat loss and deterioration being key factors in these declines. Culverts associated with road crossings are a widespread source of habitat fragmentation in salmon-bearing streams, often restricting access to upstream spawning and rearing habitats. Remediation efforts, including culvert removal or replacement to improve fish passage, are increasingly implemented as a cost-effective restoration strategy. To understand the factors leading to increased abundance following culvert remediation, we set out to perform a statistical meta-analysis. However, due to a paucity of appropriate quantitative data, we pivoted to a narrative-style review. We describe the biological responses of salmon to culvert remediation and also highlight the consequences of a lack of consistent baseline and monitoring data. Within the 11 studies reviewed here, barrier removal often resulted in increased access to high-quality habitat and increased survival and abundance of salmon populations. However, responses were not consistent across all studies. Variation in outcomes appears to be influenced by species-specific responses, hydrologic conditions, and the quality and extent of newly accessible habitat. Importantly, limited baseline data, short monitoring durations, and inconsistent study designs constrain the ability to detect biological responses and evaluate project success at a time when use of culverts as a remediation tool is increasing. We conclude that while culvert remediation can effectively restore access to habitat, its contribution to salmon population recovery remains difficult to quantify. Improved pre- and post-project monitoring, standardized methodologies, and watershed-scale planning are needed to better understand and maximize the ecological benefits of culvert remediation and funding should be allocated for this work.