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A systematic review of ecological thresholds for salmon watersheds

Finn, R., Martin, T., Tulloch, V., Murdoch, A., Lyons, S., Moir, W., Milligan, M., MacDonald, E. & Mantyka‐Pringle, C.

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Key points

• Scientists from Wildlife Conservation Society Canada, alongside partners from the University of British Columbia and the Council of Yukon First Nations, have found that current global targets to conserve 30% of lands and waters by 2030 will be insufficient to sustain salmon populations. 70-80% protection of the watersheds on which salmon depend is much closer to what's needed.
• Across North America, salmon are incredibly important for culture, subsistence food, the fisheries economy and healthy watersheds. However the ecosystems they depend on are being impacted by multiple cumulative stressors.
• These findings show the importance of identifying the true ecological tipping points that could lead to major and irreversible degradation of ecosystems and loss of species. They also point to the need for watershed-scale planning that considers what is required to conserve the functioning and integrity of the whole system. That means not orienting decision making arbitrarily around 30x30.

Abstract

Salmon watersheds are being impacted by multiple cumulative stressors. To avoid and mitigate undesirable outcomes of these cumulative effects, we require understanding of how these stressors impact watersheds and the species within. The concept of ecological thresholds describes the point at which a small environmental change triggers a disproportionately large and often irreversible shift in an ecosystem. Identifying these tipping points is critical for watershed stewardship as they can help anticipate and prevent major ecosystem degradation and loss. For Pacific salmon, cumulative effects present a particular challenge as their migratory life histories expose them to many different stressors throughout their life cycle. We conducted a systematic reviewto identify ecological and land use thresholds relevant for informing the management of salmon watersheds in North America. We extracted and summarized directly observed or cited thresholds for various biotic and abiotic features of salmon habitat, and salmon directly. We also conducted a meta-analysis of diverse responses to watershed scale land cover change. Our review provides multiple sources of support for a threshold effect around 20-30% watershed land cover change. That is, when greater than 20-30% of a watershed is deforested or converted to other land use activities, impacts to physical and biological variables related to salmon habitat integrity become consistently detectable through empirical studies. This result reflects many of the directly observed thresholds we extracted, and supports previous recommendations for implementing watershed scale land cover policies. In this context, our findings suggest that 30% conservation alone is unlikely to be sufficient for thriving salmon watersheds, and that broader land- and water-use safeguards are needed to maintain functional watershed condition before cumulative land cover change crosses the 20-30% range.

Recommended citation

Finn, R., Martin, T., Tulloch, V., Murdoch, A., Lyons, S., Moir, W., Milligan, M., MacDonald, E. & Mantyka‐Pringle, C. (2026). A systematic review of ecological thresholds for salmon watersheds. Environmental Reviews. https://doi.org/10.1139/er-2026-0074