Assessing the cumulative effects of nearshore habitat restoration actions for multiple populations of juvenile salmon in Whidbey Basin, Washington: Foundation and approach for synthesis and evaluation

Kathryn L Sobocinski^1*Michael LeMoine²Joshua Chamberlin³Letitia Conway-Cranos⁴Annelise Del Rio⁵Heida Diefenderfer⁶Correigh Greene³Jason Hall⁷Gary E. Johnson⁸Ronald M. Thom⁸Elene Trujillo⁹Todd Zackey¹⁰

• ¹Department of Environmental Sciences and Marine and Coastal Science, Western Washington University, Bellingham, United States
• ²Research and Recovery Program, Skagit River System Cooperative, LaConner, WA, United States
• ³Fish Ecology Division, Northwest Fisheries Science Center, NOAA Fisheries, Seattle, Washington, United States
• ⁴Estuary and Salmon Restoration Program, Washington Department of Fish and Wildlife, Olympia, Washington, United States
• ⁵Science and Evaluation, Puget Sound Partnership, Olympia, WA, United States
• ⁶Coastal Sciences Division, Pacific Northwest National Laboratory (DOE), Sequim, Washington, United States
• ⁷Watershed Sciences Lab, Cramer Fish Sciences, Issaquah, WA, United States
• ⁸Retired, Coastal Sciences Division, Pacific Northwest National Laboratory (DOE), Sequim, Washington, United States
• ⁹Puget Sound Partnership, Olympia, WA, United States
• ¹⁰Marine and Nearshore Programs, Tulalip Tribes, Tulalip, WA, United States

Ecosystem restoration is a common tool for re-establishing ecosystem processes, structures, and functions to improve biodiversity and services in coastal and estuarine ecosystems. In the Salish Sea, salmon habitats have been fragmented, reduced in size, and diminished in quality, and the ecosystem processes that form and sustain these habitats have been degraded and disrupted as well. This loss is especially prevalent in estuaries, where up to 90% of former salmon habitat has been lost or compromised. Salmon species are integral to the identities and cultures of people in the Pacific Northwest, yet salmon abundances remain at historic lows, especially in urbanized areas. Recent investments in restoration are creating rearing habitat and repairing lost ecosystem function. However, restoration efforts in this region have proceeded at the site scale, with less attention to big-picture thinking regarding how restoration will effectively recover degraded or lost habitats for target species. As a result, no landscape-scale evaluation program exists, and the cumulative benefits of multiple interventions are unknown. We describe innovative methods for science synthesis related to the evaluation of cumulative effects of ecosystem restoration for Pacific salmon, using years of existing, but disparate data. Building from previous work on cumulative effects evaluation and incorporating a hierarchy of hypotheses approach, we propose using causal inference across numerous hypotheses in a framework to assess the cumulative benefits to Pacific salmon from multiple estuarine restoration projects. We present the framework as a method that can be used to address many complex questions with disparate data and provide examples from the Salish Sea where the approach is being implemented. The framework draws on science synthesis from numerous fields and uses a hierarchy of hypotheses, causal analysis at multiple scales, and hierarchy of synthesis for assessing multiple lines of evidence documenting restoration effects on Pacific salmon. We propose causal inference to synthesize dissimilar data streams, in our case, to identify various manifestations of cumulative effects of restoration and benefits to salmon, and to further inform restoration and recovery planning.