SALMON ON THE LANDSCAPE: RESTORED SPAWNING HABITAT AND MARINE-DERIVED RESOURCES IN AQUATIC AND RIPARIAN ECOSYSTEMS IN THE PUYALLUP RIVER WATERSHED, WASHINGTON STATE, USA

The status of Pacific salmon depends upon a wide range of factors that influence species throughout their anadromous life history. For Puget Sound salmon populations the restoration of channel and riparian habitats in spawning streams is a key strategy for recovery. Here we focus on a large-scale restoration project on South Prairie Creek (SPC) which provides critical spawning areas in the Puyallup River watershed. The project restored the geomorphology, hydrology and ecology of SPC at a site with a legacy of agricultural impacts (channelization, erosion, deforestation). Project activities included; installation of channel spanning and floodplain wood structures, reconnection of >2,500 linear feet of floodplain side channel, and replanting of the riparian forest. Since project completion in 2023 there has been substantial streambed aggradation, flooding of side channels, and native vegetation regrowth. In the fall of 2023 both Chinook and pink salmon were observed spawning at the project site.

The goal of our study was to investigate whether the use of the restored spawning habitat influenced freshwater and riparian food webs via marine-derived nutrients and energy provided by salmon carcasses. The importance of spatial subsidies provided by spawning salmon in inland areas has been well-documented and routinely characterized using stable isotope analysis (SIA). We collected benthic macroinvertebrates and riparian vegetation in early 2024, and our sampling for SIA focused on taxa previously studied in nearby Puget Sound watersheds. Specifically, we sampled macroinvertebrate species across all ecological functional groups in order to estimate the cycling of marine-derived nutrients and energy within the restored ecosystems.

Our carbon and nitrogen isotope results demonstrated that marine resources from salmon carcasses were present in the aquatic food web, and riparian vegetation at the SPC site. We used a Bayesian mixing-model to estimate the role of the salmon subsidy across all ecological functional groups. These data provide a metric for assessing the success of management and policy decisions. In the case of the SPC project, use of the restored spawning areas by salmon, and the rapid integration of marine resources into the local ecosystem, highlight the importance of salmon recovery in Puget Sound watersheds.