Clam gardens are intertidal features modified by Northwest Coastal Indigenous people to enhance clam habitat for optimal shellfish production. The Swinomish Indian Tribal Community (SITC) recently initiated a clam garden project to address declining clam populations and community concerns regarding climate change and ocean acidification. This effort braids traditional ecological knowledge, contemporary resource management, and climate adaptation strategies to encourage local food security and sovereignty and promote sustainable seafood production.
As the first clam garden built in the U.S. in modern times, SITC’s Fisheries Department and Community Environmental Health Program co-designed a long-term monitoring study to examine the ecological changes that occur on the clam garden as it develops over time and socio-cultural changes within the community. For the ecological data, we utilized a “before-after control-restoration” design where (1) the restoration site was the modern clam garden site (Fig. 1) and the control site was a nearby beach that had not been recently modified and (2) before/after was defined by data collected prior to or after we began construction of the rock wall and tending the beach. For each site, we deployed 12 0.25m2 quadrats along various elevation stratifications and collected data on surficial substrate and benthic species. Next, the top 5cm of substrate was collected in a 4mm sieve and samples were sorted to quantify juvenile clams and other invertebrates. Once subsurface substrate data was collected, we then dug the quadrat to 0.3m depth to sample all adult clams. We also measured elevation change by deploying sediment elevation tables at both sites at 1.5m, 0.6m, and -0.6m tidal elevations relative to mean lower low.
Initial results revealed that the control site likely has higher species abundance and diversity compared to the clam garden, which would be expected because the control site is likely a relic clam garden or other human-modified shoreline. Post-installation, we found high levels of sedimentation at the high-elevation site at the clam garden, indicating that the rock wall had changed some aspects of sediment delivery in a localized area. These results and more will be discussed, with a particular emphasis on monitoring ecological change in a biocultural restoration project.