| Summary: | Maintaining food production while sustaining productive ecosystems is among the central challenges of our time, and it has been for millennia. In the Salish Sea and beyond, the ancient aquaculture strategy of constructing clam gardens, intertidal rock-walled terraces, is thought to have improved productivity of clams for coastal communities. The "Clam Garden Network," a collaborative team of First Nations knowledge holders, archaeologists, and ecologists has been examining traditional marine resource management systems throughout the Northwest Coast. Our first ecological clam garden study on Quadra Island, BC takes a closer look at how clam gardens impact the productivity of subsistence clam species. We tested the hypothesis that clam gardens enhance clam productivity by comparing the beach slope, intertidal height, and biomass and density of bivalves at replicate clam gardens and non-walled clam beaches. We also quantified the variation in growth and survival rates of littleneck clams (Leukoma staminea) as we experimentally transplanted across these two beach types. We found that clam gardens had significantly shallower slopes and greater densities of L. staminea and Saxidomus giganteus. Overall, clam gardens contained 4 times as many butter clams and over twice as many little neck clams relative to non-walled beaches. Transplanted juvenile L. staminea grew 1.7 times faster and smaller size classes were more likely to survive in clam gardens. As predicted, these relationships varied as a function of intertidal height. Consequently, we provide strong evidence that ancient clam gardens likely increased clam productivity by altering the slope of soft-sediment beaches, expanding optimal intertidal clam habitat, thereby enhancing growing conditions for clams. These results reveal how ancient shellfish aquaculture practices may have supported food security strategies in the past and provide insight into tools for the conservation, management, and governance of intertidal seascapes today.
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