Maintaining food production while sustaining productive ecosystems is among the central challenges of our time, yet, it has been for millennia. Ancient clam gardens, intertidal rock-walled terraces constructed by humans during the late Holocene, are thought to have improved the growing conditions for clams. We tested this hypothesis by comparing the beach slope, intertidal height, and biomass and density of bivalves at replicate clam garden and non-walled clam beaches in British Columbia, Canada. We also quantified the variation in growth and survival rates of littleneck clams (Leukoma staminea) we experimentally transplanted across these two beach types. We found that clam gardens had significantly shallower slopes than non-walled beaches and greater densities of L. staminea and Saxidomus giganteus, particularly at smaller size classes. Overall, clam gardens contained 4 times as many butter clams and over twice as many littleneck clams relative to non-walled beaches. As predicted, this relationship varied as a function of intertidal height, whereby clam density and biomass tended to be greater in clam gardens compared to non-walled beaches at relatively higher intertidal heights. Transplanted juvenile L. staminea grew 1.7 times faster and smaller size classes were more likely to survive in clam gardens than non-walled beaches, specifically at the top and bottom of beaches. 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.
While there is increasing recognition among archaeologists of the extent to which non-agricultural societies have managed their terrestrial ecosystems, the traditional management of marine ecosystems has largely been ignored. In this paper, we bring together Indigenous ecological knowledge, coastal geomorphological observations, and archaeological data to document how Northwest Coast First Nations cultivated clams to maintain and increase productivity. We focus on “clam gardens,” walled intertidal terraces constructed to increase bivalve habitat and productivity. Our survey and excavations of clam gardens in four locations in British Columbia provide insights into the ecological and social context, morphology, construction, and first reported ages of these features. These data demonstrate the extent of traditional maricultural systems among coastal First Nations and, coupled with previously collected information on terrestrial management, challenge us to broaden our definition of “forager” as applied to Northwest Coast peoples. This study also highlights the value of combining diverse kinds of knowledge, including archaeological data, to understand the social and ecological contexts of traditional management systems.
We test the hypothesis that Colorado River flow is important in providing nursery habitat for the Gulf corvina (Cynoscion othonopterus), a commercially valuable and endemic fish in the upper Gulf of California. We use oxygen isotopes in otoliths to determine when these fish inhabit isotopically different bodies of water (Gulf of California versus the Colorado Estuary). The δ18O values in the natal otoliths of C. othonopterus, significantly more negative than can be predicted by temperature alone, provide evidence that this species uses the brackish habitats created by flow of the Colorado River. A significant log-linear relationship between the natal δ18O values and the cumulative flow of the Colorado River during natal development confirms use of brackish habitat in years that the Colorado River water reached the Gulf. Natal δ18O values indicate that C. othonopterus seek out estuarine habitats with salinities between 26 and 38. Reduction in Colorado River flow since the construction of upstream dams has reduced the size of nursery habitat for C. othonopterus. Our results support the hypothesis that declines in commercial landings can be at least partially attributed to reduced river flow. Increased flow would increase nursery habitat and likely benefit recruitment.
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