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.

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Rock-walled archaeological features are notoriously hard to date, largely because of the absence of suitable organic material for radiocarbon dating. This study demonstrates the efficacy of dating clam garden wall construction using optical dating, and uses optical ages to determine how sedimentation rates in the intertidal zone are affected by clam garden construction. Clam gardens are rock-walled, intertidal terraces that were constructed and maintained by coastal First Nation peoples to increase bivalve habitat and productivity. These features are evidence of ancient shellfish mariculture on the Pacific Northwest and, based on radiocarbon dating, date to at least the late Holocene. Optical dating exploits the luminescence signals of quartz or feldspar minerals to determine the last time the minerals were exposed to sunlight (i.e., their burial age), and thus does not require the presence of organic material. Optical ages were obtained from three clam garden sites on northern Quadra Island, British Columbia, and their reliability was assessed by comparing them to radiocarbon ages derived from shells underneath the clam garden walls, as well as below the terrace sediments. Our optical and radiocarbon ages suggest that construction of these clam garden walls commenced between ~1000 and ~1700 years ago, and our optical ages suggest that construction of the walls was likely incremental and increased sedimentation rates in the intertidal zone by up to fourfold. Results of this study show that when site characteristics are not amenable to radiocarbon dating, optical dating may be the only viable geochronometer. Furthermore, dating rock-walled marine management features and their geomorphic impact can lead to significant advances in our understanding of the intimate relationships that Indigenous peoples worldwide developed with their seascapes.

Ancient systems of mariculture were foundations of social-ecological systems of many coastal Indigenous Peoples. However, since such systems either do not leave tangible remains in the archaeological record, and/or are hard to date, we know little about their development and use. Clam gardens, traditional mariculture features located within the intertidal zone along the Northwest Coast of North America, are composed of a rock wall positioned at the low tide mark and a flattened terrace on the landward side of the wall. Because these features are largely composed of rock and sediment, and have complex formation histories, they can be difficult to age. On northern Quadra Island, British Columbia, we identify three variations in clam garden form, constructed in different geomorphological settings, each of which require different sampling approaches to obtain ages on construction and ongoing use. To age the clam gardens, we consider radiocarbon dating of invertebrates that inhabit beach deposits (both pre- and post-garden construction), and the relationship of the gardens and clam samples to the local sea level history and taphonomic processes. Within our study area, we find clam gardens have been in use for 3500 years, likely corresponding to other social and ecological changes of the time. These data allow us to formulate guidelines on samples most suitable to constrain the age of initial and on-going wall construction and use of clam gardens, which can be extrapolated to dating other ancient mariculture features in other regions. Such dating programs are the foundation for understanding the long-term development of traditional marine management practices and how they are situated in broader social-ecological systems.

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The relationships between clam gardens and human settlement throughout the millennia reflects the inseparable links among human demographics, marine management systems, and the social-ecological contexts in which they are embedded. However, it can be difficult to assign causation between the initiation and development of eco-cultural innovations like clam gardens and the proliferation of human societies due to the temporal uncertainties associated with both. Here, we bring together data on the shape of the local relative sea level curve, clam garden wall elevation as determined by GIS and drone imagery, radiocarbon dates of clam garden walls, and ecological and archaeological field observations, to assign proxy ages for the clam garden walls of different tidal heights in Kanish and Waiatt Bay on northern Quadra Island, British Columbia, Canada. These data, combined with our mapping and dating of settlement sites, demonstrate a temporal relationship between clam garden building effort and the densification of human settlements. In Kanish Bay, where we have high resolution data, clam gardens begin to be constructed in significant numbers at least 3,800 years ago; this corresponds to a time of increased establishment of large human settlements. The corresponding increase in settlements and clam gardens reflects both the need to increase sustainable food production and the larger number of people who could sustain the ecological and social foundations of the production system. The correlation between number and area of clam gardens and the number of new, large settlements continues until ∼2000 years ago. After this time, existing settlements increase in size, but no additional large settlements were established. New clam gardens continue to be built but in seemingly lower numbers. This shift in settlements and clam gardens suggest that a threshold in social-ecological carrying capacity may have been reached in this land- and seascape. In the last few centuries, there is a dramatic decline in the number of clam gardens and evidence of human settlement, corresponding to social and ecological changes associated with European colonization. Taken together, these data demonstrate the strong linkages among Indigenous peoples, their lands and seas, and resilient food systems over the millennia.

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