Ancient Anthropogenic Clam Gardens of the Northwest Coast Expand Clam Habitat

Clam gardens are ancient mariculture features developed by Indigenous Peoples of the Northwest Coast of North America that create shallow sloping intertidal shelves where clam productivity is enhanced. We quantify the area of clam habitat created by constructing rock-walled clam gardens terraces in northern Quadra Island, British Columbia, Canada. We combined modelling, high-resolution mapping, beach sampling, and a comprehensive survey of the shoreline to document the location and areal extent of clam habitat in clam gardens today. We divided our analysis into three classes of clam gardens, which differ in substrate and thus the amount of clam habitat created. We found that Indigenous People built clam garden walls on 35% of the shoreline and that about 112,979 m(2)of flat beach terrace were created by clam garden construction. Collectively, the three classes of clam gardens increased clam habitat area between 26 and 36%. About 35% of the area of clam habitat in clam gardens was constructed de novo on bedrock shelves and rocky slopes where no clam habitat existed previously. Furthermore, about 12.0% of clam gardens are smaller than 30 m(2), reflecting the effort put into creating enhanced food production wherever possible. Our analysis demonstrates that clam management in the form of clam gardens was extensive prior to colonization and that these features still have a significant impact on today's intertidal ecosystems. Clam habitat expansion facilitated by clam garden construction encouraged a sustainable and abundant food source in the past and could do so again in today's changing environmental conditions.