There are over 15 kilometres of rock walls with more than 15 hectares of specially-built clam habitat in Quadra Island’s Kanish and Waiatt bays.
The age and sophistication of this technology for shellfish cultivation is evidence of Indigenous management systems that long predate contact with Europeans, said SFU archeologist Dana Lepofsky, a co-author.
The gardens improve the productivity of natural clam beaches and in many places create shellfish habitat where none had existed.
“This firmly establishes how old these knowledge systems are and how embedded they are in the First Nations world views and their way of being,” she said.
People generally regard beaches as wild and untouched, but nothing could be further from the truth, Lepofsky said.
Nine specially-built gardens consisting of rock walls and terraces were surveyed and radiocarbon dated with a protocol developed by the researchers using materials that would have been trapped during construction.
While material from burrowing shellfish and limpets were useful, barnacle scars preserved on rocks turned out to be key, said lead author Nicole Smith of the Hakai Institute.
“The ideal sample is a barnacle scar on the underside of a rock used in the base of the wall,” said Smith.
“If the rock came from another part of the intertidal zone and it was turned upside down during construction, it can be preserved in the sediment in an anaerobic environment.”
Because sea level has dropped over the millennia, they were also able to identify and date clam shell in the wall where it could not have grown at today’s sea level.
“It’s position lines up with what we know about the sea level back then and that gives us two lines of evidence,” she said.
Though they are difficult to detect with an untrained eye, shellfish management systems are common to West Coast beaches from Washington State to Alaska.
There are over 15 kilometres of rock walls with more than 15 hectares of clam habitat in Quadra Island’s Kanish and Waiatt bays, among the highest density of clam gardens on the coast, according to the paper published in the open-access journal PLOS ONE.
Because beaches are dynamic, ever-changing environments, it was a challenge to find the right material to accurately date structures built for mariculture.
Add to that the fact that as sea levels rose and fell over time some gardens would have been moved up or down the beach to maximize their productivity, disturbing the archeological record.
“Gardens near settlements would have been actively managed by an individual, a community or a family, and if it wasn’t productive it would just be in the way,” said Lepofsky.
Some of the gardens have been built to enhance natural shellfish beaches, while others have been built by creating rock walls, layered with sediment, gravel and shell hash on steep bedrock where no clam habitat would have existed.
Littleneck and butter clams were staple foods cultivated in the inter-tidal zone between the high and low tide marks, but the rock walls also create habitat for kelp and fin fish.
“When we talk to elders who still hold knowledge about these places they always make very clear that (the gardens) are not just about clams,” said Smith. “I’ve heard elders talk about going out to the wall to fish.”
Some gardens may turn out to be much older than the Quadra Island gardens.
Beaches near a 14,000-year-old village site on Triquet Island have been altered to create fish traps and walled clam gardens. A University of Victoria study found evidence of shellfish processing throughout the archeological record, almost to the present day.