Before the ancient Egyptians built the last of the pyramids, indigenous people along the coast of B.C. were also engineering and building stone structures that would last for thousands of years, a new study shows.

Clam gardens are undersea walls built to create terraces on the beach at just the right water level to create the ideal habitat for shellfish such as clams. The technology allows far more shellfish to be produced and harvested along a given stretch of coastline, especially when combined with other traditional management techniques, such as removal of larger clams.

Now, Canadian researchers have confirmed that the technology, which is still used along the B.C. coast today, is extremely ancient.

At least one clam garden on Quadra Island is confirmed to have been built more than 3,500 years ago, making it older than the Egyptian pyramid of Ahmose, reports the study published today in the journal PLOS ONE.

Two other clam gardens on the island are confirmed to be about 3,200 years old, based on carbon dating and data about the ancient coastline.

Still in use

And many ancient clam gardens are still in use today.

"The really neat thing is they're still there, through storms and everything," said Christine Roberts an archeologist with the Wei Wai Kum First Nation, who co-authored the paper. "They're still perfect and usable."

Dana Lepofsky, a professor at Simon Fraser University who co-authored the study, said that as archeologists, the researchers "can't help but be excited" about pinning down the age of the clam gardens.

Little neck clams, seen here at a Toronto market, are among the species that thrive in the clam gardens on the B.C. coast. (Chris Helgren/Reuters)

But she added that it wasn't a big surprise. There's evidence that indigenous people have been harvesting shellfish along the Pacific coast for 14,000 years, and that they used other food cultivation and management practices, such as cultivating an aquatic tuber called wapato and managed fish trapping, for more than 3,000 years.

It seems that clam gardens were a particularly important technology for growing food.

"Every place that people could have built a clam garden, they did," Lepofsky said. "It's wall to wall clam gardens."

Roberts, whose community is located in nearby Campbell River on the east coast of Vancouver Island, said so far, 89 clam gardens have been counted just in the local area. And others exist all along the coastline of B.C. and Washington state.

Roberts added that clam gardens are known as "low xwi we" in her language, meaning "to roll the rocks" — a reference to the way they were constructed. Each was individually owned, and even children often got small ones to learn and practise on.

Louie Wilson, Nicole Smith and Christine Smith excavate a clam garden on Quadra Island. The wall is exposed during daylight hours for only a few hours during low tide a few days a month during the summer months. Otherwise, it's underwater. (Dana Lepofsky)

While local First Nations still use clam gardens, they were unknown to western science until a couple of decades ago. In fact, Fisheries and Oceans Canada's page on the history of Canadian aquaculture still states that shellfish farming can only be traced back to the 1920s, with the cultivation of Pacific oysters in B.C.

The arrival of the Europeans decimated the local indigenous population, causing many of the clam gardens to fall into disuse.

Roberts remembers visiting them as a child in the summer, not knowing what they were. "They seemed so stinky and gross," she recalled, "but it was because they hadn't been used for a long time."

Challenging excavation

The walls themselves also remained hidden because they're underwater most of the time — something that made figuring out their age challenging.

The only time the clam garden walls on Quadra Island are exposed during daylight hours — and can be excavated — is during a few hours at a time during low tides, which occur only a few days a month, between May and August, said Nicole Smith, the independent archeologist who led the study for the Hakai Institute, an organization that supports science on the B.C. coast.

The researchers worked quickly at those rare times, searching for trapped clams and rocks that had been turned upside down during the construction of the walls, sometimes burying and preserving the barnacles attached to them. Because those clams and barnacles contain organic material, their age could be determined with radiocarbon dating.

The dates were then compared to evidence based on the location of the wall compared to the historical position of the shoreline, which has moved further out to sea on Quadra Island over the past 14,000 years.

The excavations showed that one of the clam gardens was at least 3,500 years old. (Nicole Smith, licensed under CC-BY)

Smith thinks there may be clam gardens even older than 3,500 years old out there, including some further inland and some further out to sea, depending on the way the shoreline has changed at a given location.

The excavations showed that not only did the clam gardens provide lots of habitat for clams, but the walls themselves are home to edible creatures ranging from sea cucumbers to whelks.

"I feel like this form of mariculture is very in tune with the local environment," said Smith. "There's a lot that modern shellfish aquaculture could learn."

The study was conducted on the territories of the We Wai Kai, We Wai Kum, K'omoks, Xwemalhkwu, and Klahoose Nations and supported by the Hakai Institute, the Tula Foundation, Wnner Gren, the National Geographic Society, the Social Sciences and Humanities Research Council, Simon Fraser University and the University of Victoria.