I've found another of article about this:
The tide is going out at Gibsons Beach, in the Strait of Georgia on Canada’s west coast. When the tide is low, it’s easy to spot rock walls in the intertidal zone, the area of shore land that’s exposed during low tide and hidden when the tide is in. A person can look at this beach for years and never understand that apparently random scatterings of piled rocks were actually carefully constructed to catch food from the sea. One formation, a circular shape almost 100 feet in diameter, is a clam garden, a flattened area that pools water and creates a habitat for clams to grow. Nearby, also in the intertidal zone, is a chevron-shaped collection of stones that opens into the sea and funnels fish toward the shore, a fish trap.
Dana Lepofsky, an archaeologist at Simon Fraser University in Vancouver, British Columbia, believes these gardens and traps, found up and down the coast, could be up to 2,000 years old. They were used by the indigenous population and serve as artifacts that dispute what the archaeological record has to this point claimed was the area’s primary staple: salmon.
More at
http://archive.archaeology.org/1109/features/coast_salish_clam_gardens_salmon.html
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