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Two of the three green sea turtles that have washed up on the B.C. coast in recent weeks have died. A third is being warmed up at the Vancouver Aquarium. (Parks Canada/Canadian Press)

Sighting a hard-shelled sea turtle off British Columbia's coast is extremely rare, so experts are both puzzled and concerned that three such turtles have washed ashore in the last two weeks on Vancouver Island.

Two of the hard-shelled turtles have died, while the third green sea turtle found on Wednesday is being warmed slowly at Vancouver's Aquarium but chances that it will survive are slim.

The green sea turtles, which are listed as a threatened species by the International Union for Conservation of Nature, were found hundreds of kilometres from their usual warm-water homes further south in the Pacific.

Lisa Spaven, a marine mammal biologist with Fisheries and Oceans Canada, said Sunday that this species of turtle has been known to forage in B.C. waters during warmer late summer months. She thinks the turtles may have been stranded when the water temperature dipped dramatically.

"What happens with hard-shelled sea turtles is they can't handle water that is much colder than 14 Celsius and their bodies start to shut down into what's called a cold-stunned state."

That state puts the turtles in a coma-like condition, she said, leaving them to drift for weeks or even months.

A cold-stunned state lowers the turtles metabolism and heart rate and shuts down body parts.

"That's why it's so hard to tell if these turtles are alive or dead when they wash on to shore," she said.

"It's why we take the time and energy to warm them very slowly, one or two degrees a day, over the course of a couple of days to see if they rebound back or not."

Spaven has little hope the remaining turtle will survive, saying it was in very poor condition when it was taken to the aquarium.

All three turtles washed up at Pacific Rim National Park, an obvious spot for such strandings, both because of the topography of the coast and because it's open to the ocean, Spaven said.

And while it's late in the season, Spaven said it's possible more turtles could be stranded along the coast. She is urging anyone who might spot one to call 1-800-465-4336, even if they believe the turtle is dead.

The green sea turtles, which are named so because of the colour of their skin, can grow to over 300 kilograms and 1.5 metres in length.

The turtles that washed up on B.C.'s shore were much smaller and are considered juvenile.

The adult green turtles are herbivores and eat grasses and algae, although juvenile green turtles will eat crabs, jellyfish and sea sponges.