More than 100,000 carcasses of the small, white-bellied birds have been found dead since October, including hundreds found along Long Beach in Pacific Rim National Park near Tofino, B.C.
Experts say young Cassin's often die during winter storms, but this year up to 100 times the normal number are washing up in some places along the coast.
About 80 per cent of the species' breeding population of 3.5 million birds is estimated live around the Scott Group of Islands, located off the northern tip of Vancouver Island.
Ucluelet resident Mary Christmas said that in her decades walking the beaches near her home on the west coast of Vancouver Island she had never seen anything like what she found Dec. 22.
"There were about 100 dead birds. It was quite disturbing," said Christmas.
Parks Canada staff collected 157 dead Cassin's auklets from a small section of Long Beach nearby. Some were frozen and sent for necropsies to try to understand what is causing the mass die-off.
Other have reported finding hundreds of the birds on beaches in Haida Gwaii, located to the north of Vancouver Island.
U.S. testing points to starvation
While Parks Canada staff await results, University of Washington biologist Julia Parrish said some evidence suggests the birds are not being affected by any sort of toxin, but are starving to death.
Parrish said it is unclear why, because other birds that feed on the same types of shrimp and plankton aren't affected.
"We have never seen a die-off of Cassin's like this," said Parrish.
Her best guess is several factors are at play, including too many young birds born this year, conditions at sea pushing them closer to shore than normal and perhaps something involving their prey.
"We are also hypothesizing — this is not proven, so our best guess — that these birds, which normally can go quite far out to sea — they're small birds but they're tough little things, and they'll spend the winter far out over the North Pacific — we think for some reason the whole population is much closer to the shore," said Parrish.
"So when a Cassin's is dying, it has a much greater chance of reaching the shore … than normal," she speculated.
Whether the die-off is being triggered by natural forces or if something else is at play, Parrish said, the result is still unfortunate.
"It's a tragic event. It's an untoward event.… We have never seen a die-off of Cassin's like this, so that in and of itself says something."
"I don't think it's going to cause the population to wink out, but it's enough to make me sit up and pay attention."