The sea star wasting disease that's causing mysterious and dramatic die-offs on the Pacific coast is still killing the animals — and hitting a bigger range of species over a larger area than originally thought.

Scientists investigating the disease in the U.S. and Canada met in Seattle, Wash. last week to share the latest findings on the phenomenon.

Even if the exact cause of the die-off is still being debated, the scientists agreed on the scale of the problem, said Dr. Martin Haulena, the veterinarian for the Vancouver Aquarium who attended the workshop.

"This is, if not the, certainly one of the biggest wildlife die-offs that have ever been recorded, and we're not just talking marine die-offs."

Haulena says his observation is based on the number and variety of sea stars being killed as well as the range of the die-off which stretches from Mexico to Alaska.

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Slow recovery

The wasting disease first appeared in 2013, with scuba divers and others reporting sea stars showing lesions, losing their limbs and turning to mush.

It's now spread from Mexico to Alaska, said Haulena, causing 90 to 95 per cent mortality in some areas.

"Recovery is not happening the way it should be, so I think it is still really bad."

Chris Harley, a marine ecologist at the University of B.C., has been monitoring sea star populations, including ones along a Vancouver beach in Kitsilano.

That site used to have 200 to 400 purple Pisaster sea stars — which all but disappeared in 2014 due to the wasting disease. Last year, he found about 10.

Last night, he checked again and found 50, "but of those 50, about a quarter of them were sick," said Harley.

"As it warms up I think we'll be seeing a lot more mortality."

Virus to blame?

The most likely culprit for the disease is some sort of virus, said Harley.

In 2014, researchers at Cornell University found a virus that's more prevalent in sick sea stars than healthy ones, and when they injected healthy ones with the virus it made them sick.

But that particular virus can be found in preserved sea stars from decades ago, said Harley, so there's likely something else at play.

"It could be a disease that's been in the system a long time, and something sparked an outbreak recently."

Whether warmer waters from climate change triggered the die-off — or some other factor — is still unclear, said Harley.

Keystone species

Any die-off of this magnitude is a major concern, said Harley and Haulena, not just for people who might miss seeing sea stars along the beach but for the ecology itself.

The purple Pisaster is the textbook example of something ecologists call a "keystone species." In the case of that sea star, it eats mussels — which exert a certain control over the entire rocky intertidal, said Harley.

"By having the sea stars around you're keeping the mussels in check, and that allows a lot of other plants and animals that would otherwise get covered and smothered by mussels to survive," he said.

"When a single species gets wiped out or experiences a drastic change in population, that has ramifications up and down the food chain," said Haulena.

And there are many more species — including the many-armed sun star — that are getting sick, said Haulena, as well as possible signs of disease in sea urchins and sea cucumbers.

"We knew it was a big deal around here, but to put it in context — yeah, it's a really big deal."