Scientists onboard a NOAA research vessel are beginning a survey of what could be the largest toxic algae bloom ever recorded off the West Coast.

A team of federal biologists set out from Oregon Monday to survey what could be the largest toxic algae bloom ever recorded off the West Coast.

The effects stretch from Central California to British Columbia, and possibly as far north as Alaska. Dangerous levels of the natural toxin domoic acid have shut down recreational and commercial shellfish harvests in Washington,Oregon and California this spring, including the lucrative Dungeness crab fishery off Washington’s southern coast and the state’s popular razor-clam season.

At the same time, two other types of toxins rarely seen in combination are turning up in shellfish in Puget Sound and along the Washington coast, said Vera Trainer, manager of the Marine Microbes and Toxins Programs at the Northwest Fisheries Science Center in Seattle.

“The fact that we’re seeing multiple toxins at the same time, we’re seeing high levels of domoic acid, and we’re seeing a coastwide bloom — those are indications that this is unprecedented,” Trainer said.

Scientists suspect this year’s unseasonably high temperatures are playing a role, along with “the blob” — a vast pool of unusually warm water that blossomed in the northeastern Pacific late last year. The blob has morphed since then, but offshore waters are still about two degrees warmer than normal, said University of Washington climate scientist Nick Bond, who coined the blob nickname.

“This is perfect plankton-growing weather,” said Dan Ayres, coastal shellfish manager for the Washington Department of Fish and Wildlife.

Domoic-acid outbreaks aren’t unusual in the fall, particularly in razor clams, Ayres said. But the toxin has never hit so hard in the spring, or required such widespread closures for crabs.

“This is new territory for us,” Ayres said. “We’ve never had to close essentially half our coast.”

Heat is not the only factor spurring the proliferation of the marine algae that produce the toxins, Trainer said. They also need a rich supply of nutrients, along with the right currents to carry them close to shore.

Scientists onboard the NOAA research vessel Bell M. Shimada will collect water and algae samples, measure water temperatures and also test fish like sardines and anchovies that feed on plankton. The algae studies are being integrated with the ship’s prime mission, which is to assess West Coast sardine and hake populations.

The ship will sample from the Mexican border to Vancouver Island in four separate legs.

“By collecting data over the full West Coast with one ship, we will have a much better idea of where the bloom is, what is causing it, and why this year,” University of California, Santa Cruz ocean scientist Raphael Kudelasaid in an email.

He and his colleagues found domoic-acid concentrations in California anchovies this year as high as any ever measured. “We haven’t seen a bloom that is this toxic in 15 years,” he wrote. “This is possibly the largest event spatially that we’ve ever recorded.”