By Curtis Skinner

SAN FRANCISCO (Reuters) - Oceanographers are studying whether climate change is contributing to an unprecedented bloom of toxic algae that spans the Pacific Coast of the United States and Canada, raising health concerns and leading to multimillion-dollar income losses from closed fisheries.

The bloom, which emerged in May, stretches thousands of miles from the Channel Islands off the coast of Southern California to Alaska's Aleutian Islands and has surprised researchers by its size and composition.

"It's just lurking there," Vera Trainer, research oceanographer with the Northwest Fisheries Science Center in Washington state, told Reuters on Thursday. "It's the longest lasting, highest toxicity and densest bloom that we've ever seen."

The center is a branch of the National Oceanic and Atmospheric Administration (NOAA).

The runaway bloom of pseudo-nitzschia algae is believed to have been spawned in part by unusually warm ocean water along the West Coast that scientists have dubbed "the blob."

Scientists are also concerned about the looming El Nino effect, a recurring phenomenon that alters sea currents and temperatures, Trainer said.

Researchers have yet to determine whether longer-term global climate change from rising levels of heat-trapping greenhouse gas emissions are playing a role, but the massive bloom may be a harbinger of things to come in any case, she said.

"Whether this is or is not due to climate change, I think it provides a window to the future of what we could see happen under climate change scenarios," Trainer said.

The algae bloom has produced a profusion of a chemical compound called domoic acid, which accumulates in shellfish, anchovies and sardines exposed to it, and acts as a neurotoxin on higher orders of marine life, such as sea lions and birds, that feed on them.

Researchers are investigating whether the bloom is linked to several suspicious marine mammal deaths, including a humpback whale found dead in Washington state's Grays Harbor on Monday, Trainer said.

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NOAA said contaminated seafood can also cause amnesic shellfish poisoning in humans, a serious illness that can lead to short-term memory loss, brain damage and even death. The agency last month awarded Washington state an $88,000 grant to monitor and analyze the bloom, which has prompted fishery closures in the state.

NOAA said in a statement that the closure of a Washington state razor clam fishery resulted in $9.2 million in lost income and has also damaged the state's $84 million commercial crabbing industry.

(Reporting by Curtis Skinner; Additional reporting by Courtney Sherwood in Portland, Oregon; Editing by Steve Gorman and Eric Beech)