A vast bloom of toxic algae off the West Coast is denser, more widespread and deeper than scientists feared even weeks ago, according to surveyors aboard a National Oceanic and Atmospheric Administration research vessel.

This coastal ribbon of microscopic algae, up to 40 miles wide and 650 feet deep in places, is flourishing amid unusually warm Pacific Ocean temperatures. It now stretches from at least California to Alaska and has shut down lucrative fisheries. Shellfish managers on Tuesday doubled the area off Washington's coast that is closed to Dungeness crab fishing, after finding elevated levels of marine toxins in tested crab meat.

So-called "red tides" are cyclical and have happened many times before, but ocean researchers say this one is much larger and persisting much longer, with higher levels of neurotoxins bringing severe consequences for the Pacific seafood industry, coastal tourism and marine ecosystems.

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Dan Ayres, coastal shellfish manager for the Washington Department of Fish and Wildlife, said the area now closed to crab fishing includes more than half the state's 157-mile-long coast, and likely will bring a premature end to this year's coastal crab season.

"We think it's just sitting and lingering out there," said Anthony Odell, a University of Washington research analyst who is part of a NOAA-led team surveying the harmful algae bloom, which was first detected in May. "It's farther offshore, but it's still there."