Large swaths of toxic algae have punished U.S. coastal towns at record levels this year, shutting down shellfish harvests and sickening swimmers from Maine to Texas to Seattle.

The algal blooms stretch for hundreds of miles in some areas in a phenomenon known as "red tides" and give off toxins that sicken fish and birds and can cause paralysis in humans, said Wayne Litaker, a research scientist with the National Oceanic and Atmospheric Administration.

The blooms have been getting increasingly larger and more toxic since 2004, causing an estimated $100 million a year in damage to the country's seafood and tourism industries, he said.

This year, the algal blooms:
  • Forced the closure of Maine's bivalve shellfish harvest, which includes clams, oysters and mussels, from early April to September - a first in state history, according to state biologists.
  • Killed more than 4 million fish off the coast of Texas, according to the state Parks and Wildlife Department.
  • Emitted a soaplike foam that coated the wings of seabirds off the Northwest coast, killing more than 10,000 of them, according to the Washington Department of Fish and Wildlife.
The toxic algae are ocean-dwelling creatures that have been around for centuries, said Donald Anderson, a senior scientist at the Massachusetts-based Woods Hole Oceanographic Institute. The 1970s and '80s also saw large outbreaks of red tide, he said.

The seed beds that sprout the toxic algae have grown much larger in recent years, spawning blooms that stretch for 500 to 1,000 miles in some areas, he said. Overfishing and global warming also may contribute to their widespread growth, he added.

Though the blooms affect a relatively small number of humans, they can be lethal. The algae release a neurotoxin that can reside in shellfish. When ingested by humans, it can cause death, he said.

The toxin found off the Texas coast, known as karenia brevis, is not as directly harmful to humans but has killed millions of striped mullet, ladyfish and other bait fish, washing up piles of the fish on tourist beaches, said Meridith Byrd, a biologist with the Texas Parks and Wildlife Department. The toxins ride sea spray and cause some swimmers to experience sinus or asthma attacks, she said.

In Cushing, Maine, 129 clam harvesters were forced out of work when the harvest closed, said Butch Taylor of C&S Seafood, a wholesaler. "It's been a bad year," he said. "It affects everybody."