BROWNSVILLE — Dead fish, disoriented birds, and discolored water are signaling the Texas coast's largest red tide bloom in a decade, with concentrations of the Karenia brevis organism appearing in patches from Galveston down to South Padre Island.

“We haven't had a red tide affect this much of the coast since 2000,” said Meridith Byrd, a red tide biologist with Texas Parks and Wildlife. “This is a big one.”

Blooms of toxic algae occur around the world, but the Karenia brevis species, with its distinctive red nucleus, is unique to the Gulf of Mexico. It's always present, but tends to thrive along shorelines in autumn, after months of hot, dry weather have increased salinity and nutrient levels. The blooms typically start offshore and are carried toward beaches by winds and currents.

The biggest concern for humans is the brevetoxin the algae release into the air, which can lead to itchy eyes, nose and throat, as well as coughing, wheezing and troubled breathing. The aerosol is most dangerous for asthmatics and others with respiratory problems — the 2009 bloom at South Padre Island led to an uptick of vacationers and island locals showing up at nearby emergency rooms.

The toxin can paralyze the nervous system of fish, making them unable to breathe. A coastwide bloom in 1986 caused millions to wash up on shore.

This year's bloom, while extensive, is not expected to be nearly so severe, Byrd said.

Reports of the most recent outbreak started in mid-September with reports of dead and stressed fish in the Brownsville Ship Channel area, Boca Chica beach, and the southern end of South Padre Island, with biologists confirming high concentrations of the K. brevis.

Subsequent reports led to red tide confirmations from San Luis Pass to the Brazos River, and a few days later at Galveston and Sargent Beach.

The Galveston concentrations, which made for some miserable days for Galveston Island Beach Patrol lifeguards, by Oct. 5 prompted the state Department of State Health Services to close some oyster leases to prevent harvesting of affected shellfish. A DSHS spokeswoman said the leases had not yet reopened.

Fish kills have since been reported along the middle and lower coast, affecting all sizes and species, as well as two green sea turtles.

Byrd warned that people should not eat fish that wash up dead. In fact, she said, it's illegal to pick them up. She said it's also a bad idea to swim where fish are washing up. Fish caught alive and energetic are safe.

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As of Wednesday, the tide was in some areas and had waned in others. Parks and Wildlife reported water samples with moderate to high concentrations of K. brevis at several locations along the Texas coast, including areas near Matagorda, Port O'Connor, Mustang Island/Port Aransas, and Padre Island.

The blooms will likely persist until rains, winds and currents break them up and wash them away, as they may have done in the bay side of South Padre Island.

Coastal ecologist Noemi Matos spent much of Monday surveying a Laguna Madre beach after fishermen reported dead fish. She said that kill appeared to be days old, with water samples coming up clean.

“We may have just gotten to the scene a little too late,” she said. “With the rain and everything, it may have washed out.”