Commercial fishermen along the Southwest Florida coast are reporting a massive dead zone that is almost devoid of marine life in an area of the Gulf of Mexico traditionally known as a rich fishing ground.
They have dubbed it "black water" and are demanding that government agencies find out what's causing it.
Scientists say they don't have answers yet.
"It's killed a lot of the bottom, because recently a lot of little bottom plants are coming to the surface dead and rotten out in the gulf," said Tim Daniels, 58, a Marathon Key veteran fish-spotting pilot.
Fishermen with decades on the water say they have often seen Red Tide but never anything like this. It doesn't have a foul smell. It isn't oil. They describe it as viscous and slimy water with what looks like spider webs in it.
First sighted in January, the mass of black-colored water reached from 20 miles north of Marathon Key halfway to Naples. It stretched west almost 20 miles into the gulf. Fishermen don't know whether it moved in from the north or offshore or originated in the coastal waters off Southwest Florida.
Though somewhat smaller now than descriptions from January, the mass of blackish water is moving into the Florida Keys National Marine Sanctuary. Created by Congress in 1990, the 2,800-square-mile sanctuary adjacent to the Keys is the largest coral reef in the United States, and includes the productive waters of Florida Bay, the Gulf of Mexico and the Atlantic Ocean.
Part of the ecosystem is an extensive nursery, feeding and breeding ground that supports a variety of marine species and a multimillion-dollar fishing industry that brings in almost 20-million pounds of seafood each year.
Scientists with Mote Marine Laboratory based in Sarasota said they are aware of the black water phenomenon but have not yet been able to test water samples.
Erich Bartels, staff biologist at the lab's Center for Tropical Research in the Keys, said he has only seen samples too old for testing that were brought in by crabbers.
"If you held it up to the light, it had a blackish tint to it," he said. "There is something going on. It's some kind of dead zone. We just don't know. We're trying to get samples."