The oxygen-poor "dead zone" off the Louisiana and Texas coasts is not quite as big as predicted this year, but it is still the third-largest ever mapped, a scientist said.

Crabs, eels and other creatures usually found on the bottom of the Gulf of Mexico are swimming in crowds on the surface because there is too little oxygen in their usual habitat, said Nancy Rabalais, chief scientist for northern Gulf hypoxia studies.

"We very often see swarms of crabs, mostly blue crabs and their close relatives, swimming at the surface when the oxygen is low," she wrote in an e-mail Saturday from a research ship as it returned to Cocodrie from its annual measurement trip.

Eels, which live in sediments 60 to 70 feet below the water surface, are an even less common sight, she said.

The 7,900-square-mile area has almost no oxygen, a condition called hypoxia. The Louisiana-Texas dead zone is the world's second-largest hypoxic area, she said.

This year's is about 7.5 percent smaller than what Eugene Turner, Rabalais' husband and a professor of oceanography and coastal sciences at Louisiana State University, had predicted, judging by nitrogen content in the Mississippi River watershed.

He had predicted it would be about 8,540 square miles, which would have made it the largest measured in at least 22 years. More storms than normal may have reduced hypoxia by keeping the waters roiled, Rabalais said.

Hypoxia occurs when fresh water pouring in from the Mississippi River floats above the heavier salt water in the Gulf. Algae die and fall to the bottom, where their decay uses oxygen faster than it is brought down from the surface. Eventually, the lower layer holds too little oxygen for fish and other aquatic life.

Nitrogen, from sources including fertilizer, erosion and sewage, speeds up the process by feeding algae.

The dead zone was larger in 2002 and 2001, when it covered 8,500 square miles and 8,006 square miles respectively, and was almost as big in 1999. Scientists want to reduce the zone to about 2,500 square miles.