Preliminary tests show that a lack of oxygen in part of the Gulf of Mexico caused thousands of fish to die, according to Louisiana authorities quoted by the Los Angeles Times.

The dead fish were found at the mouth of the Mississippi River, and state officials said the phenomenon—called a “fish kill” in some parts and a “jubilee” in others—was not directly related to the oil from BP's ruptured well, reported the New Orleans Times-Picayune.

“By our estimates there were thousands, and I'm talking about 5,000 to 15,000 dead fish," St. Bernard Parish President Craig Taffaro said in a news release. "Different species were found dead including crabs, sting rays, eel, drum, speckled trout, red fish, you name it, included in that kill."

Dead zones, or areas of depleted oxygen, occur every year in the Gulf because of nutrient runoff from the Mississippi River stimulating oxygen-consuming bacteria. But as we’ve noted, scientists have predicted a larger than average dead zone this year—raising questions of whether the low-oxygen conditions are being exacerbated by the oil spill.

Bloomberg pointed out that these fish kills occur every year when fish, crabs, eels and shrimp move toward the shoreline to escape the low-oxygen waters, but this year scientists have seen the kills occurring “in open water for the first time, raising concern that low-oxygen areas are expanding” because of the oil.

Other scientists cautioned against jumping to conclusions.

"A lot of things can explain a fish kill, which is not uncommon during the hot summer weather in Louisiana,” a Louisiana State University scientist, Ralph Portier, told the L.A. Times. “It could be the nutrient-rich environment with a lot of heat. It could be rainfall. It could be changes in salinity or upwelling from disturbed sediment."

But Bloomberg pointed out that another fish kill also occurred in late June off the coast of Alabama, and here’s what scientists there noted:

“Most of us believe it had something to do with the oil,” said Robert Shipp, 67, chairman of the Marine Sciences Department at the University of South Alabama. There was a “consensus” among faculty at the University of South Alabama and the Dauphin Island Sea Lab that oil played a part in the event, which was “quite different” from the naturally occurring jubilees in the Gulf’s Mobile Bay, Shipp said.

Scientists in Alabama have also reported discovering a mysterious brown residue washing up along the shoreline, reported the Alabama Press-Register. Chemical analysis has shown it is not oil, but contains hydrocarbons, leaving scientists wondering if it’s the remnant of degraded oil or something else, perhaps plankton.