The thousands of belly-up fish were discovered Friday in the Bayou Chaland area (see map) of Plaquemines Parish.
The die-off occurred during a time of year when a giant low-oxygen "dead zone" regularly forms off the Gulf, according to Prosanta Chakrabarty, a fish biologist at Louisiana State University in Baton Rouge.
Agricultural runoff into the Mississippi River contains nutrients that support the growth of oxygen-hungry algae, which can choke out other sea life.
The Gulf oil spill expanded this dead zone when a surge in oil-eating bacteria gobbled up even more oxygen, Chakrabarty said by email.
The body of water where the fish kill occurred becomes isolated during periods of low tide, which may have trapped the fish in a low-oxygen area, said Louisiana Department of Wildlife and Fisheries spokesperson Olivia Watkins.
Even so, "fish kills are not uncommon in this area, and if in fact this area was cut off from the Gulf by low tide, these fish would have suffocated," LSU's Chakrabarty said.
"We won't know the ultimate cause until further testing is done."
But LSU fish toxicologist Kevin Kleinow said he would be "very surprised if someone was able to definitely determine the cause of death" for these fish.
That's because fish necropsies are often inconclusive, partly because fish decompose so fast that it's hard to get a fresh enough sample. Additionally, there may be many causes of fish death that are difficult to rule out, he said.
And even if contaminants are found in the dead fish, that doesn’t mean the chemicals were toxic enough to have killed them, he noted.
Gulf Oil Part of Marsh "Assault"
Some pictures of the fish kill—taken by P.J. Hahn, coastal zone management department director for Plaquemines Parish—show brown residue floating near the dead fish. Hahn told National Geographic News the residue is likely oil.
But in terms of spilled oil causing the fish kill, "I would still look at it from a wider vantage point," LSU's Kleinow said.
More likely is that Gulf oil is one of many "combined insults"—such as agricultural pollutants and water diversion, which leads to wetlands loss—that weakened the ecosystem and led to the giant die-off.
"These marshlands are under assault. What we've done through our interactions with these coastal environments—we've altered them significantly," he said.
"The more we impinge on the reserve capacity of these systems, the more they become vulnerable."