The state will conduct extensive tests of the lower Neuse River, where millions of dead fish have blanketed the water in the past two weeks.
Basic tests so far have shown nothing unusual, according to the state, solidifying the theory that suffocation is the cause of the fish kill.
But the state plans a more expansive survey "looking for other indicators that would clue us in to other issues," said Susan Massengale, spokeswoman for the state Water Quality Division.
The death count in the lower Neuse River off Craven County has risen to 5 million to 6 million Atlantic men haden, Massengale said, and there's no way to know how long fish will keep dying. The fish kill is the largest in the area in at least five years, state records show.
Larry Baldwin, the Lower Neuse River keeper, said he was still seeing newly dead fish Monday. Baldwin patrols the water, investigates pollution and advocates for environmental protection for the Neuse Riverkeeper Foundation.
The state may have missed the fish-kill culprit, Baldwin said, because fish started dying in large numbers more than a week ago. "Their testing may not be timely," he said.
Fish kills are not unusual in that part of the Neuse, and they are often attributed to low oxygen. A UNC-Chapel Hill marine research team had measured low oxygen along the river bottom weeks before the fish kill began, and Massengale said dead fish collected from the area had bloodshot eyes.
"That's one of the indicators that fish are under stress from oxygen deprivation," she said.
The state has ruled out a possible poisoning because fish other than menhaden would have died, Massengale said. Menhaden are thought to be sensitive to changes in their environment, such as dropping oxygen levels.
Baldwin said dead shrimp, flounder and crabs were spotted Monday, but he did not know the cause.
A Washington environmental group put the Neuse on its 2007 list of America's 10 most endangered rivers, noting all the pollutants that flow into it from cities and farms.
The state has rules aimed at reducing Neuse pollution. Water tests showed algae growth was "within normal parameters," Massengale said. Heavy presence of algae can be a sign of pollution.
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