It’s that time of year again: the leaves change, the air temperatures drop and the fish in the Pamlico and Neuse rivers start dying.

This week, widespread reports of fish kills have been reported to Washington’s North Carolina Department of Environment and Natural Resources office. Mauls Point, Core Point, Bath Creek, Goose Creek and the north side of the Pamlico River in that area have seen the most evidence of the dead or dying.

According the DENR Environmental Supervisor Jill Paxson, the slime mold Aphanomyces invadans makes its appearance in the first few weeks of October every year. At this time of year mass migration to open water is happening, and those whose immune systems are already suppressed by a combination of factors — lack of oxygen in the water due to naturally occurring algal blooms and salinity fluctuations among them — are the first to fall prey to the mold. A. invadans is found across the globe, especially when temperatures drop in the fall, but thin-skinned Atlantic menhaden are particularly susceptible to the common water mold.

“It tends to thrive in the cooler temperatures. Right now, it’s going haywire because it’s having a good time, and it’s attacking the menhaden at this time,” Paxson said. “Other fish are affected, but it’s not causing them to die. They keep on moving.”

The estuaries serve as nurseries to large populations of fish and in the fall, the fish are moving out to sea. Those fish that have not migrated by late September and October are likely less hardy than those who made the trip earlier. As a result, the fish migrating later in the year may be more susceptible to changes in water temperature or oxygen levels, invasive bacteria and other stress factors, according to a DENR press release.

Paxson was clear: it’s not A. invadans that kills, though the lesions or red sores on the menhaden seem pretty traumatic for a small species. Rather, it’s a suppressed immune system and stress, causing fish to drop scales that gives the slime mold a chance to move in.

“Any fish that is immune-suppressed is going to kick the bucket,” Paxson said.

A. invadans does not affect humans or pose any kind of health threat.

Paxson said she’s been tracking the appearance of dead fish and welcomes any additional reports of new sightings. For more information, or to report a fish kill, call 252-948-3999.