Dozens of dead sunfish, or "Sunnies" have been found floating in Wild Duck Pond in suburban Ridgewood, N.J.
And what appears to be an identical killer has struck the Grover Cleveland Park pond in Caldwell, several miles away near Newark.
And why they have died remains a mystery.
"The fish I have seen, either dead or sick, have been dark," said fisherman Tom Abascal, 49, at the pond in Ridgewood, referring to a fish that he said is normally light colored.
On Monday, State DEP Water Enforcement Officer John Zuzeck walked the shores of Wild Duck Pond, finding between 15 and 20 dead Sunnies floating near the banks.
But it was the live one he caught that stood out.
Fish are normally skittish and either hard or impossible to catch by hand. But Zuzeck waded in, cupped his hands into the water, and on his second try easily caught the lethargic sunfish.
"White growth on the lips, sides of the body, that's pretty much what I've been seeing," said Abascal, who once sold tropical fish for a living.
The dead Sunnies were first reported last week when Bergen County environmental officials said they recovered 30 of them over two days.
The first thought was to check the water for oxygen levels, but "oxygen seems to be okay, not a problem at all at this point," said Peter Both, Bergen County's Environmental Center Manager.
Some sort of parasite seems to be the leading culprit but the fish recovered will be sent to a state biologist for further testing.
In Essex County, Executive Joe DiVincenzo told NBC New York that the ailment was described as a "white, fungus-type" growth on the Sunfish at Grover Cleveland Pond.
DiVincenzo said he will try to get DEP officials to test his dead fish as well, and said the kill to him was described as number "over 200."
All of which is disturbing to fishermen.
"That many dead fish, I guess it's got to be an environmental problem," said Scott Abascal, 21, who was fishing with his dad.
"At the same time, people come here don't even know you can fish in this lake, throwing garbage in here and things like that," he added.
Follow Brian Thompson on Twitter @brian4NY.