NORTH HALEDON — Dozens of fish are being wiped out at a local pond, but the cause of the mysterious die-off was unknown.

A stench of decaying fish — all full-grown carp — started to upset neighbors of Oldham Pond earlier this week. The pond, owned by William Paterson University, is a favorite spot for anglers.

Biologists visited the pond on Wednesday, brought fish to their labs and conducted tests on tissue samples, said Caryn Shinske, a spokeswoman for the state Department of Environmental Protection.

But, as of Thursday afternoon, testing was not done. "The DEP cannot speculate on a cause for the fish die-off, while test results are pending," Shinske said.

Mary Beth Zeman, a William Paterson spokeswoman, said local police contacted the university about the fish kill on Tuesday and that the DEP was notified.

Zeman said the university was told the cause is a virus that specifically affects carp, but Shinske would not confirm that detail.

On Thursday morning, a William Paterson employee was seen walking the eastern edge of the pond, along High Mountain Road, snapping cellphone photos. Most of the dead fish — about 35 in all — floated toward a spillway at the southeast corner of the pond.

Zeman said the university would dispose of the dead fish.

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Carp can grow up to 30 pounds and are found in almost every lake, pond and reservoir in New Jersey. Some anglers pursue them because they fight hard, but they are not considered game fish in the U.S.

Gil Hawkins, an experienced fisherman from Leonia, said he did not know about a virus contagious to carp in New Jersey.

"I've never heard of a virus," said Hawkins, environmental director for the Hudson River Fishermen's Association. "But, if there is a virus out there, I think everyone should know about it. It's an indicator, just like anything else, about the health of our waterways."

The fish kill disturbed some residents.

"Lived across the street for 30 years — never saw anything like that," said a woman, who posted her thoughts about it to Facebook.

"The smell is awful," wrote another woman, who lives "right up the street."

The pond was donated to the university by Whippany-based Bayer Corp. in 1998. Miles Laboratories, a subsidiary of the pharmaceutical giant, operated a chemical plant at the south end of the 26-acre property until 1993.

North Haledon and Passaic County turned down the gift, but the university viewed it as a chance to expand its Wayne campus.

Philip DeVencentis is a local reporter for NorthJersey.com. For unlimited access to the most important news from your local community, please subscribe or activate your digital account today.

Email: devencentis@northjersey.com Twitter: @PhilDeVencentis