Though whatever killed thousands, if not tens of thousands, of fish in a New Jersey river might be baffling locals, state officials and fishermen think a natural cause is most likely behind it.

According to WCBS-TV, the state’s Department of Environmental Protection initially thought heavy rains churned up the Shark River’s water, which could have led to an algal bloom that depleted oxygen in the water. But more recent tests of the water near Belmar, New Jersey, found that neither algae nor chemicals were present at a level that they could have been the cause.

Dead fish, which surfaced and washed ashore in the Shark River, float near a boat ramp on S Concourse in Neptune, N.J., Monday May 12, 2014. Authorities are trying to determine what's causing a massive fish kill. (AP/The Asbury Park Press, Tanya Breen)

Dead fish, which surfaced and washed ashore in the Shark River, float near a boat ramp on S Concourse in Neptune, N.J., Monday May 12, 2014. Authorities are trying to determine what’s causing a massive fish kill. (AP/The Asbury Park Press, Tanya Breen)

Another option some are considering is that a large school of fish recently spotted in the area could have sapped each other of oxygen.

“They were here last night. Biggest School I’ve ever seen,” fishing boat captain George Stella told WCBS Tuesday. “The blues and striped bass come by and chase them and they come here and run out of oxygen.”

Fisherman Nick Caruso told the news station that an incident like this seems to happen every 20 to 30 years. He too attributed it to the amount of fish, especially bunker fish.

There are those in the area who have heard of such events, but never witnessed them.

“I’ve never seen it in this river before, anything like this. Never,” Bob Matthews from Fisherman’s Den told WCBS 880.

Bunker fish “reproduce in large numbers, and typically they use up a lot of oxygen,” Michael Meddis, public health coordinator for Monmouth County Health Department, told the Asbury Park Press.

Watch WCBS-TV’s report:

At this point, Belmar Mayor Matt Doherty said that they’re cleaning up the mess and continuing to investigate the source. The Asbury Park Press reported that the mass death appears to be localized issue.

Larry Hajna, spokesman for the Department of Environmental Protection, said such mass deaths are not necessarily uncommon and it is likely a “combination of natural causes is what did it.”

This story has been updated to correct the spelling of Asbury Park Press, mistakenly written as Ashbury Park Press.

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