NEW YORK (CBSNewYork) – Possibly tens of thousands of fish have died in Belmar, the New Jersey Department of Environmental Protection said.

Footage from Chopper 2 showed thousands of dead fish in Shark River near the docks in Belmar on Monday.

The DEP believes the fish kill is a result of natural causes, the remnants of a massive influx that came into the estuary overnight, CBS 2’s Lou Young reported.

“They were here last night. Biggest School I’ve ever seen,” fishing boat captain George Stella said.

Interview With Belmar Mayor Matt Doherty

Initially, heavy rains were thought to be the cause of the massive kill.

Recent heavy rains were believed to have caused the water to churn, stirring up the sediment at the bottom of the river, the DEP told CBS 2. This could have caused algae to bloom after recent warm weather. The algae could have starved the water of oxygen, resulting in the death of the fish, the DEP said. However, that scenario now seems less likely as subsequent testing determined that oxygen levels in the water were normal, and no algae or chemicals were found.

The DEP also said that it was possible that a large school of the fish migrated into the area. The fish may have done so in close proximity to each other in a small estuary, deprived each other of oxygen, and suffocated.

Local fisherman told CBS 2’s Young that they have seen this kind of thing before.

“Every 20 or 30 years you’ll see something like this happen. It can be attributed to the amount of bunker in the area right now,” Nick Caruso said.

“The blues and striped bass come by and chase them and they come here and run out of oxygen,” Stella added.

“It is very alarming when we see something like this,” Belmar Mayor Matt Doherty told WCBS 880’s Steve Scott. “And right now, we’re cleaning up what’s here, and we’re still trying to find out what the source of this fish kill is.

“They’re coming up the docks,” Doherty added. “They’re coming up on the beaches. And they’re coming in through our marina as well.”

Doherty told WCBS 880 bunker fish, also known as menhaden, were the breed primarily turning up dead.

“Preliminary idea is that it is a low level of oxygen in the water and the bunker fish are the most susceptible to a low level of oxygen in the water,” Mayor Doherty explained.

Larry Hajna, a spokesman for the New Jersey Department of Environmental Protection, noted that in 2010 “millions” of dead menhaden washed ashore along Delaware Bay. Those are fish are more susceptible to low oxygen, he said.