Authorities aren't sure what caused thousands of fish to die in the Shark River near Belmar, New Jersey, though oxygen depletion has been named the likely cause, according to Weather.com.

Michael A. Meddis, a public health coordinator in the Monmouth County Health Department, said that the most likely cause is that the fish themselves depleted the water of oxygen.

 "This is highly unusual," Mayor Matt Doherty said, according to APP.com. "It's alarming, but we want to wait until we get the full test results to really look at what caused this."

The affected fish are mainly bunker or menhaden, a small fish usually used for bait, feed, of fish oil.

Bunker are able to "reproduce in large numbers, and typically they use up a lot of oxygen, "and most fish kills are due to oxygen depletion, according to Meddis.

Thousands of bunker were in the river, and fish use up the oxygen in the water as they die, according to Meddis. A large number of fish in one area can contribute to the oxygen depletion.

"Likely scenario: Small, shallow estuary, a huge group of fish and slightly low oxygen levels resulted in a fish kill," said Larry Ragonese, a spokesman for the state Department of Environmental Protection, according to APP.com.

The DEP has taken samples to make sure the cause wasn't disease related, according to Weather.com.

Preliminary results showed acceptable oxygen levels, and almost no algae in the water.

"We have no other complaints up the coast." Meddis said. "This looks like a local problem."

The exact death fish toll is in the thousands, and could be in the tens of thousands. Meddis said there is "no way to accurately count at this point."

The NJDEP is also conducting a chemical analysis.

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