Thousands of dead fish greeted locals of Belmar, New Jersey Monday, leaving authorities baffled at what caused these fish to die in the outwardly acceptable waters of Shark River.

Prior to the fish kill, scientists have declared the water to have an "acceptable" oxygen supply and confirmed that it is algae-free. However, reports on fish kills have sprung up in the Monmouth County, particularly in Asbury Park and Atlantic Highlands, prompting state officials to fly over the country.

"Belmar is seeing a major fish kill in the Shark River Inlet. NJDEP (New Jersey Department of Environmental Protection) has taken samples and we are cleaning up our area. Very concerning," said Matt Doherty, mayor of Belham, in his Twitter account on the day they found out of the incident.

According to Doherty, it's the "largest kill of its kind" in the river. So what caused thousands of fish to surface dead near the docks of Belmar? A lot of speculations came up.

It could have been the recent heavy rains that caused the water "to churn," NJDEP told CBS 2. It stirred up the sediment that is at the river's bottom and caused the algae to thrive, with the hot weather adding to its flourishing population in the river.

The number of the algae, a known "oxygen lover," depleted the supply in the water, tipping deaths at an astounding pace. DEP said, however, that this possibility is definitive, since the oxygen levels were found to be normal, and neither algae nor toxic chemicals were found.

Another angle could be because the fish themselves have depleted the oxygen in the water. This is according to Michael Meddis, a public health coordinator in the Monmouth County Health Department.

He said the bunkers, also known as menhadens, "reproduce in large numbers, and typically they use up a lot of oxygen." Since these fish are quite large, a huge school of them in a body of water could deplete the supposedly adequate amount of oxygen supply, hence, causing them their own demise.

Bunkers are known forage fish gathering in big schools. They are flat-bodied fish, bright silver in color, and their soft bodies are rich in oil. These fish are usually found in the Gulf of Mexico and the Atlantic Ocean feeding on plankton.

To humans, bunkers are important input for fishmeal, fish oil, and animal feed.