Menhaden Fish in the Shark River in New Jersey died off by the thousands, and investigators are trying to piece together the cause. Tens of thousands of fish were found dead in one large pile, in the Shark River, near Belmar. 

Wildlife officials in the Garden State are saying the deaths may be the result of too many fish, exhausting the oxygen supply in the river. However, tests of the water revealed normal levels of the gas, and very little algae. 

Michael Meddis is the public health coordinator of Monmouth County, where the deaths took place. He agrees there may have simply been too many of the animals in a small estuary of the river at one time. The species have been known to lower oxygen levels in sections of rivers where they congregate. The incident took place near a dock, in shallow water, which would have reduced the availability of oxygen. 

"Likely scenario: Small, shallow estuary, a huge group of fish and slightly low oxygen levels resulted in a fish kill," Larry Ragonese, spokesman for the New Jersey Department of Environmental Protection, wrote in an email to the Asbury Park Press. 

The deceased fish are mostly moss bunkers, also called menhaden. This species of fish re-produces readily and consumes large quantities of oxygen. They can be found living throughout the eastern seaboard of North America, from Canada to Mexico reaching a maximum of 15 inches long, and are bright silver, with an oily flesh. Commercial fishermen often capture the animals for production of fish oil and meal. 

Weather in the region may have also played a role in the mass deaths. Heavy rains and warm weather may have driven algae and nutrients to the surface, along with environmental toxins. 

"It is very alarming when we see something like this, [a]nd right now, we're cleaning up what's here... They're coming up the docks. They're coming up on the beaches. And they're coming in through our marina as well," Matt Doherty, mayor of Belmont, told WCBS 880. 

State officials are testing the fish for possible poisonings, as well as disease. There is no reason to believe yet that such causes could be behind the deaths. Many of the bodies are now sinking to the riverbed. 

In 2010, millions of moss bunkers were found dead of asphyxiation in Delaware Bay, as per Larry Hajna, spokesman for the New Jersey Department of Environmental Protection. 

Biologists believe the incident in New Jersey is limited to that area only, and not likely to be seen in other regions.