Tens of thousands of bunker fish, known as menhaden, were found dead in Shinnecock Bay in a one-week span.

They were found in three different parts of the body of water. Officials say it looks like they suffocated due to a lack of oxygen.

Dr. Chris Gobler, a professor at Stony Brook University, says during evening hours, oxygen levels can get down to 1 milligram per liter.

"The DEC would tell you they want to see at least three (milligrams per liter)," he told News 12.

Scientists say nitrogen from fertilizers and antiquated septic tanks makes its way into the waters and ultimately depletes oxygen levels.

"People have on-site septic tanks and those add more nitrogen," says Gobler. "That then elevates algae levels, which then create an oxygen demand. There's a big initiative now to reduce nitrogen from septic tanks. This is just one more reason we would want to do that."

The fish that died are known as menhaden. They migrate in large schools along the East Coast and are a food supply for larger predatory fish.

There have been several of these fish kills in recent years including in 2015 in the Peconic River in Riverhead.