What killed 10,000 Fish?

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The US Department of Environmental Protection (DEP) confirmed that the cause of the massive menhaden die-off in the Delaware Bay this week was low oxygen levels. This is based on the most recent water sampling results.

What does “low oxygen levels” mean? It’s when the water contains less than 5 micrograms per liter of oxygen. The scientific term is “biological stress.” The New Jersey DEP also found the lowest oxygen level was at 3.4 micrograms per liter — this water sample was from Pierce’s Point in Middle Township, the area with the most dead fish.

On Monday, residents on an island close to Fairhaven, Mass., were struck with a terrible odor: thousands of dead fish on the shores of their beach. Two days later and 200 miles away, beachgoers on the Jersey Shore stumbled across an even worse sight: Tens of thousands of dead menhaden fish splayed ashore for more than eight miles along the Delaware Bay, with tons still floating in the lapping waves.

When the state DEP began investigating the cause, experts said one reason the sensitive species died is “warm waters that have depleted the oxygen,” a marine fisheries employee told KSAT.

New Jersey’s Bureau of Marine Water Monitoring observed water samples taken Wednesday and concluded there were no signs of toxic phytoplankton species, such as red tide. After inspecting the remains, marine fisheries said the fish had been deceased for at least a few days before washing on and around the shore.

According to Eric Stiles, New Jersey Audubon’s vice president for conservation and stewardship, the menhaden are a “critical fish” for the area’s ecology, and the fact that so many died is “highly significant.”

Residents in the area have never witnessed such a large population dying at the same time. Thirty members of the Fishermans Beach Association own the beach, according to one member, KSAT reported. She added, “This is quite sad for us. … We’ll have to get down here and clean it up.”

“If the fish schooled very tightly in shallows very close to shore for any reason, they may have simply used up all the oxygen that was available to them and died,” said Robert Van Fossen, the DEP’s assistant director for emergency management.

Major cleanup operations started Friday and continue through the weekend.