The sudden summer weather over the weekend is likely to blame for the death of millions of fish in the Shark River recently.

Fish kill in Belmar (S.A.N.D.)

"The hot weather cooked the nutrients" in the water according to Larry Ragonese of the New Jersey Department of Environmental Protection, forming an algae that "loves oxygen." He explains that menhaden, a kind of bait fish, "are very susceptible to depletion of oxygen" which leads to a natural fish kill.

It led to the site of millions of fish coming to the surface in Belmar early on Monday morning. “For two hours straight, there were millions and millions of fish that were coming out of the river when the tide had changed,” Jesse Thomas of the Fisherman's Den told CBS New York .

In a tweet, Belmar Mayor Matt Doherty called the fish kill "very concerning" and later added that the DEP was cleaning up the water. Doherty told CBS New York the fish were washing onto the docks and beaches.

NJ.com reported that the DEP is also testing the water for dissolved oxygen levels and chemicals.

Ilya Hemlin contributed to this report.

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