NEW YORK (CBSNewYork) — State officials are investigating a fish die-off in the Hudson River.

Dead bunker fish started washing up first on the shores of the Long Island Sound, and now on the Hudson River from Peekskill to New York Harbor.

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The fish will be analyzed, but Riverkeeper Paul Gallay believes they suffocated.

“This is nature’s way of warning us to take better care of our coasts and rivers,” Gallay said.

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Nitrogen and fertilizer promotes algae blooms that deplete oxygen from the water. Gallay said the best way to cut down on these die-offs is to invest in waste water treatment plants and manage storm water better.

“After it rains, bacteria levels go up, oxygen levels go down. It’s less safe for the fish. It’s less safe for the people who swim and boat, and you can fight that by investing in your water treatment facilities,” Gallay said. “You can fight that by doing a better job of controlling the runoff from your property if you are using fertilizers.”

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The Riverkeeper said it is early in the season to see fish die off.