MANATEE -- The birds were the first thing that tipped Frank Riccio off.

At first there were just a few vultures. Then more. And more.

Soon, several dozen were flying overhead, perching in trees and skimming the surface and shore of the stormwater retention pond behind his house near Palma Sola Park.

“I noticed a lot of birds flying right over the water and picking up fish right off the top of the water,” he said Monday.

It was nature’s classic reaction to a fairly common event during Florida winters: a fish kill.

The fish, predominantly blue gill, died from a lack of oxygen caused by recent cold weather, a Manatee County official said. The normal oxygen level in freshwater bodies is six to seven parts per million; tests showed the Palma Sola pond was at one part per million.

“That’s very insufficient to maintain aquatic life,” said Greg Blanchard, environmental program manager in the county’s watershed management division.

Blanchard called the fish kill a “minor incident” and said the county had not received any other recent reports of large-scale fish kills elsewhere.

Nor has the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute in St. Petersburg, which operates a fish kill hotline, spokeswoman Carli Segelson said.

But Blanchard said more reports are likely as a result of the current cold snap.

“Given that we’ll be colder tonight (Monday), we expect to have more lakes and ponds behave the same way,” he said.

He urged people to call the county’s Citizens Action Center at (941) 742-5800 to report fish kills, especially small ones in retention ponds and lakes. Larger kills in rivers and bays should be reported to the state research institute’s hotline at (800) 636-0511.