There’s a mystery in the water in St. Petersburg: something's killing pelicans and making other birds sick.

It's happening on Riviera Bay Lake, just south of Gandy Boulevard, and follows a massive fish kill there.

Riviera Bay neighbors like Chuck Gall love their peaceful place on the lake.

“You see every bird known to mankind almost, at least in Florida. They seem to come around it now. It's so cool,” said Gall.

Gall says all of the birds are also a sign of typical trouble: a fish kill, which usually happens after a cold snap, like the one last weekend, and changes in the chilly water.

So far this week, John Norris, director of Stormwater, Pavement and Traffic Operations, says the city has cleaned up 4,900 pounds of dead fish, but this year is different - birds are dying, too.

“We see the birds coming. That's how we know the fish are dying, because one leads to the other. The thing is, we've never seen birds die. That's the really sad part,” Gall said.

While out at the lake, 10News spotted a pelican that was clearly struggling to survive and called bird rescuer Bill Riskus with Seaside Wild Bird Rescue.

“Just get in there, buddy,” Riskus said as he scooped the pelican into a net. "He's not in good shape. He can't hold his head up,’ he said. 10News asked if the bird needed the call for help, and Riskus replied, “Yeah, he would’ve been dead by tonight.”

The city says that this week alone more than a half dozen pelicans have died. The Seaside Seabird Sanctuary rescued 15 pelicans and an egret. The rehabilitation facility says not all of the birds survived.

“We've had a pretty classic thermal inversion at the Riviera Bay Lake. In typical thermal inversion you get the first cold temperatures, cold water on the top, and that's more dense than the warm water that rises to the surface," Norris said.

"You get deficient dissolved oxygen levels in the lake and that causes the fish kill,” said Norris. “If it was just a fish kill, we would not be nearly as concerned, because it is a fairly common practice or occurrence with the thermal inversion. It happens most years, but with the birds dying it's a different element. We want to find out if there's something else causing it,” Norris said.

The city asked FWC to test if toxic algae in the water is to blame.

“It’s not likely red tide, but they're also looking for other types of algae that could be toxic,” Norris said.

FWC hopes to have water test results back by early next week. The dead pelicans have been sent to the University of Georgia for necropsies to uncover what is unexpectedly killing them.

Some good news for the pelican 10News came across: he's with St. Pete wildlife rehabilitator Penny Boehme. She says he’s getting fluids and a toxin removal treatment.

If you see an injured bird you can call:

Seaside Seabird Sanctuary Injured Bird Rescue Line: 727-391-6211

Seaside Wild Bird Rescue, Bill Riskus: 727-210-5313

Area help for sick, injured or orphaned wildlife (All Mammals: Including but not limited to Raccoons, Fox, Coyotes, Opossums, Armadillos, and Squirrels):