Why are there so many dead fish in Tempe Town Lake?

Thousands of fish — mostly tilapia, with some shad and striped bass — are belly up in Tempe Town Lake.

Crews have been scooping out the dead fish since about mid-January, and the effort continues, officials said.

Lake-goers are buzzing on social media about what's killing the fish.

One post inaccurately theorized the dead fish were due to chemical contamination from rain runoff or illegal dumping. Others suggested the growth of toxic golden algae could be the cause. The latter is partially correct.

MORE:5 things to know about Arizona's drought this winter 

The shad and striped bass have died because of an outbreak of golden algae, said Rick Amalfi, vice president of Aquatic Consulting and Testing, Inc., which contracts with the city.  

The tilapia deaths are more of an annual situation. Tilapia, which are stocked in the lake to help control algae, can't take the cold mornings, he said. 

The cold weather kills the fish initially; they sink and warm weather causes gas to build up in the fish, which then float to the surface, Amalfi said.

'Unless you have gills you have nothing to fear'

Chemicals emitted by the golden algae render fish unable to use their gills, asphyxiating them, Amalfi said.

The presence of golden algae, which is absent from the lake "probably 99 percent of the time," is harmless to humans, he said.

"The chemical the algae produces has no effect on humans or domestic animals," he said. "Unless you have gills, you have nothing to fear."

Fish in the lake are still edible, he said.

MORE:fish that clean Phoenix's canals are gorging themselves, getting bigger

When will it be cleaned up?

As the clean-up progresses, Amalfi said the amount of dead fish ebbs and flows with the weather. 

"We made great headway getting all the tilapia out, and then we had a number of days where it was very nice; where we saw hardly any dead fish," he said.

He said this past weekend was particularly bad. "A wave of dead fish came through."

So when will dead fish stop surfacing? It all depends on the weather, Tempe spokesperson Kris Baxter-Ging said. 

"It can take a few weeks, it can take a few days," she said.

Baxter-Ging noted it's not just a Tempe Town Lake problem. “This is something that happens throughout the Valley this time of year,” she said.

She said it's currently uncertain how much the cleanup effort will cost the city.

Reach reporter Angel Mendoza at amendoza@gannett.com, or follow him on Twitter @angelnmendoza_ and on Facebook.

READ MORE:

Non-native pacu fish, relative of the piranha, found in Arizona lake

2017-18 Arizona Fishing Reports

The fish that clean Phoenix's canals are gorging themselves and getting even bigger