When dead fish washed up along Lake Hollingsworth last week, readers had questions.
Was it mosquito spraying? Herbicides? Toxic algae? Something more serious?
So LkldNow kept asking.
After multiple calls and follow-ups with city and state officials, the clearest answer is this: the fish kill appears to have been a relatively small, natural event caused by several stresses hitting an especially sensitive species at once.
What officials say killed the fish
The dead fish were gizzard shad, a silvery baitfish in the herring family.
Kyle Luba, a biological scientist and coordinator of the Florida Fish and Wildlife Conservation Commission “Fish Kill Hotline,” said FWC worked with the city after reports of dead fish at Lake Hollingsworth.
“FWC’s Harmful Algal Bloom staff analyzed the water sample and found it was dominated by unidentified filamentous cyanobacteria,” Luba said in an email. That’s a type of blue-green algae.
“The cyanobacteria abundance observed could be high enough to lower the dissolved oxygen overnight,” he said.
But he said the fish kill did not appear tied to one single cause.
“Since only Gizzard Shad were affected, we suspect that there were multiple factors in play,” Luba said. “Spawning stress, fluctuating dissolved oxygen levels, and recent rainfall, which can influence temperature, all likely played a part in this mortality event.”
Why only gizzard shad?
Gizzard shad are especially sensitive to environmental stress, and this is also spawning season when shad gather in shallow water and burn a lot of energy releasing eggs. That can leave them more vulnerable if temperatures swing quickly or oxygen dips overnight.
Laurie Smith, who leads Lakeland’s Lakes and Stormwater Division, said temperature changes of five degrees or more can affect gizzard shad.
“We went from average 75-, 80-degree days often up to the mid-90s,” Smith said. Lake Hollingsworth’s shallow depth — about 4 feet on average — likely made that worse because shallow lakes warm quickly.
Smith also said it was likely only a few hundred fish — considered small in a Florida lake.
“If there was a significant kill, you could almost walk across the lake with all the dead fish,” FWC regional fisheries administrator Eric Johnson said. “Based on the pictures I saw, I thought it was a relatively minor kill.”
So a few hundred gizzard shad likely died because algae, warm water, low oxygen, spawning stress, and rapid weather shifts all lined up at the wrong time.
“The fact that we didn’t see any crappie or bluegill or bass or anything along with that is actually a good thing,” Johnson said.
Was it mosquito spraying?
This was one of the biggest concerns from readers.
Lakeland does not spray for mosquitoes. Polk County does. But Jackson Mosley, Polk County’s mosquito control manager, said the current drought has left the county’s mosquito control fleet largely idle because mosquito activity has been relatively low.
“We haven’t had the numbers of mosquitoes collected to go out and treat,” he said.
When the county does spray, it applies larvicide to large puddles, flooded fields, ditches, and standing water after rain. Those are where mosquitoes breed — not in lakes.
For adult mosquitoes, county trucks focus “on neighborhoods and places where people are.”
“We don’t spray it over bodies of water,” he said.
What about herbicides in the lake?
Smith said Polk County — which handles larger aquatic vegetation treatment — last treated Lake Hollingsworth in January 2025.
The city did a localized herbicide application near drainage outfalls in early April but not broad treatment in the lake itself.
For larger plant issues, Smith said the city usually uses mechanical removal. “Our aquatic harvester has been in Hollingsworth quite a few times,” Smith said. “We physically remove the vegetation out of the lake without use of pesticides.”
Johnson said approved aquatic herbicides are used carefully.
“The only sometimes chain-reaction type event that could potentially happen is if you have massive amounts of plants … and you treated a very large amount,” he said, because decaying plants can pull oxygen from the water. But herbicides used properly “should never cause a fish kill.”
FWC did not specifically test for herbicides or insecticides following last week’s kill. But officials said the fish pattern, timing, and treatment history did not point there.
How common are fish kills?
Florida experiences fish kills regularly enough that FWC maintains a statewide fish kill hotline and tracking system.
Luba said he gets about 2,000 fish kill reports a year, and fewer than 5% are typically linked to herbicides or other chemical-related problems.
He said there were several reports of other freshwater fish kills last week, including a similar shad-only kill at Lake Bonny.
Johnson said small, isolated kills are common during hot, dry weather and seasonal transitions. “There’s always something going on around the state,” he said.
What about the smell?
Reader Mike Young, who walks around the lake daily, said, “The smell is horrendous. I would have thought the town would try and do some cleanup, but the fish are being left to rot.”
Johnson said cleanup usually happens naturally, especially with a smaller kill.
“The alligators and the turtles and the birds will take care of those shad species,” he said, adding that gizzard shad “decompose pretty quickly compared to other species.”
He said shad are small, oily fish without much meat, so they break down faster than larger fish. “The turtles and the birds and the gators will do a pretty quick good job, especially in the low volume of them,” he said.
Cindy Glover is a reporter for LkldNow, a nonprofit newsroom providing independent local news for Lakeland. Read at LkldNow.com.
