FWC News Release

Northwest Region - Stan Kirkland (850) 265-3676
North Central Region - Karen Parker (386) 758-0525
Northeast Region - Joy Hill (352) 732-1231
Southwest Region - Gary Morse (863) 648-3206
South Region - Dani Moschella (561) 625-5122

Reports of dead fish, ranging from a few individuals to hundreds of thousands, have prompted dozens of calls per week to the Florida Fish and Wildlife Conservation Commission (FWC) this summer.

Apprehensive callers to the FWC’s Fish Kill Hotline worry something must be terribly wrong; others just want to know the causes.

Fortunately, the mechanism at the root of most summertime fish kills in brackish estuaries, freshwater lakes and man-made retention ponds is well understood by scientists. Ironically, a lack of sunshine in the Sunshine State is where problems begin. The process starts with overcast skies, hot summer days and rainy weather. Low water levels, brought on by drought, add to the mix.

Most of the oxygen fish breathe is created when waterborne microscopic plants use sunlight to produce and add dissolved oxygen to the water, a process known as photosynthesis. But when overcast skies persist for several days, there is often not enough sunlight to power the oxygen-making process that supports fish life.

Heavy rain and wind from thunderstorms adds to the problem by stirring up bottom sediments that muddy the water, further reducing sunlight penetration in the water column. Rainwater runoff also washes large amounts of decaying plant and animal material into the water body. In the decay process, bacteria use dissolved oxygen in the water to break down plant and animal matter, further dropping oxygen levels.

“If these conditions persist, and dissolved oxygen levels drop for extended periods, fish suffocate,” said Jeff Willitzer, FWC fisheries biologist.

In short, during extended periods of overcast, rainy or cloudy weather, the biological system uses dissolved oxygen faster than it can produce it. Heavy weather aggravates the situation by creating and intensifying all the conditions that lower dissolved oxygen levels in the water.

The good news is that most water bodies don’t suffer repeat performances every year. In fact, most water bodies go years without suffering a major fish kill.

“In spite of summer fish kills, most Florida lakes are in relatively healthy shape and fish populations remain stable. Angling success generally is not affected over the long haul, even after events that might appear serious to the untrained eye,” said Willitzer, who is also an avid angler.

Still, it’s important for biologists like Willitzer to keep track of the location and extent of fish kills in natural lakes and estuaries to see if there are serious problems developing in an ecosystem that might require investigation or restorative measures.

Floridians can report fish kills in natural water bodies to the FWC’s Fish Kill Hotline at 1-800-636-0511, the FWC’s Lakeland office at (863) 648-3200 or online at MyFWC.com by clicking on “Contact FWC.” It is not necessary to report fish kills in man-made retention ponds.