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Common Causes of Fish Kills in Florida

Fish kills occur frequently in Florida. In most cases they are the result of natural causes. However they can be sometimes be attributed directly to human actions. This page is a brief overview of the most common causes of fish kills occur in Florida.

Low Dissolved Oxygen

Fish “breathe” oxygen that is dissolved in the water. Plants, algae, wind, and wave action all contribute dissolved oxygen to the water. During the day plants and algae use sunlight for the process of photosynthesis, which results in the production and addition of oxygen to the water. At night those same organisms consume oxygen. After the sun has been down for several hours, especially by the early morning, so much oxygen may have been consumed that there is no longer enough for fish to “breathe” and they suffocate. This may especially be a problem in warm, stagnant water bodies like ponds. Just before a low dissolved oxygen kill occurs, fish may be seen “gasping” at the surface.

Algae Blooms

Excess nutrients in the water (e.g. nitrogen, or phosphate) can cause rapid and prolific growth of algae. We call this an “algae bloom”. These blooms can cause low dissolved oxygen kills, as described above. If an algae bloom is extremely dense, the shape and sheer number of algae cells can clog a fish’s gills, causing suffocation.

Harmful Agal Blooms

Some species of algae can produce toxins. When these species are found in large amounts in a waterway the algae bloom may be considered a Harmful Algae Bloom (HAB). The most common species that causes HABs in saltwater habitats in Florida is red tide (Karenia brevis) and the type of algae that causes most HABs in fresh water is a blue-green algae called Microcystis. Toxins from HABs can enter the bloodstream through the gills or accumulate in the fish’s organs from food they eat. Effects of the toxins vary but usually inhibit “breathing”, cause erratic swimming, and death.

Extreme Weather Events

Extreme temperatures and severe storms such as hurricanes can sometimes kill fish and other aquatic animals.

Low Temperature

Many species of fish found in Florida are adapted to tropical and sub-tropical temperatures. However, Florida does occasionally experience freezing or near freezing temperatures. Suden decreases in water temperature, or extended periods of cold, can cause fish adapted to warm waters to become lethargic. Biologists call this behavior “cold-shock” and it is sometimes fatal. While cold water kills are most often reported in winter, cold water upwelling events sometimes occur in the summer months along the central-east coast of Florida. During an upwelling event, cold water from deeper areas off the coast is brought to the surface.

High Temperature

Fish kills due to high temperatures most often occur in July and August. Most animals can live only within a relative narrow range of temperatures. Scientists call this range an animal’s thermal tolerance. During extreme heat events, abnormally high temperatures, near the edge of the animal’s thermal tolerance place stress on the fish. To make matters worse, warmer water does not hold as much oxygen —which makes it harder for the fish to breathe.

Severe Storms and Named Storms 

Severe storms can cause dense cloud cover for several days can heavy rains can erode soil and stir up bottom sediments, making the water muddy. This blocks sunlight and may prevent aquatic plants and algae from producing oxygen, causing a low dissolved oxygen kill. After storm surge and flooding recede, fish can become stranded in areas that are normally dry land.


Fish kills caused by diseases usually occur when fish are already stressed by poor water quality. Low dissolved oxygen, toxins from harmful agal blooms, cold temperatures, or other factors place strain on fish and make it harder for them to handle disease.


Herbicides are used to control noxious weeds or algae in ponds and lakes. Most fish kills related to herbicide, are not caused directly by toxicity of the chemical. When many plants die at once, they decompose and use up the oxygen that fish need to breathe, causing a low dissolved oxygen fish kill.

Sewage Spills

Sewage spills introduce large amounts of nitrogen and phosphate to the water fueling algae blooms, and low dissolved oxygen fish kills.

Petroleum Spills

Vigilance and strong regulations keep these spills rare, but are quite serious when they occur. Oil, gasoline, and diesel are all acutely toxic to all animals, including humans.

It is difficult to predict when a fish kill will occur. Reporting fish kills to our Hotline helps us better understand when and where the events happen in Florida.