Notes

Fish kills often occur in Florida because of our warm and hot weather. They also occur because of changes in the biological and chemical environment of the water. Fish kills occur for a variety of reasons. They are often connected to a decline in dissolved oxygen in the water, the presence of an algae bloom, or some pathogen that may be affecting the fish. In March 2016, a large fish kill was reported in the Indian River Lagoon.

Currently the Indian River Lagoon is experiencing the same conditions as the fish kill that occurred in 2016. There is a brown algae bloom in the northern Indian River Lagoon and Banana River. The air and water temperatures are also warming. Warm water holds less oxygen than cooler water. Many agencies are watching and waiting to see if a fish kill will occur.


In the March 2016 fish kill, there were many reports of fish gulping for air at the surface. This was an indication of low dissolved oxygen (DO) in the water. Warm water fish need DO levels of at least 5 parts per million or 5 milligrams per liter (5 mg/L). Most fish become stressed when DO levels drop between 4 mg/L and 2 mg/L and they try to leave the area. Fish begin to die when DO levels drop below 2 mg/L. They may not be able to escape if the low DO levels cover a vast area. Fish kills caused by low DO affect fish of all different sizes and species. Some fish species are more sensitive to low DO levels than others. Large fish tend to be affected first, and more severely, than smaller fish.

What causes dissolved oxygen to drop?

DO becomes depleted when consumption of oxygen exceeds production of oxygen in the water. An increase in consumption can occur because of too many plants or algae in the water (e.g., the algae blooms in the Indian River Lagoon). Plants and algae are photosynthesizing during the day and releasing oxygen into the water. But at night they are respiring and taking up oxygen from the water. In addition, fish are respiring and consuming oxygen during both the day and night. If the amount of algae or plants in the water is too great (e.g., during an algae bloom), then they will consume more oxygen through respiring than they create through photosynthesis.

Low oxygen can also occur because of decomposition of organic matter, such as when plants, algae, or animals die. The bacteria that feed on dead and decomposing animals or plants uses up oxygen in this process. With more food available, the bacteria increase in number and continue to use oxygen in the water. Less oxygen means more fish will die, the oxygen will continue to be consumed by the bacteria, and the cycle continues. In March 2016, the Brevard County government organized cleanup and removal of the dead fish found along the shoreline, which is an important step in stopping the cycle.

Who are the agencies involved when a fish kill occurs?
The Florida Fish and Wildlife Conservation Commission (FWC) is the first agency informed when large fish kills occur. They have a Fish Kill Hotline at 800-636-0511 and an online reporting system where anyone can report a fish kill. Scientists with FWC and the Fish and Wildlife Research Institute (FWRI) will often come out to a large fish kill to take samples of the fish and water to determine the cause of the fish kill.


Dissolved oxygen concentration fluctuates on a 24-hour basis. This fluctuation is called the diurnal oxygen cycle. Dissolved oxygen increases during daylight hours when photosynthesis is occurring and decreases at night when respiration continues but photosynthesis does not.
Low DO will be one of the first things that scientists will measure and look for when determining the cause of a fish kill. DO in the water can change over time. The levels of DO in the water at say 10:00am on Tuesday won’t be the same as at 3:00pm on Wednesday. In general, DO levels are greatest later in the day because the plants and algae have been photosynthesizing all day and adding oxygen to the water. In contrast, oxygen levels are lowest before sunrise because the plants and algae have been respiring and taking up oxygen during the night. This is called the diurnal oxygen cycle.

Local governments are also involved when a fish kill occurs. Especially if there’s any type of cleanup of fish that might occur. Generally, during a fish kill, the fish are left in the water to decompose naturally. But often times local governments will try to remove the fish from the water, especially if the fish kill covers a large area. The Save Our Indian River Lagoon Project Plan team will be working with agencies to get fish cleaned up and removed from the water if a fish kill occurs this year. Since the goal of the plan is to remove nutrients from the water, removing fish is an important step in making sure those nutrients don’t stay in the system.

Was the fish kill in March 2016 caused by low dissolved oxygen levels?

There are several remote water quality monitoring stations in the Indian River Lagoon. One of these is ORCA Kilroy. The ORCA Kilroys monitor and measure physical, chemical, and biological indicators in the water. There is a Kilroy located in Sykes Creek, Merritt Island. Historical data shows that DO levels fell below 2 mg/L starting late in the day on March 17, 2016 and continued to be below this level until the morning of March 21, 2016. You can look at this historical data and current water quality data yourself at their public Kilroy data website. This evidence suggests that low DO for an extended period of time was the most likely cause of the fish kill. This was later confirmed by other agencies.

What can I do?
If you see a fish kill, contact the FWC’s Fish Kill Hotline at 800-636-0511 or submit a report online. There is also a phone app called FWC Reporter where fish kills can be reported. Also contact Terry Williamson, Lead Environmental Scientist for Brevard County Natural Resources Department, at 321-633-2016 so local government can start mobilizing clean up efforts. Try to volunteer for local fish cleanup efforts if you’re able. The more fish that can be removed, the less nutrients that will stay in the water.