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Post Hurricane Fish Kills

Freshwater Fisheries biologists explain why fish kills occur after hurricanes

    • Bacteria break down plant material in the water and consume oxygen.
    • Often, this process results in oxygen levels that are lethal to fish.
    • Longer periods of overcast weather cause algae and aquatic plants to slow oxygen production during the day while still consuming oxygen from the water at night.
    • As weather and flood levels return to normal algae and plants will resume regular oxygen production.
    • Biologists have observed this occurrence in the past following hurricanes and tropical storms. Historically, fish  populations return to normal within a few years.

Following Hurricane Irma, Florida Fish and Wildlife Conservation Commission (FWC) biologists monitoring the St. Johns River found multiple fish kills (or one long, continuous fish kill) along the middle St. Johns River from Puzzle Lake to Astor. Fish kills are commonly observed with hurricanes and tropical storms and their associated flooding.    

Many people believe that tannic acids, pushed into the river from swamps, causes oxygen levels to plummet and kill fish. That’s not exactly the case. The dark waters are stained by tannins and lignins, compounds that are a byproduct of the decomposition of plant materials. When bacteria are involved in decompostion, oxygen is consumed, creating what’s called a Biological Oxygen Demand (BOD). This BOD depletes oxygen in the water often to levels that are lethal to fish.

With each hurricane, there are usually a few days of overcast weather before and after the storm hits. This overcast weather causes the algae and plants in the water to lower oxygen production during the day (through photosynthesis) while they continue to consume oxygen at night. This drives down the oxygen levels even further. After the hurricane, dead vegetation is flushed out and bacterial decomposition continues to deplete oxygen. Darker, tannic stained waters that are flushed out of the local swamps also add to the shading effect and prolong the time it takes for regular oxygen production to resume.

Fish generally need a minimum of 2.0 mg/L of dissolved oxygen (DO) in the water, though most fish need higher oxygen levels. While some species of fish can tolerate lower oxygen levels for a period of time, fish kills can occur when oxygen levels drop to 2.0 mg/L. Fish become stressed and may begin swimming at the surface seemingly “gulping” the air. They’re actually trying to take in the water that’s on the surface and is highest in dissolved oxygen.

FWC Biologists, along with the Army Corps of Engineers measured oxygen levels at several points along the St. Johns River and DO levels ranged between 0.09 and 1.5 mg/l due to these associated impacts from Hurricane Irma. Once the flood waters recede,  plants and algae in the waters will be able to produce enough oxygen to bring the oxygen levels up to a tolerable level for fish. How long this will take depends on how long it takes for the waters to recede and conditions to return to normal. As long as DO levels are not optimal, fish kills will continue.  It’s unfortunate, but natural.  We’ve observed this in the past with other hurricanes that have hit the state, and within a few years fish populations return to normal.