Red Tides and Florida Manatees

Recently, we received questions regarding red tides and Florida manatees – What are red tides? Are red tides harmful to manatees? If so, what is being done to help our County’s manatees?

All great questions!

Red tide blooms have been persistent in waters along Florida’s Gulf Coast since September of 2015. Although these blooms have not reached Hernando County, its beautiful Weeki Wachee River serves as a winter home for many endangered Florida manatees!

What are red tides?

Red tides are harmful algal blooms caused by an accumulation of the microscopic algae, Karenia brevis, a naturally occurring organism found throughoutthe Gulf of Mexico, occasionally on the east coast of Florida, and as far north as North Carolina. This organism can be found in both marine and brackish waters, but cannot tolerate low-salinity waters such as lakes and rivers. Although red tide blooms normally begin in September and extend through winter and spring, blooms can occur every month of the year. Blooms originate 10-40 miles offshore and are then transported inshore where it is possible for the algae to use man-made nutrients to grow and intensify.

Despite their name, red tides are not always red not are they associated with actual tides. Both visible and hidden effects, such as dead fish and seafood contamination, often occur during red tides and impact the environment, and local and state economies.


Are red tides harmful to Florida manatees?

Unfortunately, persistent red tides can be harmful to Florida manatees (Trichechus manatus latirostris). While red tides are not always strictly identified as cause of death, they can contribute by various routes through exposure to brevetoxins produced by the Florida red tide organism. Because these toxins are neurotoxins, they damage nerve cells and tissues of many organisms.

Florida manatees are herbivorous – they feed on aquatic vegetation – and seagrasses compose a large portion of their diet. Epiphytes, such as small crustaceans and barnacles, grow on the blades of seagrasses and feed by filtering out particles from the surrounding waters. During persistent red tide blooms, epiphytes remove large amounts of red tide cells from the water and concentrate the toxin-producing algae in their gut. These toxins are then transferred to the manatee through seagrass consumption and can be extremely harmful to the health of the manatee.

Manatees are warm-blooded mammals who seek warmer waters during the winter months. They frequently surface to breathe air through their nostrils, but can hold their breath for approximately 20 minutes. If red tide blooms occur during the winter months, manatees will pass through the blooms on their journey to reach warmer, inshore waters. As waves break inshore, they open red tide cells releasing airborne toxins. Manatees are then exposed to these toxins when they surface to breathe and can develop fatal respiratory infections.

What is being done to help our manatees?

Red tide events, especially persistent blooms during winter months, can be one of the largest killers of Florida manatees. Boat strikes are another major contributor. There is currently no large-scale cure or treatment for red tides, but research, rescue, and rehabilitation are actively occurring.

Preliminary results from January 1, 2016 through December 31, 2016 attribute 48 manatee deaths to a persistent red tide bloom along Florida’s Gulf Coast waters since September 2015. Researchers at the Fish and Wildlife Conservation Commission’s (FWC) Fish and Wildlife Research Institute (FWRI) collaborate with citizenscientists, the University of South Florida, and Mote Marine Institute to monitor and track red tides. Biologists at FWRI partner with Lowry Park Zoo, Sea World Florida, and Miami Seaquarium to rescue, rehabilitate, and release Florida manatees affected by red tides back into the wild.


Report any dead, sick or injured manatees to the FWC Wildlife Hotline: 888-404-3922




Capper, A., L.J. Flewelling, and K. Arthur. 2013. DietaryExposure to Harmful Algal Bloom (HAB) toxins in the Endangered Manatee (Trichechus manatus latirostris) and Green Sea Turtle (Chelonis mydas) in Florida, USA. Harmful Algae 28:1-9.

Flewelling, L.J. 2008. Vectors of Brevetoxins to Marine Mammals. Graduate Theses and Dissertations

Florida Fish and Wildlife Conservation Commission Marine Mammal Pathobiology Laboratory Report. 21 December 2016.

Landsberg, J.H., L.J. Flewelling, and J. Naar. 2009. Karenia brevis Red Tides, Brevetoxins in the Food Web, and Impacts on Natural Resources: Decadal Advancements. Harmful Algae 8:598-607.



Figure 1. Scanning electron microscope image of Karenia brevis. Photo credit: Florida Fish and Wildlife Conservation Commission, Harmful Algal Blooms.

Figure 2. Boundary between a red tide bloom and normal waters. Photo credit: Florida Fish and Wildlife Conservation Commission, Harmful Algal Blooms.

Figure 3. Successful release of rehabilitated manatee from red tide related illness. Photo credit: Florida Fish and Wildlife Conservation Commission, 2013.




Posted: January 4, 2017

Category: Coasts & Marine, Conservation, Natural Resources, UF/IFAS Extension, Water, Wildlife

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