Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida
High Concentrations of Red Tide on Sanibel

Onshore winds pushed a red tide bloom that has been offshore the islands for the last few months onto shore earlier this week.
On Sunday evening and Monday morning, SCCF Marine Lab scientists found very high levels of Karenia brevis, the organism that causes red tide, at Algiers Beach and Tarpon Bay Road Beach.
“Our samples found more than 22 million cells per liter in the water just off the beach at both locations,” said SCCF Research Associate Mark Thompson. Concentrations of more than 1 million are considered high.
Karenia brevis is a single-celled organism belonging to a group of algae called dinoflagellates. Large concentrations of this organism, called blooms or ‘red tides,’ can discolor water red to brown, giving it its name.
Karenia brevis produces brevetoxins, which may lead to adverse health impacts for humans and potentially fatal impacts for wildlife. The severity of these impacts depends on the concentration of K. brevis cells per liter of water, with impacts to humans and wildlife becoming likely at concentrations of 10,000 and above.
Two weeks ago, SCCF reported on red tide impacts starting to show up on the islands.
Last week, the Florida Department of Health in Lee County issued a health alert for a medium-level presence of red tide near Turner Beach, at the southern end of Captiva Island. They also issued a Health Alert for Alison Hagerup Beach Park on Captiva and today added one for Bowman’s Beach on Sanibel. Signs are posted at public access points.
Residents and visitors in areas with medium to high levels are advised to take the following precautions:
- Do not wade or swim in or around red tide. Red tide can cause skin irritation, rashes, and burning/sore eyes.
- Keep pets away from water and dead fish. If your pet swims in waters with red tide, wash them with soap and water as soon as possible.
- Wash your skin and clothing with soap and fresh water if you have had recent contact with red tide, especially if your skin is easily irritated.
- Remain cautious and stay away from these locations, especially if you have chronic respiratory conditions.
- Residents living in affected beach areas are advised to close windows and run the air conditioner.
Bloom began after Milton
The current red tide bloom started near Tampa Bay shortly after Hurricane Milton in October. It has been offshore and showing up in the satellite imagery for the last few months.
Samples collected along the beaches have been showing background concentrations during this time even though the satellite images and reports from aerial photography showed the red tide west of Sanibel for the past month.
Satellite imagery as well as aerial photography and video shows that the bloom is large and stretches from Sanibel and Captiva to Naples.
Marine Lab found high counts offshore earlier this month
Samples collected by the Marine Lab in early January confirmed the offshore presence of red tide in high concentrations.
“The runoff from recent rain events have provided additional nutrients which help sustain the bloom in the waters near the barrier islands,” said Thompson. “The conditions of Sunday evening and Monday morning — with light onshore winds and no waves in the surf zone — caused it to become more concentrated along the shoreline leading to more potential adverse effects.”
The SCCF Marine Lab will continue to monitor the bloom and provide updates.
SCCF has also received reports of dead fish from beachgoers this week. If you see dead fish or experience any impacts of red tide, please email sccf@sccf.org.
Please also report any fish kills to the Florida Fish & Wildlife Conservation Commission online here or by calling their Fish Kill Hotline at 1-800-636-0511.
Visit SCCF’s Red Tide Resources webpage for more information.