Florida Red Tide History

Florida red tide goes back at least to the 1840s, when blooms were documented along Florida's Gulf coast, and fish kills near Tampa Bay appear in the records of Spanish explorers.

The Florida Fish and Wildlife Conservation Commission (FWC) answers the question of whether red tide is new with a flat no: red tides were documented along Florida's Gulf coast in the 1840s, and fish kills near Tampa Bay were even mentioned in the records of Spanish explorers (FWC red tide FAQ). The agency also says red tides occurred in Florida long before human settlement and that severe blooms were observed in the mid-1900s, before the coastline was heavily developed.

The continuous scientific record starts later. The FWC Fish and Wildlife Research Institute (FWRI) HAB Monitoring Database documents Karenia brevis blooms from 1953 to the present, one of the longest records of red tide data, and holds more than 200,000 records from samples provided by more than 190 state and county agencies, research institutions, universities and FWC staff (FWC HAB Monitoring Database). The database is updated daily, and data requests go to FWRI.

The organism itself never fully leaves Florida waters. FWC says Karenia brevis is found year-round at background concentrations of 1,000 cells per liter or less, and a red tide is a higher-than-normal concentration of the alga; the cells produce brevetoxins capable of killing fish, birds and other marine animals (FWC).

The Florida blooms that stand out in FWC and NOAA records include the 1950s copper sulfate experiment, the 2004 to 2006 bloom, the 2007 northeast coast event and the 2017 to 2019 bloom.

In the 1950s, U.S. Fish and Wildlife Service and State of Florida scientists used copper sulfate to attempt to eliminate a red tide in coastal Florida waters; it killed some cells but released toxins that, with the copper, harmed other marine life (FWC). A study of three red tide blooms that occurred in the 1970s and 1980s estimated losses from each to be between $15 million and $25 million (FWC).

The FWC Past Florida Events list covers the mid-2000s in detail (FWC). A dolphin mortality event was first reported from St. Joseph Bay in northwest Florida in March 2004, and by the end of April dolphin mortalities totaled 107. In 2005 a Florida red tide became trapped in bottom waters and killed bottom-dwelling animals across a wide area. In January 2006 aquatic animal deaths in Choctawhatchee Bay were traced to brevetoxin exposure, and from September 2007 through January 2008 a Karenia brevis bloom occurred on the northeast coast of Florida.

NOAA and FWC date the 2017 to 2019 bloom differently. NOAA says it started in October 2017, resurged in five southwest Florida counties by early summer 2018, spread to the Florida Panhandle in September 2018 after Tropical Storm Gordon, and was observed along the east coast of Florida at the beginning of October 2018, with some shellfish harvesting areas closed since November 2017; its summary says the bloom dissipated in the winter of 2018/2019 (NOAA National Ocean Service). FWC refers to the November 2017 to February 2019 red tide bloom and notes that, while persistent, it was not the longest on record (FWC). NOAA also compares it with a bloom that lasted from 2004 to 2006 with the greatest impacts in 2005.

The economic toll of 2018 was measured afterward. A 2020 University of Florida survey of marine businesses in Charlotte, Collier, Hillsborough, Lee, Manatee, Monroe, Pinellas and Sarasota counties found charter and for-hire operators averaged a 61 percent revenue decrease when red tide was present locally, and the UF/IFAS Economic Impact Analysis Program conservatively estimated approximately $184 million in tourism-sector losses from a review of Airbnb reservations (UF/IFAS).

Florida red tides most often start in late summer or early fall off the central and southwest Gulf coast between Clearwater and Sanibel Island, then persist through winter.

FWC says Karenia brevis blooms occur in the Gulf almost every year and are most common off the central and southwestern coasts of Florida between Clearwater and Sanibel Island, though they may occur anywhere in the Gulf (FWC). Blooms begin in nutrient-poor water 18 to 74 kilometers (11 to 46 miles) offshore, a finding that overturned the pre-1970s belief that they started inshore near passes and barrier islands. The FWC red tide FAQ gives a different range, 10 to 40 miles offshore, so the agency's own pages do not agree on the exact distance (FWC red tide FAQ). Most blooms last three to five months and affect hundreds of square miles, but they can continue sporadically for as long as 18 months.

NOAA researchers sharpened the calendar in a January 2022 study of more than 60 years of southwest Florida cell counts: blooms typically form in August and continue through the winter, with October and November being the months most frequently impacted (NOAA NCCOS). The same study found that strong offshore winds can suppress the breathing irritation people notice on the beach, and some Florida bloom years, such as 2008, 2009, 2011, 2013 and 2014, produced almost no respiratory impacts.

Geography limits where Florida red tide can go. It reaches bays and estuaries but not freshwater lakes and rivers, because Karenia brevis cannot tolerate low-salinity water for very long, and some red tides have been carried by the Gulf Stream into the Atlantic as far north as Delaware (FWC). Florida freshwater lakes and rivers instead see other harmful algae, including cyanobacteria (blue-green algae).

FWC says its Florida red tide data cannot show whether blooms have become more frequent or severe, because sampling effort has changed so much since the 1950s.

Most historical samples in the HAB Monitoring Database were collected in response to blooms, so effort rose and fell with bloom size and state funding; since the mid 1990s sampling efforts have increased substantially, with three times as many samples collected compared to early years, and that has produced the detection of more blooms (FWC). For that reason scientists cannot use the data to determine if red tides have increased in frequency or severity in the past 60 years. Before 1998, 80 percent of Florida samples were surface samples collected inshore; after 1998 the federally funded ECOHAB Florida program sampled more than 70 inshore and offshore stations monthly.

On causes, FWC finds no direct link between nutrient pollution and the frequency or initiation of Karenia brevis blooms, which develop offshore away from man-made nutrient sources, but says blooms carried inshore can use land-based nutrients, including man-made ones, to grow (FWC). Severity can still be compared year to year within modern monitoring: NOAA NCCOS found calendar year 2018 had the most extensive red tide on the southwest Florida coast, and the respiratory irritation index measured in 2018 in Manatee and Sarasota Counties was the highest on record since routine monitoring started in 2006 (NOAA NCCOS).

Florida shellfish harvesting areas close when Karenia brevis reaches 5,000 cells per liter, and FWC reports beach effects in its weekly status using cell-count categories.

The FWC key on the Florida status page sets the closure trigger: shellfish harvesting closures when cell abundance equals or exceeds 5,000 cells/L (FWC). The Florida Department of Agriculture and Consumer Services (FDACS) Division of Aquaculture runs the closures. An approved area is normally open but may be temporarily closed under extraordinary circumstances such as red tide events, hurricanes and sewage spills; FDACS monitors harvesting areas with FWRI in St. Petersburg and reopens them only after water or shellfish meat sampling meets National Shellfish Sanitation Program standards (FDACS).

For recreational harvesters the rule is strict: harvesting hard clams, oysters and mussels from approved or conditionally approved Florida areas is banned during red tide closures, while commercially available shellfish harvested locally are tested for red tide toxins before they are sold (FWC). Health guidance sits with the Florida Department of Health, which gathers FWC marine sampling and DEP freshwater data to develop health cautions and alerts issued through county health departments (Florida Department of Health).

Current Florida red tide status comes from the FWC weekly report, its daily sample map and the statewide recording at 866-300-9399.

FWC posts the status report on Friday afternoons, adds Wednesday updates when available, and keeps a daily sample map that shows the last eight days of sampling and is updated daily at 5 p.m. (FWC red tide current status). The Red Tide Status Update for October 9, 2026 found Karenia brevis at background to very low concentrations in 11 samples from and offshore of the Florida Gulf Coast, in Pinellas, Manatee, Sarasota, Charlotte and Collier counties and in Bay and Gulf counties to the northwest; it was not observed along the east coast, and no red tide fish kills or respiratory irritation were reported that week. The next report was scheduled for Friday, October 16. Callers can reach the statewide recording at 866-300-9399 at any hour.

Some official history pages are out of date. The FWC Past Florida Events list, as fetched on October 15, 2026, lists no event that began after 2012 and has no entry for the 2017 to 2019 bloom (FWC), and the NOAA 2018 event page now opens with a note that the bloom dissipated in the winter of 2018/2019 but keeps its duration notes as written at the beginning of October 2018 (NOAA). For later blooms, FWC points to archived status maps in its Flickr gallery and previous regional status reports on its red tide resources page (FWC), and the HAB Monitoring Database runs to the present. Last verified: October 15, 2026, on the FWC website.