Skip to main content

Effects of Florida's Red Tide on Marine Animals

Karenia brevis produces toxins called brevetoxins that affect a variety of marine wildlife. Aquatic organisms can be exposed to brevetoxins in several ways: coming into contact with K. brevis cells or toxins in the water, ingesting cells, inhaling toxins and consuming toxic prey. Brevetoxins can accumulate in primary consumers such as zooplankton, fish, bivalves and other filter feeders. From these organisms, the toxins can be transferred to predators such as fish, crustaceans and other bottom-dwelling organisms. The accumulation and persistence of brevetoxins in these marine animals and environments such as seagrass and sediment can eventually lead to the transfer of these toxins to larger animals, including sea turtles, birds and marine mammals. Here’s a more detailed look at how K. brevis toxins affect different animals.

dead fish in water


Fish are most affected by
red tide, and fish kills are often the only noticeable effects of blooms. Large scale die-offs have been observed with red tides since 1844, but the organism responsible, K. brevis, was not identified until 1946. Brevetoxins affect fish regardless of species or developmental stage. Signs of intoxication in fish include violent twisting and corkscrew swimming, defecation and regurgitation, pectoral fin paralysis, caudal fin curvature, loss of equilibrium and convulsions. Ultimately, fish die because their gills stop functioning. In most instances, death from brevetoxin exposure is immediate; however, death can occur after long-term exposure to toxins. Fish typically die when exposed to 250,000 K. brevis cells per liter of water, although this threshold varies with bloom concentration and exposure. Shark species have also been affected during multi-species fish kills, but large scale die-offs of sharks are rare.


Red tides can also lead to die-offs of invertebrates, including shrimp, sponges, sea urchins, crabs and certain shellfish. In most cases, it is unclear whether invertebrate die-offs occur because of exposure to toxins or the low-oxygen conditions resulting from blooms. Although most bivalve shellfish appear to be unaffected during K. brevis blooms, bay scallop (Argopecten irradians) kills have been documented. Lab studies have shown that survival of bivalves depends on maturity; larvae are more susceptible than developed invertebrates.


Strandings of live and dead sea turtles increase during K. brevis blooms. Signs that a sea turtle has been exposed to brevetoxins include swimming in circles, lack of coordination, head bobbing, muscle twitching, jerky body movements and extreme lethargy. It is possible to successfully rehab sea turtles; however, it can take up to 50 days for the toxins to clear their system. During prolonged blooms along Florida’s central-west coast in 2005 and 2006, more than 300 sea turtles stranded, representing a four-fold increase over the previous 12-year average. Loggerheads (Caretta caretta), Kemp’s ridley (Lepidochelys kempii) and green turtles (Chelonia midas) were the species affected. Loggerheads were most affected and took much longer to recover than the others.


Mass die-offs of coastal seabirds and shorebirds are common during K. brevis blooms because 
birds consume fish and crustaceans contaminated with toxins. Some of the commonly reported species affected include double-crested cormorants (Phalacrocorax auritus), lesser scaup (Aythya affinis) and brown pelicans (Pelecanus occidentalis). Sick birds exhibit an inability to stand, slumping of the head, weakness, reluctance to fly, seizures, nasal and oral discharge, labored breathing and dehydration.


The first record of a marine mammal die-off caused by a red tide occurred off the southwest coast of Florida during 1946 and 1947. Marine mammals are affected by consuming contaminated prey or by inhaling toxins. K. brevis blooms that span a large range and persist for long periods are most harmful to marine mammals. For example, when a red tide spanned across 150 miles of Florida’s shoreline between 1953 and 1955, catastrophic die-offs of marine animals were recorded. 


The largest dolphin die-off caused by a red tide resulted in more than 740 bottlenose dolphin strandings, many of which were already dead, along the U.S. Atlantic coast from June 1987 to May 1988. Four other notable bottlenose dolphin die-offs have been recorded since that time, three of which occurred in the Florida Panhandle. The first Panhandle event, in 1999, unfolded during a 40-week period. In 2004, another dolphin die-off occurred in the same region, but during a much shorter period (four weeks). Although this event did not coincide with an identifiable K. brevis bloom, brevetoxins were found in all of the dolphins tested. Baitfish, Menhaden (Brevoortia sp.), containing high concentrations of brevetoxins were the most common prey identifiable in dolphin stomach contents, revealing the source of the brevetoxins. In a third Panhandle event, almost 100 bottlenose dolphins died, with strandings initially coinciding with dense K. brevis blooms. Despite differences in bloom conditions, 93 percent of the dolphins tested were positive for brevetoxins.


Suspected red tide-related deaths of the endangered Florida manatee (Trichechus manatus latirostris) were first reported in 1965, and several notable, extensive die-offs have occurred within the last three decades. In 1982, numerous sick manatees were observed during a K. brevis bloom, and deaths continued for three weeks after the bloom subsided. Researchers believe brevetoxins accumulated in filter-feeding organisms attached to seagrass blades, which were then eaten by manatees. A similar, but larger-scale event occurred during a bloom in 1996. In this case, there were fewer reports of sick manatees, but researchers recorded an unprecedented number of manatee deaths. Manatees were recovered in areas with high K. brevis cell concentrations, and the end of the die-off closely followed the dissipation of the bloom. The stomach contents of several animals were found to be toxic, again indicating ingestion as a primary route of brevetoxin exposure. Further manatee die-offs from exposure to red tide toxins were recorded in southwest Florida in 2002, 2003, 2005, 2007, and most recently in 2013.