Red Tide Kills Dolphins and Manatees
Brevetoxin found in marine mammals and in the food they eat

June 12, 2005

Red tides are produced in the ocean by microscopic algae called Karenia brevis (K. brevis). When there are high concentrations of this algae, the water can become poisoned when the algae release a toxin called "brevetoxin." Brevetoxin can be eaten or inhaled. This toxin affects the nervous system by opening sodium channels on neurons. This can lead to paralysis and loss of coordination. Red tides kill many fish and can harm people who eat contaminated shellfish or inhale air containing the toxin.

Although brevetoxin poisoning has been suspected in the deaths of marine mammals, scientists have not known how these animals come in contact with the toxin. Therefore, researchers studied 27 manatees and 36 bottlenose dolphins that died in Florida in the spring of 2002 (manatees) and 2004 (dolphins).

High concentrations of brevetoxin were found in tissues (e.g., liver, kidney, lung) and fluids (e.g., blood, stomach contents, urine) of all animals. Because brevetoxin was found in the stomachs of these animals, the researchers concluded that the manatees and dolphins must have eaten contaminated food. Seagrass, a primary part of the manatee diet, contained brevetoxin in the areas where the manatees died. Menhaden, a fish that feeds on plankton, was found in the stomachs of half of the dolphins that died. These fish had high levels of brevetoxins.

This is the first evidence that fish are the source of brevetoxin poisoning. Although most people get sick from brevetoxin when they eat contaminated shellfish, these findings suggest that people are also at risk if they eat contaminated fish.

Did you know?

  • The governor of Massachusetts declared a state of emergency because of a red tide in early June 2005.
  • Red tide is produced by Karenia brevis, a dinoflagellate.
  • The brain of a bottlenose dolphin weighs 1,500 to 1,600 grams.
  • The brain of a manatee weighs about 360 grams.
  • The brain of an adult human weighs about 1,400 grams.

References and more information:

  1. Flewelling, L.J. et al., Red tides and marine mammal mortalities, Nature, 435:755-756, 2005.
  2. Manatee photographs on this page are courtesy of Jim P. Reid and James A. Powell (manatee/diver photo), U.S. Fish and Wildlife Service.
  3. About Red Tide

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