Red Tide: how it develops, and how it can effect your health

Red tide is a natural phenomenon caused by algae blooms of reddish-brown phytoplankton (microscopic algae) in the sea, often in coastal waters. These blooms can effect your health. Most red tides are harmless, but a few species of phytoplankton cause red tides that are poisonous to marine animals and to humans.

Red tide occurs when appropriate ocean conditions allow for massive overgrowth of phytoplankton. These microscopic organisms undergo such rapid growth that the phenomenon, called algal bloom, sometimes will discolor the waters. Several of these microorganisms produce toxins and store them in their cells, hence the term harmful algal blooms. Shellfish such as clams, mussels, scallops, oysters, and cockles feed upon the phytoplankton, concentrating the poisons within their bodies. The poisons can sicken people who consume the shellfish (paralytic shellfish poisoning, or PSP) and create severe respiratory irritation to humans along the shore.

Red tides occur throughout the world, drastically affecting Scandinavian and Japanese fisheries, Caribbean and South Pacific reef fishes, and shell fishing along U.S. coasts. Most recently, it has been implicated in the deaths of hundreds of whales, dolphins, and manatees in North American waters.

If a bloom approaches a shoreline, one may see its effects of dead fish, a characteristic burning sensation of the eyes and nose,and dry, choking cough. When the bloom is severe, fish die rapidly from the neurotoxic effects of the red tide which enter their bloodstream through the gills. Because the fish die so quickly, these toxins do not have time to build up in their tissue. Fish exposed to lower (sub lethal) concentrations, however, may accumulate these toxins in their body. The toxins can sicken people who consume the affected fish (ciguatera).

Red tide populations well below the fish kill level pose a serious problem for public heath through shellfish contamination. Bivalve shellfish, especially oysters, clams and coquinas, can accumulate so much toxin that they become toxic to humans. Public health concerns also emerge from studies that show the presence of airborne toxins have an impact on the human respiratory system.

When visiting your favorite beaches, be sure to heed warnings about red tide and any fish/shellfish consumption advisories, and check Beaches 911 for current beach advisories!

Helpful Links & Resources

More in-depth information on red tide can be found using the following resources:

• FDA Various Shellfish-Associated Toxins
• Florida Fish and Wildlife Research Institute
• Harmful Algal Bloom Forecasting System
• MSNBC TV News “Sea Sick”
• The Harmful Algal Page from Woods Hole Oceanographic Institution