Red Tide: What to Know

Medically Reviewed by Michael W. Smith, MD on July 19, 2021
4 min read

A “red tide” is a type of harmful algal bloom, or HAB. It happens when algae grow so much in an area of the ocean that it discolors the water, often with a reddish or reddish-brown color. These red tides can make substances that are toxic to fish, marine animals, birds, and humans. They usually happen in the late summer and early fall.

Not all algae blooms are bad. Sometimes they are part of the natural cycle of the ocean and simply provide a food source for certain types of sea life.

Algae types known to produce red tide include Karenia brevis, Alexandrium catenella, and Alexandrium fundyense. A red tide happens when one of them grows out of control.

Scientists still don’t know the exact combination of things that cause this uncontrolled growth. In general, higher temperatures plus more rain combined with less wind than normal make changes in the seawater. They make it warmer, saltier, and allow more nutrients to collect. Runoff that has agricultural pesticides or fertilizers in it can also drain into this water. All this leads to a massive growth of algae.

Scientists continue to study ways in which humans might be able to control such blooms.

When the algae grow to these levels, they use up oxygen in the water, killing many of the animals that live in it. The bodies of these animals contaminate the waters and shorelines in the area of the red tide.

Red tide algae also make toxins, though scientists don’t know why. These toxins can damage the breathing and digestive systems of wildlife and people in the area.

They can also build up in certain “grazing” fish, like krill, and in shellfish. This can make larger fish, mammals, and humans sick if they feed on them.

Fish and other seafood that have high levels of these toxins don’t look or taste different from uncontaminated ones. Cooking or freezing the fish doesn’t kill the toxins.

Especially in warmer climates, red tide toxins in the air and water can cause problems with allergic reactions, minor breathing problems, and irritation of skin and other areas.

In cooler areas, food poisoning from eating shellfish that contain the toxins from the algae is more likely. This can cause:

  • Paralytic shellfish poisoning (PSP): You get this from shellfish that eat PSP toxins made by the alexandrium species of algae in red tide. It can change the way your nerves work and even paralyze you. It happens in the U.S. on both the West and East Coasts. It can be quite serious. It sometimes causes death.
  • Diarrhetic shellfish poisoning (DSP): This comes from dinophysis algae. It’s more common in Europe and Japan but happens in the U.S too. You might notice diarrhea, nausea, cramps, vomiting, and belly pain.
  • Amnesic shellfish poisoning (ASP): You get this from certain types of mussels and soft-shell clams, typically along the east coast of Canada. You might have an upset stomach along with dizziness, brain fog, and memory loss.
  • Get your seafood only from trusted sources. Online resources like the Monterey Bay Aquarium’s can help you make healthy, safe, and sustainable choices.
  • Don’t eat any shellfish like oysters and clams that come from an area with a red tide. See your doctor if you believe you have eaten contaminated shellfish or you feel sick after eating shellfish.
  • Stay away from known red tide areas. This will help prevent the eye, nose, and throat irritation that can happen when you swim in the water or breathe the air. And don’t swim if you notice discolored water or dead fish on the shore.

The symptoms of illness from exposure to red tide don’t generally last long. They usually go away when you leave the red tide area. Still, it’s best to avoid contact with these algae blooms, especially if you have breathing problems like asthma.

Over-the-counter allergy medicines (antihistamines) may help with minor breathing problems and skin irritation linked to red tide. Over-the-counter medicines can help with belly issues. Be sure to drink plenty of fluids so that you don’t get dehydrated.

If you suspect that you’ve gotten sick from eating seafood that’s been contaminated with red tide toxins, let your local department of health know and tell them where so officials can follow up and issue a warning for that area if necessary.

The National Centers for Coastal Ocean Science maintains a webpage that provides forecasts of harmful algal blooms by U.S. region. You can also check the websites of local municipalities like Florida, Texas, California, and other areas with regular red tides.