What is a Red Tide?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 13 October 2019
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A red tide is a type of oceanic algae bloom which is characterized by a red tinge in the water. Some of the algae involved in this phenomenon are harmful to fish, and can cause fish to die off. Several species are also dangerous for humans, and can cause symptoms ranging from discomfort to serious illness. Most nations keep a watch on their marine environments so that they can notify citizens when the water is unsafe due to a red tide.

A red tide happens when naturally occurring algae bloom, or grow very quickly and accumulate into a large mass. While this mass of algae is harmless in many cases, it can look startling because certain phytoplankton in the algae bloom are red, and can cause the water to change color. In other instances, the red tide is harmful because it harbors organisms like Alexandrium tamarense and Dinophysis norvegica, which can release toxins when they are consumed. Biologists test the water to determine whether or not the tide is dangerous.

As we have seen, a red tide actually has nothing to do with tides at all. In addition, an algae bloom can be dangerous without being red. Therefore, many biologists prefer the term “harmful algae bloom,” which they feel is more precise. Because the term “red tide” is commonly understood and accepted, most organizations which focus on getting information about ocean conditions to consumers continue to use it.


During a red tide, the toxic algae can poison fish, causing bodies to wash up on shore. This is often used as an indicator by biologists who study harmful algae blooms. The toxins that kill fish are usually not dangerous for humans. Shellfish, however, can accumulate toxins that are very dangerous to people, and can cause conditions like paralytic shellfish poisoning, which can result in death if left untreated. Other types of shellfish poisoning are not uncommon and have symptoms ranging from diarrhea to neurological damage.

The toxin cannot be eliminated from shellfish through cooking, freezing, or any other method. This means that shellfish harvested during a red tide are not safe to eat. Fish from an area with a harmful algae bloom should also not be consumed, due to the chance that they may harbor dangerous toxins. For this reason, it is strongly recommended that people collecting their own seafood check prevailing ocean conditions before going out, and dispose of unsafe seafood.


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Post 7

I think I caught the red tide. What should I do?

Post 6

What would be the symptoms of a person who is infected with red tide?

Post 4

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Post 2

Thanks! sure helps a lot with assignments! Great site!

Post 1

this helps a lot! good job!

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