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I'm Phoebe Zimmermann with the VOA Special English Agriculture Report.
Different events can change the balance of sea life. Red tides are an example. These can happen in oceans, rivers or lakes anywhere in the world. What happens is that algae suddenly increase in numbers. Algae are single-celled organisms. They are a normal part of sea life.
As they spread, or bloom, the water may turn red or brown. Sometimes the water does not change color at all. But deadly algae could still be present.
Some kinds of algae produce a strong poison. This can build up in shellfish that eat the algae and make them poisonous, but not kill them. Other kinds of algae may kill sea life by reducing oxygen levels in the water. In some red tides, thousands of dead fish appear on beaches.
Scientists do not know exactly why red tides happen. But they say a combination of conditions all play a part. These include water temperature, nutrients in the water and water flow. Pollution could also play a part.
A number of different algae can cause red tides. A common form has the scientific name Karenia brevis. It is often linked with red tides in the Gulf of Mexico and the Caribbean area.
Algae poisons build up in the tissue of shellfish like clams, oysters and mussels. This can make them unsafe for several weeks after a red tide goes away.
People are also advised not to eat the organs of fish or shellfish like shrimp, crab or lobster. But expert say the poison produced by K. brevis does not normally enter the meat of these creatures.
United States' health officials say scientists knew little about the possible risks of swimming in red tides or breathing algae poisons in the air. Some swimmers report skin problems or shortness of breath, but the dangers of eating poisonous shellfish are well known. the center for food safty and applied nutrition lists four kinds of shellfish poisoning by the effects they cause, these include diarrhea, fever, stomach and muscle pain, and breathing problems. In serious case people can die unless they receieve treatment. The center says older people are especially likely to experience severe effects from algae poisons. The center for food safty is part of the United States food and drug administration. Internet users can learn more at fda.gov.
This VOA Special English Agriculture Report was written by Mario Ritter. I'm Phoebe Zimmermann. | 
