Red Tide and Shellfish Safety


Summer is a great time to savour fresh seafood and shellfish. As the temperature rises, “Red Tides” may occur in some heavily polluted coastal waters where the shellfish species inhabit, absorb and accumulate the toxins from their surroundings. Consumption of these toxic shellfish may lead to food poisoning or even death. This article explains how red tide affects the shellfish safety and the potential risk of eating shellfish1.


What is red tide?

Red tide is the common name for “algal blooms”, which are natural phenomena that occur in the sea caused by rapid multiplication of tiny algae. When the algae reproduce in dense concentrations, they form visible discoloured strips or patches near the water surface and turn the water in red or reddish-brown2.


How red tide affects shellfish safety

Red tides occur naturally but not all of them contain toxic matters. The red tides occurring in coastal waters are largely caused by discharge of industrial wastes and pollutants into the water due to urbanisation. According to previous records, red tides had occurred in Hac Sá Beach, Cheoc Van Beach and Nam Van Lake because of water pollution in the Pearl River Delta region3. The red tide phenomenon is a sign of declining water quality that can threaten to marine life. For an example, humans who come into contact with red tide water can irritate the skin, eyes, nose and cause respiratory discomfort. When the red tide is triggered by an algal bloom of toxin-producing algae (e.g. dinoflagellates), the toxins produced by the algae will accumulate in the bodies of fish and shellfish along the food chain. Consumption of toxin-containing fish and shellfish leads to food poisoning with symptoms of gastroenteritis, and even death in severe cases. It is apparent that red tide is an abnormal ecological phenomenon that affects human health as well as the marine ecosystem.


Potential risk of eating shellfish

Shellfish do not produce toxins naturally, but they are able to accumulate toxins through feeding on toxic algae or through a symbiotic relationship with algae. In recent years, the high-risk shellfish found in China are linked to mussels (such as Asian green mussels, raw and processed blue mussels), oysters, scallops and Manila clams, and all of them are filter feeders. The amounts of toxins present in contaminated shellfish are determined by the amount of toxic algae ingested, and the levels of toxicity vary from part to part of shellfish. Toxins are usually concentrated in the digestive and reproductive glands.


The main types of shellfish poisoning are paralytic, diarrheal, neurotoxic and amnesic, of which paralytic shellfish poisoning is the most common. The symptoms of paralytic shellfish poisoning typically appear within minutes to hours after ingestion of contaminated shellfish. It includes tingling sensation, numbness of the mouth and limbs, as well as gastrointestinal discomfort. In severe cases, the victims may have difficulty swallowing, respiratory arrest and even result in death.


It is difficult to determine whether shellfish is toxic by its appearance solely; since, shellfish toxins are colourless, odourless and tasteless. The toxins accumulated in shellfish are not destroyed by normal cooking or processing, and dissolve in the liquid used for cooking shellfish. Ingestion of these toxins can endanger to our health.


Important notes on purchase and consumption of shellfish

  • As the quality of the coastal waters of Macao is not suitable for cultivation of aquatic products intended for human consumption, the locally farmed seafood may not be safe to eat. The food sector and consumers should buy aquatic products from reputable suppliers and shops, and be informed about the source of the shellfish to make sure the shellfish be safe for human consumption;
  • In handling shellfish, wash and scrub the shell with water and remove their viscera and gonads;
  • Shellfish must be thoroughly cooked before eating;
  • Avoid drinking the liquid used for cooking shellfish;
  • Maintain a balanced diet and avoid eating too much shellfish at a time;
  • If experience nausea, vomiting, diarrhea, numbness of limbs and other symptoms after consumption of shellfish, please seek immediate medical attention. 

For further reading:



(1)   Reminder on risk of shellfish consumption, 2020:

(2)   Preliminary studies on the causes of unusual red tides in Macao waters:

(3)  News report: Red tides occurred frequently in heavily polluted Macao waters, the creation of the “fourth space” coupled with three sophistications to improve water quality: