Red means DANGER

Serious Red Tide Wave

Red Tide should set alarm bells ringing

We’ve always associated colours with meaning; blue = cold, yellow = sunny and red = danger. So following that train of thought, the words “Red Tide” should set alarm bells ringing as a sign of something dangerous.

Red Tide is a colloquialism for the out of control growth of harmful algal blooms, these are small, single cell plants that live in our oceans and freshwaters, which then produce toxic effects on fish, shellfish, marine mammals, and birds and have been known to cause illness in humans, if the contaminated fish / shell fish is consumed.

These “blooms” will most often discolour the water tainting it red, reddish brown and sometimes orange, bringing us back to the common name; “Red Tide.” At night and with a florescent light, this phenomenon, can actually look quite beautiful.


Though every coastal town / city has at some point in their history and more than once, experienced “Red Tide”, not all algal blooms are harmful. On the contrary, most blooms are actually beneficial as these tiny plants are food for the oceans creatures and are in fact, the major source of energy that fuels the ocean food web. There are a small marinefoodwebpercentage of algae, however, that produce powerful toxins that kill fish, shellfish, mammals, and birds, and may directly or indirectly cause illness in people. These also include blooms of non-toxic species that have harmful effects on marine ecosystems. For example, when masses of algae die and decompose, the decaying process can deplete oxygen in the water, causing the water to become so low in oxygen that animals either leave the area or die. I’m not saying this is the case, but perhaps this could explain why certain marine mammals beach themselves at times? It’s just a thought.

Red Tide is poisonous and can make you very ill if ingested

When eaten, poisonous marine animals can make you very ill, and may even cause muscular paralysis and death. Below is a guide to poisonous marine animals that can be found in the South African seawaters. A leading South African toxicologist Dr Gerbus Müller, was instrumental in its compilation. Read through it and know the potential dangers before you indulge in the seas delicacies this summer.

Mussels, oysters and other bi-valve shellfish, strain the harmful blooms from the water then digest them, and accumulate their poison. Shellfish do not seem to be harmed by the poison, but are toxic when eaten. Shellfish affected by red tides can be extremely poisonous and have been known to cause paralysis and even death, in humans.

I realise that this is a lot to look at but let’s take a look at the different types of “Red Tide” poisoning one could be faced with in South Africa and their symptoms. (Please note, this is not meant as a medical diagnosis, only information, and should there be any thought at any time that there is “Red Tide” poisoning present, it is strongly recommended that you go immediately to your nearest medical practitioner, hospital or poison centre.)

  1. Paralytic shellfish poisoning Eating mussels that contain the poison saxitoxin, produced by Alexandrium catenella, causes paralytic shellfish poisoning.

Signs and symptoms: Symptoms occur within 30 minutes to two hours after eating the food.

These include a tingling sensation or numbness around the lips (which spreads to the rest of the face and neck), a prickly sensation in fingertips and toes, headache, dizziness, a floating or gliding sensation, vertigo, visual disturbances and a weakness of the legs.

In severe cases, progressive muscular paralysis, with pronounced breathing difficulty, develops. Death due to respiratory failure may occur within 2-24 hours after eating affected food.

Treatment: Symptoms and signs normally clear within 36-48 hours. Luckily, the toxic effect is completely reversible

Treatment is symptomatic and supportive.

  1. Diarrhetic shellfish poisoning This type of poisoning is caused by okadeic acid, which is produced by Dinophysis acuminata.

Signs and symptoms: Symptoms usually develop within four hours after eating a meal.

These include nausea, vomiting, diarrhoea, abdominal pain and rigors.

Treatment: People usually recover within three days of eating the affected food.

Treatment is symptomatic. Pay particular attention to fluid retention by drinking copious amounts of fluids.

  1. Neurotoxic shellfish poisoning This type of poisoning is caused by Gymnodinium species.

Signs and symptoms: Exposure, usually in the surf close to the beach, causes irritation of the eyes, nose and throat with coughing, sneezing and difficulty breathing.

It may lead to asthma attacks in certain people.

Treatment: Treatment is symptomatic and supportive.

How to prevent poisoning: Be aware of red tide warnings and never eat shellfish that originated from the affected area during this time. Red tides usually occur during the late summer and early autumn, but may occur at any time of the year.

An obvious sign to look out for is discolouration of the water.

Check the safety of picking your own mussels (and other bivalve shellfish) by dialling the Red Tide Alert desk in Cape Town on (021) 434 4457.

  1. Scombroid poisoning Scombroid poisoning is caused by the consumption of fish that has undergone autolytic changes which increases its histamine content.

These changes may be tied to the food chain of the fish itself. However, the exact mechanism is unclear.

This is a problem that occurs from time to time, particularly in Cape yellowtail, although tuna and mackerel have also been implicated. Several incidents of scombroid poisoning have occurred in South Africa and all over the world in recent years.

It is not necessarily linked to breaks in the cold chain or improper storage conditions.

Signs and symptoms of poisoning: The onset of the illness occurs rapidly, usually within minutes after the affected food is ingested.

Symptoms include a hot, blotchy flushing of the skin, gastrointestinal symptoms, such as vomiting and diarrhoea, mouth sensations and palpitations. These symptoms seldom last for more than six hours.

Most cases of scombroid poisoning are mild and not life-threatening. The illness can be more serious in patients suffering from heart disease or asthma, or in elderly people.

Treatment: Seek medical attention. Your doctor will prescribe antihistamine treatment.

How to prevent poisoning:

Avoid eating Cape yellowtail if you suffer from heart disease or asthma.

Unfortunately, there is no way of telling whether a helping of fish is affected just by looking at it. Although there have been reports that affected fish tastes “peppery”, “pungent” or “bitter”, this is not an accurate measure to go by.

Let’s have fun and enjoy the ocean in all it’s forms, with understanding and a knowledge of it’s possible dangers.

I hope this has been helpful.

friendly shark






Until we meet again, keep that toothy grin!

By Nadine Bentley

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