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Not every plant poisoning is caused by a large, terrestrial plant. Several species of microscopic marine algae are notoriously poisonous to hapless humans who consume them in shellfish.

Red tide is a phenomenon that has been observed for thousands of years. The Bible may arguably include the first account of red tide. In Exodus (Chapter 7, verses 20–21) it is written, “... and all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river ...” The Red Sea is not red in colour, but it might have been so named because of seasonal blooms of red-coloured cyanobacteria. It is not surprising that the eating of shellfish is forbidden in the Muslim and Jewish faiths and, to some extent, some Christian sects.

People all over the world have long been aware of the potential dangers of eating contaminated shellfish. A well-known quotation, “It is unseasonable and unwholesome in all months that have not an R in their names to eat an oyster,” was written in the late 16th century by the English author Henry Buttes in his cookbook Dyets Dry Dinner. He was referring to the possible dangers of eating shellfish during the time of year when blooms typically occur.

In the 1600s, explorers noted that native tribes in the St. Lawrence River basin avoided eating shellfish at certain times. In 1776, some of the crew of the English explorer Captain Cook became sick after eating contaminated reef fish in Tahiti.

The first recorded case in Canada occurred in 1793, during Captain George Vancouver’s expedition to what is now British Columbia, when John Carter, a seaman, died from the effects of eating mussels, presumably infected with toxic algae. An eyewitness account of his death occurs in the June 17 diary entry of the expedition naturalist and surgeon, Archibald Menzies.

In 1987, cultivated mussels from Prince Edward Island contaminated with Pseudo-nitzschia multiseries caused three deaths and sickened more than 100 people in Montreal. The incident also threatened the livelihoods of mussel farmers. Consumer confidence has returned with continued stringent monitoring and testing, which makes shellfish on the Canadian market as safe as humanly possible.

Amnesic Shellfish Poisoning (ASP)


Scientists at the Canadian National Research Council showed that the species Pseudo-nitzschia multiseries was responsible for the 1987 poisoning incident in Montreal. It was the first time a diatom was found to produce toxins. Now it is known that many species of diatoms can produce these compounds.

Diarrhetic Shellfish Poisoning (DSP)

Prorocentrum lima

Causative organism: In 1990, 13 people in eastern Nova Scotia became ill with DSP poisoning. Unlike previous incidents in Europe and Japan, this episode was caused by Prorocentrum lima rather than the typical Dinophysis species. This was the first time a species of Prorocentrum was implicated for this type of poisoning.

Paralytic Shellfish Poisoning (PSP)

Alexandrium tamarense

Causative organism: The effects of PSP toxins have been known for centuries. This large family of toxins are produced by species of Alexandrium. The most common and potent one is saxitoxin.

Many new toxins have been discovered in Nova Scotia since the late 1980s, including spirolides (SPX), pectenotoxins (PTX), azaspiracids (AZA), and yessotoxins (YTX). There have so far been no reports of illnesses due to these toxins. Monitoring practices are important to ensure that our shellfish continue to be safe to eat.

There is also increasing concern about freshwater species that produce toxins such as saxitoxin, cyanotoxins, and microcystins, which could potentially threaten drinking water supplies. As in the case of poisonous fungi, scientific knowledge of poisonous algae is incomplete; indeed, new and alarming toxic algae are still coming to light.

Public Safety Measures

Public Safety Measures

Cultivated shellfish in Canada are safe to eat because of stringent monitoring by the Canadian Food Inspection Agency (CFIA). Today, poisoning incidents involve the ingestion of toxic mussels, clams, or oysters collected by people from beaches around Nova Scotia.

Since there is no visual method for determining whether particular shellfish are affected, Canada has been carrying out extensive laboratory monitoring of shellfish toxicity since 1943. As soon as these routine studies detect dangerous concentrations of toxic algae, CFIA posts notices on affected beaches and estuaries advising against shellfish harvesting. Signs are commonplace in Nova Scotia during the warmer months. It is dangerous to ignore these signs.


Algae are tiny, single-celled plants that, like plants on land, capture and use the sun’s energy to grow. The growth of algae is an essential life process, as it is the first step in transferring solar energy into aquatic food webs. The huge variety of marine algae are typically subject to annual cycles of growth and decay. These organisms thrive and multiply principally during the spring and summer, in response to increased light intensity and favourable levels of salinity and nutrients in ocean water. During periods of rapid growth, or blooms, many types of single algal cells may replicate themselves one million times in two to three weeks.


Shellfish Poisons

Shellfish poisoning occurs when filter-feeding shellfish (mussels, clams, oysters, and scallops) eat certain kinds of tiny marine algae. Toxins produced by the algae accumulate in the shellfish, making them poisonous when eaten.

In Nova Scotia, three main types of shellfish poisoning are known to occur: amnesic shellfish poisoning (ASP), diarrhetic shellfish poisoning (DSP), and paralytic shellfish poisoning (PSP). Generally, these are caused by marine algae called diatoms.



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