Red Tide of 2008: the Mass Mortality of Marine Fauna Linked to a Toxic Algae Bloom

Experts from Fisheries and Oceans Canada and those from organizations that cooperated in monitoring the "red tide" observed in the St. Lawrence Estuary in 2008 have just published an article in Plos One, a prestigious international scientific journal. In the article, these experts exhaustively describe this exceptional red tide, which resulted in the death of a large quantity of fish, birds and marine mammals, including several beluga whales—a threatened species at the time but classified as endangered since May.

The red tide was caused by a dinoflagellate bloom of Alexandrium tamarense, a microalgae producing very powerful neurotoxins called “paralytic shellfish toxins” (PST). The accumulation of these toxins in filter feeders—such as shellfish—and the risks to human health represented by contaminated shellfish are well known. The Government of Canada ensures close monitoring of shellfish sanitation along the Canadian coastline and prohibits shellfish harvesting when toxin thresholds exceed the limit. Until recently, however, the consequences of red tides on the health of marine ecosystems remained unclear and was poorly documented.

Important discoveries
In August 2008, following heavy rainfall, an intense algal bloom of Alexandrium was observed in the St. Lawrence Estuary. At the same time, a record number of deaths among many species of fish, birds and marine mammals was reported to the Quebec Marine Mammals Emergency Response Network.

When experts began to examine many tissues of marine organisms washed up on the beaches, they noticed significant levels of PST, particularly in the liver or the gastrointestinal system of approximately half of the carcasses examined. They also detected the presence of PST in samples of living organisms collected during the red tide, including various fish and shellfish as well as in plankton (microalgae and zooplankton).

These scientific data are proof that the PST produced by Alexandrium were transferred in the food chain to the upper trophic levels and led to mass mortality of marine fauna during the red tide. This conclusion by the experts is strengthened by the fact that the hub of the deaths followed the drift of the red tide along the coast of the St. Lawrence Estuary. No other cause of death was identified in most of the animals examined during the necropsies performed by pathologists from the Université de Montréal Faculty of Veterinary Medicine. Finally, additional evidence that PST was responsible for the mass mortality is found in the large number of reports relating signs of neurological dysfunction—such as partial paralysis—among fish, birds and marine mammals.

The discoveries made by the team of experts are important because they shed new light on:

• the accumulation of toxins in various tissues of marine organisms;
• the transfer of these toxins and their biotransformation in food chains;
• the possibility of a transplacental transfer of toxins causing prenatal deaths;
• the behavioural signs of organisms indicating PST intoxication.

This new information is proving to be of great interest in a context where the frequency, intensity and the geographic extent of algal blooms producing PST are increasing around the world. What are the causes of this worrisome phenomenon? As one might expect, climate change is one cause—but also the enrichment of coastal areas with nutrients, called eutrophication—as well as other environmental disturbances.

To learn more about this subject, read the following article: Multispecies mass mortality of marine fauna linked to a toxic dinoflagellate bloom.

Michel Starr
Pelagic and Ecosystem Science Branch