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Impacts of climate change on harmful algal blooms and seafood safety

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posted on 2023-05-22, 15:16 authored by Gustaaf HallegraeffGustaaf Hallegraeff

In a strict sense, harmful algal blooms are completely natural phenomena that have occurred throughout recorded history. However, even non-toxic algal blooms can have devastating impacts when they lead to kills of fish and invertebrates by generating anoxic conditions in sheltered bays. Other algal species, although non-toxic to humans, can produce exudates that can cause damage to the delicate gill tissues of fish (raphidophytes Chattonella, Heterosigma, and dinoflagellates Karenia, Karlodinium). Whereas wild fish stocks are free to swim away from problem areas, caged fish in intensive aquaculture operations are trapped and, thus, can suffer devastating mortalities. Of greatest concern to human society are algal species that produce potent neurotoxins that can find their way through shellfish and fish to human consumers where they evoke a variety of gastrointestinal and neurological illnesses. One of the first recorded fatal cases of food poisoning after eating contaminated shellfish happened in 1793, when Captain George Vancouver and his crew landed in British Columbia (Canada) in an area now known as Poison Cove. He noted that, for local Indian tribes, it was taboo to eat shellfish when the seawater became bioluminescent due to algal blooms by the local dinoflagellate Alexandrium catenella/tamarense, which is now known to be a causative organism of PSP. The increase in shellfish farming worldwide is leading to more reports of PSP, DSP (first documented in 1976 in Japan), NSP (reported from the Gulf of Mexico as early as 1844) and ASP (first identified in 1987 in Canada). The explorer Captain James Cook already suffered from the tropical illness of CFP from fish when visiting New Caledonia in 1774. Worldwide, almost 2 000 cases of food poisoning from consumption of contaminated fish or shellfish are reported each year. Some 15 percent of these cases prove fatal. If not controlled, the economic damage through the slump in local consumption and exports of seafood products can be considerable. Whales and porpoises can also become victims when they receive toxins through the food chain via contaminated zooplankton or fish. In the United States of America, poisonings of manatees in Florida via seagrasses and, in California, of pelicans and sea lions via contaminated anchovies have also been reported (Hallegraeff, Anderson and Cembella, 2003).

In the past three decades, harmful algal blooms seem to have become more frequent, more intense and more widespread. Four explanations for this apparent increase in algal blooms have been proposed: (i) a greater scientific awareness of toxic species; (ii) the growing utilization of coastal waters for aquaculture; (iii) the stimulation of plankton blooms by domestic, industrial and agricultural wastes and/or unusual climate conditions; and (iv) the transportation of algal cysts either in ships’ ballast water or associated with moving shellfish stocks from one area to another (Hallegraeff, 1993).

Few long-term records exist of algal blooms at any single locality; ideally, at least 30 consecutive years of data would be needed. Therefore, whether or not the apparent global increase in harmful algal blooms represents a real increase is a question that will probably not be answered conclusively for some time to come.

The growing interest in using coastal waters for aquaculture is leading to a greater awareness of toxic algal species. People responsible for deciding quotas for pollutant loadings of coastal waters, or for managing agriculture and deforestation, should be made aware that one probable outcome of allowing polluting chemicals to seep into the environment will be an increase in harmful algal blooms. In countries that pride themselves on having disease- and pollution-free aquaculture, every effort should be made to quarantine sensitive aquaculture areas against the unintentional introduction of non-indigenous harmful algal species. Nor can any aquaculture industry afford not to monitor for an increasing number of harmful algal species in water and for an increasing number of algal toxins in seafood products – using increasingly sophisticated analytical techniques such as LC-MS (see Section 3.2.5). Last, global climate change is adding a new level of uncertainty to many seafood safety monitoring programmes, as are range extensions of harmful algal bloom species through their being transported in ships’ ballast water and as a consequence of increases in sea surface temperatures (Hallegraeff, 2010).


Publication title

Assessment and Management of Seafood Safety and Quality: Current Practices and Emerging Issues




J Ryder, K Iddya, L Ababouch






Institute for Marine and Antarctic Studies


Food and Agriculture Organization of the United Nations

Place of publication

Rome, Italy



Rights statement

Copyright 2014 FAO

Repository Status

  • Open

Socio-economic Objectives

Fisheries - aquaculture not elsewhere classified

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