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Harmful Algal Bloom Events Increase Along with Seafood Demand, Coastal Development

An unprecedented analysis of almost 10,000 Harmful Algal Bloom (HAB) events worldwide over the past 33 years was launched today by UNESCO's Intergovernmental Oceanographic Commission.

The first-ever global statistical analysis examined ~9,500 HABs events over 33 years and found that the harm caused by HABs rises in step with growth of the aquaculture industry and marine exploitation and calls for more research on linkages.

Conducted over seven years by 109 scientists in 35 countries, the study found that reported HAB events have increased in some regions and decreased or held steady in others. A widely-stated view that HABs are on the rise throughout the world, perhaps due to climate change, isn't confirmed.

However, the study, "Perceived global increase in algal blooms is attributable to intensified monitoring and emerging bloom impacts," published in the Nature journal Communications Earth & Environment, creates the world's first baseline against which to track future shifts in the location, frequency and impacts of HABs, which differ depending on which of the 250 harmful marine algae species is involved and where, requiring assessment on a species-by-species and site-by-site basis.

A public webinar on Global HAB Status Report will take place Tuesday Jun 15, 2021 at 1 PM, Paris time. To register:

Databases mined

The scientists mined both the global Harmful Algae Event Database (HAEDAT), consisting of 9,503 events with one or more impacts on human society, and the Ocean Biodiversity Information System (OBIS) database, containing 7 million microalgal observation records, including 289,668 toxic algal species occurrences.

The study found that regionally-recorded HAB events, after being corrected for higher levels of monitoring effort, have


  • Central America/Caribbean
  • South America
  • Mediterranean
  • North Asia


  • West Coast America
  • Australia/New Zealand

No significant change:

  • East Coast America
  • South East Asia
  • Europe

The 9,503 events' impacts on humans break down as follows:

  • 48% involved seafood toxins
  • 43% high phytoplankton counts and/or water discolorations with a socio-economic impact
  • 7% mass animal or plant mortalities
  • 2% caused other impacts (including foam and mucilage production)

(As well, in 11% of events, a single incident had multiple impacts, e.g. both water discoloration and mass mortality)

Of the event records linked to seafood toxins:

  • 35% were Paralytic Shellfish Toxins (PST)
  • 30% Diarrhetic Shellfish Toxins (DST)
  • 9% Ciguatera Poisoning (CP)
  • 9% marine and brackish water cyanobacterial toxins
  • 7% Amnesic Shellfish Toxins (AST)
  • 10% others, including Neurotoxic Shellfish Toxins (NST), Azaspiracid Shellfish Toxins (AZA), and toxic aerosols

By region, the largest number of records came from, in order:

  • Europe
  • North Asia
  • Mediterranean
  • The east and west coasts of North America
  • Caribbean
  • Pacific/Oceania
  • Southeast Asia

With more limited data sets for South America, and Australia/New Zealand

All geographic regions were impacted by multiple HAB types, but in varying proportions.

  • 50% of regional HAEDAT records in the Caribbean, Benguela, Mediterranean Sea, North and South East Asia related to high phytoplankton density problems.
  • Seafood toxins and fish kill impacts dominated in all other regions

Among toxin-related impacts:

  • Paralytic Shellfish Toxins (PST) prevailed in North America, the Caribbean, South America, South East Asia, and North Asia
  • Diarrhetic Shellfish Toxins (DST) were the most frequently recorded in Europe and the Mediterranean (and are an emerging threat in the USA) Neurotoxic Shellfish Toxins (NST) were confined to the US State of Florida, with a single outbreak also reported from New Zealand
  • Human poisonings from Ciguatera were prominent in the tropical Pacific, the Indian Ocean, Australia and the Caribbean.

For the most part, however, the impacts were confined to shellfish harvesting area closures; rarely to human poisonings. The exception: Ciguatera event records are almost exclusively based on medical reports of human poisonings.

HAB events over time

Eight of nine regions used in the study showed increases in reports logged via HAEDAT of harmful events per year, of which six were statistically significant.

The OBIS dataset, meanwhile, generally showed an increase in sampling effort in five of the nine regions.

When all the information was combined, the researchers could find no statistically significant global trend overall.

They also found, however, that aquaculture production increased 16-fold from a global total 11.35 million tonnes of seafood in 1985 to 178.5 million tonnes in 2018, with the largest increases occurring in Southeast Asia and South America/Caribbean and Central America, with North America and Europe stabilising.

The number of recorded harmful algal bloom events over time was strongly correlated with intensified aquaculture production in all regions with data suitable for the study.

However, says lead author Gustaaf M. Hallegraeff of the University of Tasmania: Intensified aquaculture clearly drives an increase in HAB monitoring efforts essential to sustaining the industry and protecting human health.

"And, just as clearly, a secondary effect of aquaculture is nutrient pollution. But a major data gap exists here. Conducting a meta-analysis of HABs vs aquaculture we had data on HAB monitoring efforts using OBIS records as a proxy but data on nutrient pollution is inadequate. The relationship between aquaculture-related nutrients and HABs therefore represents an important direction for further research."

Greater monitoring efforts

The study revealed

  • A 4-fold increase from 1985 to 2018 in observations of organisms mainly responsible for Diarrhetic Shellfish Poisoning (84,392 OBIS records)
  • A 7-fold increase in observations of organisms mainly responsible for Amnesic Shellfish Poisoning (128,282 OBIS records)
  • A 6-fold increase in observations of organisms mainly responsible for Paralytic Shellfish Poisoning (9,887 OBIS records)

(Note: Some observations may include non-toxic species or strains.)

In each case, the clear increase in the number of observations of problematic organisms paralleled an increase in records of associated toxic syndrome impacts.

They also found that the presence of toxic HAB species doesn't always accurately predict cases of human shellfish poisonings, which the study credits to the food safety risk management strategies in many affected countries. Some 11,000 non-fatal events related to Diarrhetic Shellfish Poisoning were reported worldwide, mostly from Europe, South America and Japan, with impacts consisting mostly of shellfish harvesting area closures.

Also, the study says, despite widespread distribution of the responsible algal species, there have been no human fatalities from Amnesic Shellfish Poisoning since the original 1987 incident in Prince Edward Island, Canada (150 illnesses, three fatalities). But ASP-associated mortalities of important marine mammals are of growing concern in Alaska and other parts of western North America, and ASP toxins have been linked to marine mammal calf mortalities in Argentina.

Of the world's 3,800 human Paralytic Shellfish Poisonings from 1985 to 2018, the largest number (2,555 from 1983 to 2013, including 165 fatalities) occurred in the Philippines, which depends strongly on aquaculture for human food protein.

DNA and other advanced detection methods have improved knowledge of the global distribution of ciguatera- causing organisms. Ciguatera poisonings, rarely fatal but annually affecting 10,000 to 50,000 people, have been decreasing in Hawaii and remained stable in French Polynesia and the Caribbean but constitute a new phenomenon in the Canary Islands.

Farmed fish killed by algal blooms: Largely a human-generated problem.

Aquacultured finfish mortalities account for much greater economic damage than HAB-contaminated seafood. Notes the study: wild marine finfish can simply swim away from blooms but those held captive in intensive aquaculture operations are vulnerable. Recorded losses include US $71 million in Japan in 1972, $70 million in Korea in 1995, $290 million in China in 2012, and $100 million in Norway in 2019.

A 2016 Chilean salmon mortality event caused a record $800 million loss, causing major social unrest.

Again, the presence of fish-killing HAB species doesn't accurately predict economic losses, the study shows. For example, Heterosigma blooms occur on the west and east coasts of Canada and the US, but fish mortalities are mostly confined to the west coast. In large part, the difference reflects the differences between sites where blooms occur and the relative location and size of aquaculture operations.

A harmful algae species that caused no problems in Australian lagoons killed 50,000 caged fish in Malaysia in 2014. It is now also known in Japan and the Philippines.

The authors note that some troublesome algal species may thrive, others decline, as ocean waters warm and acidify.


"There has been a widely-stated contention that HABs worldwide are increasing in distribution, frequency or intensity, so a quantitative global assessment is long overdue," says lead author Prof. Hallegraeff of the Institute for Marine and Antarctic Studies, University of Tasmania.

"While some of the HAB literature over the past 30 years has handpicked selected examples to claim a global increase and expansion in HABs, this new big data approach shows a much more nuanced trend," he adds.

"Our study concludes that the health and economic damages caused by harmful microalgae -- seafood poisoning, water discolouration that blights tourism, and the death of finfish in aquaculture operations, for example -- differ between regions."

Adds co-author Adriana Zingone: "We also found that overexploitation acts as a natural multiplier of the effects of HABs, leading to an increase in impacts independent of an actual trend in HABs."

"It should be noted that over the last 40 years capacity and monitoring efforts to detect harmful species and harmful events have also increased, thus increasing the reporting of harmful events across the world's seas," she says.

"The absence of events and decreasing trends, like all negative results, are rarely published. Whether or not HABs are increasing globally, however, their impacts are a growing concern all around the globe."

Says co-author Henrik Oksfeldt Enevoldsen: "As the human population continues to increase in tandem with resource demands, HABs will predictably constitute a serious threat in terms of seafood safety and security, a hindrance to recreational uses of the sea, and a problem for the tourism industry."

"Occurrences of harmful species over time and their human impacts can be expected to change locally, regionally and globally alongside the effects that climate, hydrography and human pressure impose on the coastal environment."

"Understanding the trends and distribution patterns of harmful species and events at multiple spatial and temporal scales will help predict whether, where and when to expect HABs, their frequency and intensity. This knowledge is fundamental for effective management of HABs and to optimise the uses and values of the maritime space in coastal areas."

Johan Hanssens, Secretary-General Flanders Department of Economy, Science and Innovation, a sponsor of this report, concluded: "This status report is a very timely reminder, at the start of the UN Decade of Ocean Science for Sustainable Development, that a thorough understanding of natural and ecological processes in the ocean is crucial for the development of the blue economy, now that many coastal countries are turning to the sea for additional resources, including food provisioning. International scientific collaboration is essential and most efficient to address the associated challenges."


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