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Basic Information on Red Tide and Harmful Algal Blooms (HABs)

Red tide is a term used to describe all phenomena which the water is discolored by high algal biomass or concentration of algae. The discoloration may not necessarily be red in color but it may also appear yellow, brown, green, blue or milky, depending on the organisms involved. It may either be harmful or harmless.

What is Harmful Algal Blooms (HAB)?

Harmful algal bloom (HAB) is a term used to describe events associated with the blooms of algae, which cause harmful effects to the environment, living organisms and humans. It refers to blooms of toxic and non-toxic algae that discolor the water, as well as to blooms that are not sufficiently dense to change the color of the water but which are dangerous due to the algal toxin they contain or the physical damage they cause to other diota

Types of Harmful Algal Blooms

1. Species that produce basically harmless water discolorations, however, under exceptional conditions in sheltered bays, blooms can grow so dense that they cause indiscriminate kills of fish and invertebrates due to oxygen depletion.



2. Species which produce potent toxins that can find their way through the food chain to humans, causing a variety of gastrointestinal and neurological illnesses, such as:

Paralytic Shellfish Poisoning (PSP)
Diarrhetic Shellfish Poisoning (DSP)
Amnesic Shellfish Poisoning (ASP)
Ciguatera Fish Poisoning


Even when no bloom or discoloration is observed in the water, this type of microalgae, via the food chain can cause shellfish poisoning syndromes to humans. The toxins responsible for these poisoning syndromes are not single chemical entities but are families of compounds having similar chemical structures and effects. For example, the saxitoxins that cause paralytic shellfish poisoning (PSP) are a family of at least 18 different compounds with widely differing potencies. Most algal toxins cause human illness by disrupting electrical conduction, uncoupling communication between nerve and muscle, and impeding critical physiological processes.

3. Species that are non-toxic to humans but harmful to fish and invertebrates (especially in intensive aquaculture systems) by damaging or clogging their gills.


The Cause

In general, most harmful algal blooms are caused by plants (photosynthetic organisms) that form the "bas" of the food chain. These include both microscopic species of algae, referred to scientifically as phytoplankton and the microphytobenthos, as well as the larger macroalgae. These plant life provide the source of primary productivity of the ocean; contributed significantly to the production of oxygen through photosynthesis and are critical food for bivalve shellfish as well as larvae of commercially important crustacean and fish. They range in size from 1/1000 of a milliliter 50 2-milliliters and live a floating existence in the upper 200-m of the ocean, where sublight is available for photosynthesis. They include representatives of as many as 13 algal classes, of which more than 10,000 species are described and only about 50 of the thousands of known algal species actually produce toxins. Other HABs are caused by accumulations of non-chlorophyll-containing cells (heterotrophs) that are similar in form to microscopic algae.

Some species recur in the same geographic regions each year, while others are episodic, leading to the unexpected deaths of local fish, shellfish, mammals, and birds.

A bloom occurs when an algae or hetrotroph rapidly increases in numbers to the extent that it dominates the local planktonic or benthic community. Such high abundance can result from explosive growth, caused, for example, by a metabolic response to a particular stimulus (e.g., nutrients or some environmental condition like a change in water temperature), or from the physical concentration of a species in a certain area due to local patterns in water circulation.


Effects of Red Tide

  • Health Impact
  • Economic Impact
  • Environmental Impact

Harmful algal blooms have adverse effects on marine resources, human health and economy of the country. Harmful algal bloom in the Phlippines does not only pose a public health hazard but a major economic threat as well. Between 1983 and 2002, a total of 2,122 paralytic shellfish poisoning (PSP) cases with 117 deaths affecting all age group have been reported (Fig.1).

The economic losses associated with red tides and/or harmful algal blooms are not easily assessed due to the broad range of sectors in society that are affected. Furthermore, data onlosses in the seafood industry are often not released to the public and in many cases the losses are not quantified.

The occurrence of HABs has been reponsible for the economic losses in the fisheries sector of the country particularly the shellfish industry. The 1983 outbreak in central Philippines resulted to a lose of 2.2 million pesos and there was a dramatic decline in demand for fishery products and during 1988 in Manila Bay, it caused extensive economic damage since prices of all seafoods dropped to almost 40% of the normal price.

Figure 1


Shellfish bans during outbreaks pose problem to international trade. During the 1988, 1992 and 1993 outbreaks, Japan and Singapore have banned shrimp import from the Philippines. During outbreaks, shellfish closures result to unemployment for both the fisherment and the secondary industries such as processing, middlemen and, suppliers. In 1992 outbreak in Manila Bay around 38500 municipal fishermen were displaced from their livelihood for almost four months. The human impact caused by harmful algal blooms in the Philippines extends beyond human illness. When harmful algal blooms contaminate coastal resources, the livelihoods, social structure of local residents and food security of artisanal fishers are threatened and there is a possible collapsed of the local fishing communities.

The cost of monitoring potentially toxic areas and inspection and analysis of marine products are another consequence of harmful algal blooms. Regular shellfish monitoring programs are expensive. In a developing country, like the Philippines wherein financial resources are scarce, shellfish monitoring program is a luxury considering the high cost of laboratory equipment and supplies for analyzing samples.

Mitigation and Control

Management strategies and options for mitigation or impact prevention and control of harmful algal blooms are necessary to protect public health, ecosystem health and fisheries resources.

Strategies for mitigation are actions undertaken to reduce the losses of resources and economic values and minimize human health risks that occur as a result of harmful algal blooms. These include better monitoring and surveilance to reduce the risk of ingestion or exposure to toxins, improved forecasting to allow more time to protect resources and avoid risks, restoration of affected resources and a variety of alternative actions to minimize effects which might occur.

HAB control methods are still limited in scope and remain largely untested in major blooms since it is premature to conclude whether control methods are feasible, applicable and advisable due to lack of knowledge on the side effects of those methods and research studies are needed to validate the methods.

To monitor toxic micro algae, plankton net collects seawater samples.
Toxic micro algae analyzed using light microscope
Shellfish samples are collected to monitor toxin
Shellfish toxicity is determind by mouse bioassy


Harmful algal blooms are one of the marine environmental problems and resource management issued that confront the Philippines. It has been recognized as a catastrophic phenomenon that affects public health and economy of the country since 1983. The first recorded occurrence of blooms of Pyrodinium bahamense var. compressum, a toxin-producing dinoflgellate was in 1983 in central Philippines, and since it was the first time that the country has experienced such kind of phenomenon, its impact in terms of public health and economy was so great.

Apparently, harmful algal blooms in the country, particularly Pyrodinium have expanded both in time and space. Blooms of Pyrodinium spread to around 22 coastal areas of the country. Paralytic shellfish poisoning due to Pyrodinium has increased in severity during the last two decades, the country has experienced more than 540 outbreaks of harmful algal blooms with subsequent shellfish poisoning episodes between 1983 and 2002. Figure 2 shows the various coastal areas in the Philippines where harmful algal blooms occurred.

Figure 2. Occurence of Harmful Algal Blooms in the Philippines
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