Red tide and harmful effects on seafood production

Red tide is the phenomenon of algal blooms, which causes mass deaths in many parts of the world.

1. Phenomena of algae blooms or "red tide"

In the study, it was found that some microalgae species, when meeting favorable environmental conditions, were able to develop at a density of up to millions of cells / liter (Smayda, 1990; Hallegraeff, 1993). This bloom changes the color of the whole sea, with colors like blue, red, brown, yellow . often called toxic algae blooms or "red tide". The species produces toxins or does not produce toxins and is a major cause of death for aquatic animals.

Of the 5000 species of marine flora, about 300 species are capable of causing " red tide". About 40 species are capable of producing toxic toxins for fish, crustaceans, molluscs and humans (Shumway, 1990). Algae have once caused serious economic losses (mainly in aquaculture), the environment and human health. It is worrisome that some species can cause immediate harm at very low density through their toxins.

2. Types of poisoning caused by algae

The toxin produced by algae includes many different groups, which are accumulated in aquatic animal meat through the food chain and are a direct cause of various forms of poisoning. People have statistics of the following forms of poisoning:

1. Poisoning causes PSP (Paralytic shelfish poisoning) due to species of Alexandrium genus and Gymnodiniums, accumulated in bivalve mollusks.
2. DSP diarrhea (Diarrhetic shellfish poisoning) is caused by a number of species belonging to genus Dinopysis, accumulated in bivalve mollusks.
3. Poisoning caused amnesic shellfish poisoning (Amnesic shellfish poisoning) due to some species of the genus Pseudo-nitzschia, accumulated in bivalve mollusks.
4. The poisoning of CFP coral (Ciguatera shellfish poisoning) is caused by an algae group that lives in the bottom of reef fish.
5. NSP neurotoxicity (Neurptoxic shellfish poisoning) is caused by several species, including Gymnodinium breve, which causes nerve paralysis.

Red tide, also known as blooming algae, is the phenomenon of too many algae reproducing in water quickly.

3. The cause of the "red tide" phenomenon

In nature, toxic algae exist in the environment and are a natural food for aquatic animals like other useful microalgae. Usually they exist with a certain density and less harm or their harms are unclear. But in suitable environmental conditions (nutrition, salinity, temperature .) they can flare in a short time with a density of up to millions of cells / liter, creating a flowering phenomenon. And it comes with serious economic and environmental damages.

There are many causes of algal blooms, but according to Hallegraff (1993), human activities play an important role. Specifically:

1. The increase in nutrient content in water environment caused by human activities of aquaculture and domestic wastewater.This is considered to be the most important reason for toxic algal blooms.
2. Poor water flow exchange
3. Unusual changes in climatic conditions
4. Marine transport and transport of aquatic breeds carry resting spores of toxic algae from one crib to another and when there are favorable conditions, they cause outbreaks.

4. Harms caused by "red tide"

Three types of algae damage when flowering are: (1) growing with high density depleting oxygen in the water body or causing aquatic animals to die due to gills or gills; (2) Production of toxins that kill organisms in the ecosystem and (3) produce toxins accumulated in seafood species (through the food chain) that are harmful to users. The algae can carry one, two or all of the above effects.

- Toxic algae causes shrimp and fish to die: Many common algae groups exist in deadly water bodies for aquatic animals. Some Ceratium algae are known to be involved in the death of bivalve larvae. Notiluca scintillans is a large-sized algae, capable of eating floating fish eggs, and harming aquatic animals through very high levels of ammonia accumulated in the cell. Some studies also show that some toxic algae species can kill shrimp including Pyrodinium sp .; Alexandrium sp .; Protogonyulax sp .; Protoperidium sp. and Gymnodinium sp. Aquatic animals in lagoon cages and ponds are more affected by toxic algae than animals that live naturally in water bodies because they are unlikely to flee from the water affected by toxic algae.

Red tide causes clams to die mass.

- Toxic algae: Currently, about 40 species of toxin are capable of producing toxins, most of them belong to 3 algal groups: Cyanobacteria, Cyanobacteria (Dinophyta) and dinoflagellates (Haptophyta or Prymnesiophyta) ). Besides these three main groups, some other algae species are also capable of producing toxins such as silica algae, Raphydophyta algae group. Many studies have demonstrated that toxins produced from toxic algae with Dinoflagellata can damage the gills, affect the respiratory activity of aquatic animals, causing bleeding and rupture of blood vessels. or affect the nervous system of aquatic animals. In addition, the flowering of toxic algae can greatly alter the environmental indicators such as: DO, pH, alkalinity, changing the content of heavy metals in seawater through the metal ion exchange process of Algae cells. A study in China found that some of the flagellate algae (Dinoflagellata) of Psymneseum can release toxins that break blood vessels, causing death of shrimp and fish.

- Accumulated toxins in marine products: We know that molluscs are aquatic animals capable of absorbing and accumulating toxins from water bodies. Therefore, in addition to the direct lethal effects of toxic algae to mollusks, most countries now conduct monitoring and have strict regulations on mollusk import and export. For mollusks, toxins can accumulate in the body for several months. The elimination of toxins takes place in 2 stages, the first stage the level of toxin content is reduced very quickly to moderate levels on the safe level allowed and maintained for a long time.

The phenomenon of "red tide" is a phenomenon that occurs in nature for a long time, but recent surveys show that the frequency of toxic algal blooms causing damage to production tends to increase in the range of the past two decades. Partly, scientific awareness of this issue is increasingly being raised. More and more research programs with the participation of many scientists and monitoring systems are also strengthened so now there are many research and observational data on the phenomenon of "red tide" in self Of course.

Statistics show that there is actually an increase to the level of concern about the frequency and extent of damage caused by "red tide" waves. The implementation of toxic algae research and control programs in aquaculture areas ensures the benefits of these areas because it ensures the environmental safety and ensures the safety of coastal ecosystems. and ensure fisheries production is not affected by toxic blooms of toxic algae. More importantly, it ensures that seafood products produced in that area are safe for consumers and thus contribute to ensuring the reputation of aquatic products.

The phenomenon of "red tide" is a phenomenon that occurs in nature for a long time.

5. Some remedial measures when the "red tide" phenomenon occurs

When the "red tide" phenomenon appears, solutions should be quickly implemented:

1. Immediately stop providing food for shrimp and fish to reduce the oxygen demand of aquatic animals.
2. Move the cage to a safe place or immerse the cage near the sea floor to avoid the surface layer of water affected by toxic algae.
3. Pumping seawater from the bottom to the surface where cages and rafts are used to provide clean water.
4. A measure has been applied in some countries such as Korea, Japan and China to use clay (in powder or liquid form) sprayed on the red tide with the amount of 20-200g / m ² to precipitate the Hazardous algae cells sink to the bottom.However, this measure is only tested but not widely used.
5. Scientists are now developing a method to use chemical to dissolve mucous membranes caused by toxic algae in fish, support fish breathing or use chemicals such as ozone to kill algae.Or use biological measures such as filtered organisms (bivalve molluscs), zooplankton, viruses, bacteria or another species of algae to limit the outbreak of toxic algae.

(Excerpt from Hanoi 2007 Agricultural Science and Fisheries Encyclopedia - Nguyen Van Nguyen, Le Thanh Tung).