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How do red tides/HABs affect you?

How do red tides/HABs affect you?

Impacts on Aquaculture

Some algal species may produce mucus that clog the gills or toxins that damage the gills, both can impair the fish's ability to respire. The fish may die of suffocation.

Fish kills in fish cage

Mucus clog the gill

Red tide may also kill fish indirectly by depleting the oxygen in water. The massive number of algae will consume a large amount of oxygen in the water during the night. Once the massive red tide dies off, bacteria may degrade the algal cells and further deplete the oxygen in water. Fish cultured in fish cages, will die of oxygen depletion.

Algal species causing oxygen depletion
Prorocentrum minimum
Prorocentrum micans

Fish grasping for air at the surface due to oxygen depletion; some fish are dying.

Impacts on Human Health

Humans who consume shellfish, particularly the bivalves such as clams, mussels, oysters, fan shells and scallops containing toxins produced by harmful algae may experience poisoning syndromes. Avoid eating the viscera of shellfish where biotoxins are concentrated. The common shellfish toxin poisoning syndromes include: Paralytic Shellfish Poisoning (PSP), Diarrhetic Shellfish Poisoning (DSP), Neurotoxic Shellfish Poisoning (NSP), Amnesic Shellfish Poisoning (ASP).

Some algal species may produce toxins that can kill fish. In general, human consumption of fish killed by harmful algal blooms should be avoided.

Some algae species produce chemical toxins that may affect swimmers by causing minor eye, nose or skin irritation and respiratory distress.

Impacts on Ecosystem

Other marine organisms may suffer from the adverse impacts caused by HAB such as;

killed by toxins released by some algal species e.g. Manatees in Florida, whales in New England.

suffocation resulted from gill clogging by mucus secreted by some algal species.
fish/shellfish/corals/benthic/sessile organisms, killed by low oxygen conditions as the high biomass bloom decays
sublethal effects such as reduced growth due to hypoxia induced stress, increased susceptibility to disease in cage cultured fish

Impacts on Recreational and Aesthetic Value

Formation of red tide also degrades the aesthetic value of an environment by discolouring the seawater. Also, the occurrence of red tide in a bathing beach or water sport venue may reduce its recreational value.

Wong Shek Water Sport Centre
Hoi Ha Marine Park
Tai Tam Bay
Silverstrand beach

Some red tides/HABs may produce large and visible colonies, extensive foams or fouling smell when washed on beach.

Phaeocystis globosa

If large number of marine organisms die off at coastal area, the dead bodies may be washed to the shore and decompose, producing bad smell and hygienic problems.

Red tide with fish kill
Red tide (Slime)


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