Toxic algal blooms

Algal blooms occur when there is a rapid increase in the number of algae in water. Blooms may show as large red or brown patches in the sea but sometimes you can't see them. Find out more about the causes of algal blooms and what MPI does to monitor their effect on shellfish.

Algal blooms are a build-up of organisms

Algae are a diverse group of organisms ranging from large seaweeds to tiny phytoplankton. Phytoplankton are microscopic photosynthetic organisms that float freely with the currents or have a limited ability to swim. Algal blooms are simply the result of so many of these tiny phytoplankton being present. They are often visible as coloured patches in the water, usually as a brown or red bloom.

Most phytoplankton are harmless, and are a food source for some marine life. However, about 2% of algae species are capable of causing harm by:

  • producing toxins (called biotoxins or phycotoxins) that can kill other organisms, including marine mammals, birds, and humans
  • forming large blooms (high concentrations of cells) that may clog the gills of fish, cover beaches, or deplete oxygen in the water as they die and decompose
  • disrupting the ecosystem, for example, by reducing the ability of herbivores to graze.

Causes of algal blooms

Algal blooms usually occur naturally when wind and water currents are favourable. Causes can include slow water circulation or unusually high water temperatures. Some blooms have happened after extreme weather events such as cyclones, floods, or drought.

Sometimes algal blooms are caused by 'overfeeding'. This happens when nutrients (mainly phosphorus, nitrogen and carbon) build up at a rate that 'overfeeds' the environment's algae. Nutrient pollution from human activities can make the problem worse, leading to blooms occurring more often.

MPI shellfish monitoring programme

MPI manages a monitoring programme of the main recreational shellfish harvesting areas in New Zealand. The programme involves regularly testing shellfish for toxins, and seawater for toxic algae.

During a bloom, extra monitoring is carried out. This tracks the progress and spread of the bloom, and shows what is happening to the toxicity of different kinds of shellfish. If toxins exceed safe limits, they can poison humans.

When a warning is issued

If a shellfish biotoxin alert is issued, warning signs will be posted at the affected areas, media statements released, and this website will be updated.

Safety of commercially sold shellfish

All commercial shellfish growing areas have a strict sampling programme in place to monitor biotoxins and ensure shellfish bought from retailers and wholesalers are safe to eat.

Some shellfish carry more toxin than others

Mussels accumulate toxins more quickly than other types of shellfish, and are a good indicator species, alerting us to rising levels.

Tuatua, pipi, cockles and toheroa store toxins longer than others and can remain toxic for a long time after a bloom subsides.

Toxicity in shellfish can change quickly when a bloom is present. Shellfish that were safe yesterday may not be safe a few days later.

Food safety and fish that feed on shellfish

MPI advises that it is safe to eat the muscle or flesh of fish, but not the guts of fish that feed on algae or shellfish. The gut contents of these fish should be disposed of carefully, so that animals like cats and dogs can't eat them.

Cats and dogs can get seriously ill from eating toxic shellfish, and possibly from the guts of fish. Cooking does not destroy the toxin, and can cause it to spread from the gut to other parts of the fish. So it's important to gut species that may feed on shellfish or plankton before cooking.

Crabs and crayfish that feed on shellfish may accumulate toxins in the hepatopancreas. This is the area under the top shell (carapace) of the crab, or the 'mustard' in crayfish. This should not be eaten until MPI confirms it's safe. Crabs and crayfish should be gutted prior to cooking, as cooking the animal whole may spread the toxins from the gut to the flesh.

Algal blooms and other animals

Algal blooms are known to kill fish and other marine animals on occasions. The algae usually does this in 3 ways.

  • They produce toxins which kill animals.
  • The bloom uses up all oxygen in the water, creating a dead zone.
  • The bloom damages or clogs the gills, killing the fish.

Bay of Plenty gets frequent algal blooms

The Bay of Plenty is a sunny place and has some of the warmest water in New Zealand, which circulate down from the Pacific Ocean. Blooms often form and disperse in the currents of this region due to these growing conditions.

Some algae form resting cysts when conditions worsen and lie dormant on the sea floor until conditions improve. A weather or ocean event may stir the cysts back into life and form another bloom. Besides the Bay of Plenty's favourable growing conditions, some shellfish like tuatua and pipi can hold onto the toxin for a long time, even when a bloom has disappeared. Make sure you obey biotoxin signs if they're in place.

Who to contact

If you have questions about toxic algal blooms, email

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