What are toxic algal blooms?

Shellfish can become unsafe to eat from toxic algal blooms. Find out about toxic algal blooms, the causes, and what we do to monitor shellfish to ensure they're safe to eat.

Toxic algal blooms explained

Algae are aquatic plants ranging from large seaweeds to tiny phytoplankton. Phytoplankton are microscopic and feed from the sun. When a lot of these are in the water, it's known as an algal bloom. Blooms often appear as coloured patches (usually brown or red).

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

  • producing toxins (called biotoxins or phycotoxins) that can kill animals, including marine mammals, birds, and humans
  • forming large blooms that can clog the gills of fish, cover beaches, or use up oxygen in the water as they die and break down
  • 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 like 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" algae in the environment. Nutrient pollution from human activities can make the problem worse. This leads to blooms occurring more often.

Build-up of toxins in different kinds of shellfish

Toxins build up in mussels more quickly than in other types of shellfish. This makes them a good species for alerting us to rising toxin levels.

Tuatua, pipi, cockles, and toheroa storfe toxins longer than other species 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.

MPI shellfish monitoring programme

The Ministry for Primary Industries (MPI) manages a monitoring programme of the main recreational shellfish harvesting areas in New Zealand. The programme involves regularly testing shellfish for toxins and sea water for toxic algae.

During a bloom, we do extra monitoring. This tracks the progress and spread of the bloom and shows what is happening to the toxicity of different kinds of shellfish. If toxin levels are above safe limits, they can poison humans.

Check where it's unsafe to collect shellfish

We update our shellfish biotoxin alerts webpage when testing shows that shellfish in an area are unsafe to eat.

We also issue public health warnings, media releases, and put up signs at affected beaches.

Safety of commercially sold shellfish

We have strict water and flesh monitoring programmes for commercial shellfish to ensure it's safe to eat. If commercial shellfish become toxic, they cannot be harvested or sold and the growing areas are closed.

Can you eat fish that have eaten toxic shellfish or algae?

Except for the guts, it's safe to eat fish that have fed on toxic algae or shellfish. Make sure to dispose of the guts or toxic shellfish safely so that animals like cats and dogs can't eat them.  Both things can make them seriously ill.

Cooking doesn't destroy toxins

Cooking will not make toxic shellfish safe to eat, as it does not destroy the toxin. Cooking can also cause the toxins to spread from the guts to the flesh of the fish. Before cooking, make sure to gut any species that may feed on shellfish or algae.

Eating crabs and crayfish that have fed on toxic shellfish

Toxins can build up in crabs and crayfish that feed on toxic shellfish. They can build up in hepatopancreas, in:

  • the area under the top shell (carapace) in crabs
  • the "mustard" in crayfish.

Crabs and crayfish should be gutted before cooking. Cooking them whole may spread the toxins from the gut to the flesh.

Remember, crayfish and crabs are protected under the Animal Welfare Act 1999. You must not cause them any unnecessary pain or distress and you must kill them humanely. Our fact sheet explains how to kill them humanely.

Fact sheet on animal welfare regulations for crustaceans [PDF, 485 KB]

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. These conditions make it easier for blooms to form and disperse there.

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 warning signs if they're in place.

Who to contact

If you have questions about toxic algal blooms, email info@mpi.govt.nz

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