8 Fast Facts About Toxic Shellfish Poisoning

With warmer weather on the way, Kiwis all over the motu will be heading to the beach to enjoy the sea’s bounty. But before you go gathering shellfish, New Zealand Food Safety deputy director-general Vincent Arbuckle has some fast facts to share about toxic shellfish poisoning and how to keep yourself, your whanau and friends safe. 

1. New Zealand hasn’t had a toxic shellfish poisoning outbreak for a decade

Toxic shellfish poisoning can cause serious illness and can even be fatal. Fortunately, we haven’t had an outbreak of toxic shellfish poisoning since 2014, when 13 people fell ill. New Zealand Food Safety (NZFS) works hard to keep it that way by monitoring key recreational shellfish harvesting areas around the country for biotoxins.

2. NZFS regularly tests water and shellfish for biotoxins to keep the public safe

NZFS has more than 40 monitoring sites, with water tested weekly for toxic algae and shellfish tested fortnightly for biotoxins. The commercial shellfish industry also provides their own biotoxin monitoring data from more than 30 sites, giving us even greater coverage. If shellfish are found to contain biotoxins at unsafe levels, we issue public health warnings and post signs at affected beaches. In 2023 we issued 17 biotoxin alerts. You can check for current biotoxin warnings on our website, subscribe to our alerts here,  or download our free NZ Fishing Rules App.

3. Cooking does not destroy biotoxins

It’s important to know that cooking will not make toxic shellfish safe to eat, because cooking doesn’t destroy biotoxins. If anyone becomes ill after eating shellfish from an area where a public health warning has been issued, phone Healthline for advice on 0800 61 11 16, or seek medical attention immediately. We advise that you also contact your nearest public health unit and keep any leftover shellfish for testing.

4. Shellfish become poisonous by feeding on toxic algae

Shellfish with two shells (bivalve shellfish) can each filter more than 200 litres of water a day. They do this to filter out microscopic algae (called phytoplankton) and other particles as a food source. If these algae are toxic, the biotoxins can build up in the shellfish, making them poisonous. Generally, the more toxic algae there are in the water, the more toxic the shellfish get.

5. Bivalve shellfish are the most affected by biotoxins

Shellfish affected by biotoxins usually include bivalves like mussels, oysters, tuatua, pipi, toheroa, cockles and scallops. But pūpū (cat’s eyes) and Cook’s turban and can also have elevated levels of some biotoxins. Generally, you can still eat pāua, crab and crayfish, but only if you remove the gut before cooking, as biotoxins accumulate in the gut. Finfish are not affected by biotoxins from algal blooms, but we advise discarding the liver before cooking.

6. Not all species of phytoplankton are toxic

Phytoplankton are photosynthetic microscopic algae, meaning that, just like plants, they use the sunshine to grow. And, like plants, they also need nutrients (which they get from the water) to grow.  If conditions are right, phytoplankton can multiply to high numbers and form algal blooms. Most species of phytoplankton are harmless, but some can naturally produce highly toxic chemicals we call marine biotoxins.

7. Algal blooms occur naturally under particular conditions

Algal blooms usually occur naturally when wind and water currents are favourable for their growth. Causes can include slow water circulation or unusually high water temperatures. Some blooms have happened after extreme weather events like cyclones, floods, or drought. NZFS does extra monitoring during a toxic algal bloom to track its spread and see how it is affecting different kinds of shellfish.

8. There are four main types of toxic shellfish poisoning in New Zealand

Paralytic shellfish poisoning (PSP) is potentially the most serious of the four main types of toxic shellfish poisoning found in NZ. PSP symptoms usually appear within 10 minutes to 3 hours of eating and may include numbness and tingling around the mouth, face, hands, and feet; difficulty swallowing or breathing; dizziness and headache; nausea and vomiting; diarrhoea; paralysis and respiratory failure; and, in severe cases, death. If you suspect someone has PSP poisoning after eating shellfish from an area where we have issued a public health warning, seek medical attention immediately. You can read more about PSP - as well as amnesic, diarrhetic and neurotoxic shellfish poisoning - on our website.

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