Keeping kiwis who collect and eat shellfish safe

Alexandrium pacificum paralytic shellfish toxin bloom in Nydia Bay, Pelorus Sound, 2018. Photo credit: New Zealand Food Safety
Haumaru Kai Aotearoa, New Zealand Food Safety

Locally gathered shellfish are currently off the menu in some parts of Aotearoa due to marine biotoxins being above safe levels.

“It’s really important that people check for any warnings to make sure that shellfish are safe to eat before they collect and consume – shellfish biotoxins can make people very sick and can potentially be deadly,” says deputy director-general New Zealand Food Safety Vincent Arbuckle.

Information about how to stay aware of current alerts, including how to receive automatic email alerts, is on our website here: mpi. govt.nz/shellfish What are marine biotoxins?

Right at the bottom of the food chain in the ocean, we have microscopic algae called phytoplankton. Phytoplankton are photosynthetic; like plants, they use the sunshine to grow. And, like plants, they also need nutrients to grow. If conditions are right, phytoplankton can multiply to high numbers and form algal blooms, which may or may not be visible from the surface. Some phytoplankton species can naturally produce these highly toxic chemicals we call marine biotoxins.

Shellfish with two shells (bivalve shellfish), such as mussels or oysters, can filter 2- 10 litres of seawater an hour. That’s more than 200 litres a day for each shellfish. These shellfish filter out the phytoplankton and other particles as a food source. If these algae are toxic, then the toxins can build up to harmful levels in the shellfish.

“These toxins can cause serious illness if you eat affected shellfish, especially in children, the elderly, immune-compromised, and pregnant people. Importantly, cooking does not remove the poison,” says Mr. Arbuckle.

Algal blooms will die off once the phytoplankton runs out of nutrients or conditions change, such as a water temperature drop or if a storm disturbs the water column. The phytoplankton will usually all sink to the bottom of the ocean as the bloom collapses, which can mean any bottom-dwelling shellfish can become even more toxic.

Once the bloom has disappeared, shellfish will naturally cleanse themselves of the toxin. This may take a few weeks or months, depending on the type of shellfish. Tuatua and pipi have been known to hold on to the toxins for months in their siphon (the tube that sticks out of the sand).

What is included?

Public health warnings for shellfish apply to bivalve shellfish such as mussels, oysters, tuatua, pipi, toheroa, cockles, and scallops, as well as pūpū (cat’s eyes), Cook’s turban and kina (sea urchin).

The warning doesn’t apply to finfish, pāua, crab, and crayfish, which may still be eaten if the gut has been completely removed before cooking, as toxins may accumulate in the gut. If the gut is not removed, its contents could contaminate the meat during the cooking process.

Types of shellfish poisoning

Different types of shellfish poisoning have different symptoms. There are four main types of shellfish poisoning in NZ with varying safe limits – you can find more information about these on the NZFS website: Amnesic shellfish poisoning, Diarrhetic shellfish poisoning, Neurotoxic shellfish poisoning and most concerning of all for New Zealand, Paralytic shellfish poisoning.

Monitoring shellfish for marine biotoxins

New Zealand Food Safety (NZFS) has monitoring in place for the main recreational shellfish harvesting areas around the country, especially in areas where biotoxins have been a problem in the past. The programme involves regularly testing shellfish for toxins and sea water for toxic algae.

There are more than 40 regular monitoring sites around the country, with weekly water samples taken to analyse for toxic algae and fortnightly shellfish sampling to analyse for toxins. These samples are sent to a laboratory in Nelson for testing, who then report the results to NZFS. The commercial shellfish industry also contributes its own invaluable biotoxin monitoring data from more than 30 sites around the country, giving us great nationwide coverage.

Extra monitoring is undertaken during a toxic algal bloom to track the progress and spread of the bloom and show how it impacts different kinds of shellfish.

“If the shellfish are not safe to eat, NZFS will issue public health warnings and post signs at affected beaches. Generally, signs aren’t placed at beaches where there is little or no shellfish, so it’s important to check the shellfish biotoxin alerts on the NZFS website or in the NZ Fishing Rules app,” says Mr Arbuckle.

Why do you need to know?

“Public health warnings are an important mechanism in our food safety system here in Aotearoa – it’s how we spread the word about foodborne illness to help people prevent getting sick.

“You’ll see signs up at affected beaches, there’ll be information on our website, the New Zealand Fishing Rules App, and in local media. You can have these warnings automatically emailed to you by subscribing to our alerts page.

“The warnings stay in place until re-testing confirms that the shellfish is safe to eat. This can sometimes take months. We usually need two clear samples taken over two weeks to remove a warning.”

There have been no outbreaks of toxic shellfish poisoning in the previous eight years. The last outbreaks were in 2014 (13 cases) and 2012 (29 cases). NZFS is working hard to keep it that way.

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