Harvesting shellfish from the wild is a much-loved tradition for many New Zealanders, but NZ Food Safety (part of the Ministry for Primary Industries) has warned that toxin alerts should always be heeded.

Bivalves (like mussels, pipi/tuatua and oysters) generally pose a greater risk than 'grazing' species such as pāua, kina and catseyes.

Several types of biotoxins have been found in New Zealand shellfish, and seawater samples from around the country are tested weekly to check for contamination.

If toxin levels are found to be unsafe, the local public health unit and New Zealand Food Safety issue a warning against collecting and eating shellfish from the affected area.

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The status of any given area can be checked by signing up for email alerts at www.mpi.govt.nz/travel-and-recreation/fishing/shellfish-biotoxin-alerts.

Warnings are removed once the area is deemed safe, but can remain in place for several months, or longer, as some shellfish retain toxins long after a toxic algae bloom has gone.

People with low immunity, like young children, the elderly, pregnant women and their unborn babies, or anyone who has a chronic illness or disease, are more likely to suffer serious issues after eating contaminated shellfish.

Shellfish should not be taken from areas marked with either temporary or permanent warning signs, usually near sewer or stormwater outlets.

Bacteria and viruses found in shellfish generally derive from human and animal waste that gets into the water from sewage and stormwater drains, or rivers and streams after rain.

Shellfish contaminated with sewage may contain norovirus, hepatitis A, shigella and salmonella, which can result in diarrhoea and vomiting, or more serious symptoms.

Dangerous levels of heavy metals were very rare in New Zealand, but it was still advisable to avoid collecting shellfish near slipways and marinas where fuel, paint, heavy metals and solvents were used.

Shellfish should be kept cool in shade in a bucket of fresh seawater, and transported in a chilly bin with ice wrapped in a towel (to protect them from becoming getting too cold). Freezing shellfish will kill them, but keep them chilled until they are to be eaten.

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Refrigerate as soon as possible after harvesting, covering them with a clean wet towel (not in an airtight bag or container) at 2-4 degrees Celsius, in the lowest, coldest part of the fridge, below cooked and prepared food.

If freezing, shuck immediately and freeze in small amounts. Always defrost frozen shellfish before cooking.

Shellfish should be eaten within two days of being collected. Don't eat any that have died. Live bivalves will shut their shells when touched; live oysters keep their shells tightly closed. Dead shellfish won't respond, and should be discarded.

Do not cook or eat any with broken shells.

Avoid cross-contamination from bacteria and viruses by keeping hands, chopping boards, knives and utensils clean. Thorough cooking will help kill bacteria, viruses and parasites.

NZ Food Safety also advises checking the maximum take and size regulations, which can differ between regions.

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For the most up-to-date information, go to the Fisheries website or sign up for the free NZ Fishing Rules app on an Apple or Android device. You can also text 9889 with the name of a species (eg kina, or snapper), for legal bag and size limits for that species by return text.

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The first public health warning against taking potentially toxic shellfish in the Far North was issued last week, covering the east coast from North Cape to Cape Karikari, including Parengarenga, Houhora and Rangaunu harbours.

The Ministry for Primary Industries warned that cooking or freezing shellfish, including mussels, oysters, tuatua, pipi, toheroa, cockles, scallops, catseyes and kina do not make them safe to eat.

Pāua, crabs and crayfish may be eaten if the gut has been completely removed prior to cooking.

Symptoms of paralytic shellfish poisoning, which typically appear between 10 minutes and three hours after ingestion, include numbness and tingling around the mouth, face, hands and feet, possibly with dizziness and difficulty swallowing or breathing, vomiting, diarrhoea, paralysis and respiratory failure. It can potentially be fatal.

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Anyone who becomes ill after eating shellfish should phone Healthline (0800 611-116) or seek medical attention immediately. They may also contact the Northland District Health Board Public Health Unit at Whangārei Hospital, on (09) 430-4100, and ask to speak with the on-call health protection officer.