Look Out for Summer Biotoxin Warnings

A toxic algal bloom affecting almost a third of the South Australian coast serves as a timely reminder for Kiwis heading to our beaches these holidays to look out for biotoxin warnings when gathering shellfish.

Since March, the Australian bloom has been making people sick, killing marine life and affecting industries and activities that depend on a healthy ocean.

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Just like plants, the phytoplankton that forms algal blooms uses sunshine and nutrients to grow. If conditions are right, phytoplankton can multiply until it becomes an issue. Most species of phytoplankton are harmless, but some can naturally produce highly toxic chemicals called marine biotoxins.

Bivalve shellfish filter out phytoplankton and other particles from the water as a food source. If these algae are toxic, biotoxins can build up in the shellfish, making them poisonous.

“Fortunately, the three toxic algal blooms New Zealand has had so far this year have been on a much smaller scale than what we’re seeing in Australia,” says New Zealand Food Safety (NZFS) deputy director-general Vincent Arbuckle. “Nonetheless, these toxins can cause serious illness or even be fatal, so please stay vigilant and avoid eating shellfish from any area where we have issued a warning.”

To help prevent people getting ill from eating shellfish containing these biotoxins, NZFS monitors 42 recreational shellfish harvesting areas around the country. Water and shellfish are tested weekly or fortnightly for algae or biotoxins respectively. The commercial shellfish industry also provides their own biotoxin monitoring data from more than 58 sites, giving even greater coverage.

If shellfish are found to contain biotoxins at unsafe levels, NZFS issues public health warnings and posts signs at affected beaches.

“To make sure you know what the current biotoxin warnings are, check our website, subscribe to our shellfish biotoxin alerts, or download our free NZ Fishing Rules App.”

Algal blooms die off once the phytoplankton run out of nutrients or conditions change, such as a water temperature dropping or a storm disturbing the water column. Once the bloom disappears, shellfish naturally cleanse themselves of the toxin. This may take a few weeks or months, depending on the type of shellfish.

Shellfish affected by biotoxins include bivalves like mussels, oysters, tuatua, pipi, toheroa, cockles and scallops. But pūpū (cat’s eyes) and Cook’s turban can also have elevated levels of some biotoxins. Kina are still safe to eat.

“It’s important to know that cooking will not make toxic shellfish safe to eat, because cooking doesn’t destroy biotoxins,” says Mr Arbuckle.

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 flesh is not affected by biotoxins from algal blooms, but NZFS advises discarding the liver and gut before cooking.

“If anyone becomes ill after eating shellfish, 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.

“Keep yourself and your whānau safe this summer by not collecting or eating shellfish from areas where we’ve issued a biotoxin warning.”