29 Jan 2023

Scientists observe warming seas for harmful algal blooms

5:50 pm on 29 January 2023
A noctiluca scintillans bloom in Blackwood Bay, 2012.

A noctiluca scintillans bloom in Blackwood Bay, 2012. Photo: Supplied / Cawthron Institute

A watchful eye is being kept on Aotearoa's oceans for any unannounced, and potentially harmful, visitors.

Scientists believe rising sea temperatures may entice more harmful algal blooms from the sub-tropics to New Zealand's shores.

Aquaculture research scientist Dr Anne Vignier said while the exact impact of warmer ocean temperatures was not yet known, it was likely to have negative consequences for seafood.

"We're already seeing reduced resilience in commercially important fish and shellfish species with increased water temperatures, and then if you put harmful algal bloom exposure on top of that, it's an extra stress," Vignier said.

Aquaculture research scientist Dr Anne Vignier

Aquaculture research scientist Dr Anne Vignier. Photo: Supplied / Cawthron Institute

Warmer sea temperatures may change how harmful algal blooms act too, she said.

"We're likely to see range expansions of some bloom species and increases in those bloom durations and intensities. We also might see changes to toxicity. Blooms that previously weren't toxic or had low toxicity may have much more toxicity."

New species entering New Zealand, especially in the North Island, were on the cards as well.

But Cawthron Institute senior scientist Dr Lesley Rhodes said the country was well prepared for that.

"We might get new species coming in from the tropics that could be of concern. We're already doing a lot of work with species commonly found in the Cook Islands and Kermadec Islands so that we've got methods in place if they do arrive and start to form blooms," she said.

The researchers had been taking samples from Pacific Island blooms that attach to seaweed or coral on the harbour floor, and were often missed in water monitoring.

A bloom in Nydia Bay, Marlborough Sounds in 2019.

A bloom in Nydia Bay, Marlborough Sounds in 2019. Photo: Supplied / Cawthron Institute

Gambierdiscus was one of those, found about the Kermadec Islands, that could make people ill, Rhodes said.

"We've got really good methods to predict it being in the water and we've also got good toxin tests to be able to see if there's any toxin in any of the seafood."

Harmful algal blooms could be hard to get rid of, Rhodes said.

Cawthron Institute senior scientist Dr Lesley Rhodes

Cawthron Institute senior scientist Dr Lesley Rhodes. Photo: Supplied / Cawthron Institute

"In South Korea, they dump clay into the water and that drags the cells out. It also binds up the toxins. But I don't think New Zealanders will tolerate having loads of clay on the sea floor," she said.

"So it's really good to be pre-emptive because we do think it is likely it will come to Northland. We already have a closely related species, which is non-toxic and quite commonly found in the Bay of Islands."

Plenty of monitoring was already in place for both commercial and recreational use, Rhodes said.

"We have really good monitoring programmes in New Zealand. The seafood industry know what algae are in the water and if they're starting to bloom. And the Ministry for Primary Industries also does it for recreational harvesting."

Aquaculture research scientist Anne Vignier said new technologies predicted where different blooms were likely to spring up by testing for tiny DNA fragments.

Informed site selection, not putting ropes or cages near the back of bays where blooms are likely to accumulate, putting physical barriers in the water or carrying out emergency harvests were all options marine farms could also use to mitigate the risk of harmful algal blooms, Vignier said.

Recreational seafood gatherers could also take measures to keep themselves safe this summer.

"If you're going to go out harvesting shellfish, keep an eye out - check what the toxicity status of where you're harvesting is. Check that the harvest areas are open and there are no bloom warnings in place," she said.

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