Scientists are closely watching the unusually warm waters around the North Island’s East Coast – and predict an “ocean heat wave” could develop near Canterbury and Otago within a few days.

Described as an extended period of extremely warm ocean temperatures at a particular location, ocean heat waves can last for several months and cover thousands of square kilometers.

“Scientifically, ocean heat waves are defined when the ocean temperature at a particular location is in the top 10 percent of the temperature normally recorded during that time of the year for five or more days,” explains University of Otago marine scientist Dr Robert Smith.

Events that have never happened before 2017-18 sparked New Zealand’s hottest summer and came with dramatic consequences.

Glaciers are melting as some pockets of sea off the South Island’s West Coast warmed to 6C above average, while elsewhere, seashells are suffering from flowing losses and vineyards are experiencing early harvests.

While sea surface temperatures around New Zealand have been near normal for much of last summer, Smith said a strong ocean heat wave developed during late February to the east of the country – and is still going on.

“This ocean heat wave is currently impacting the coastline of Wairarapa, Hawke’s Bay and Chatham Islands,” he said, adding that it had pushed temperatures more than 2 degrees Celsius above normal.

“The event was somewhat unusual in that it also covered much of New Zealand’s sub-Antarctic zone during the month of March, which is not, as the name implies, an area normally considered to experience a heat wave.”

At these ecological points, warmer oceans can disrupt all kinds of species, from plankton and seaweed to marine mammals and seabirds.

“They may also impact regional fisheries, including for the pāua around the Chatham Islands.”

As part of The Moana Project – a five-year, $ 11.5 million collaboration aimed at making New Zealand the world leader in ocean forecasting capabilities – Smith and fellow Otago researchers are working to gain a clearer understanding of heat waves and their effects.

The project has also seen the file launch free ocean heat wave forecasting system.

“Currently, we provide short distance forecasts of up to seven days, where and when ocean heat waves are most likely to occur, for specific coastal locations around New Zealand,” he said.

“These sites include Hauraki Bay, Bay of Plenty, Cook Strait, and Banks Peninsula. While the current tool provides us with short-term estimates, we are looking for ways to extend this estimate to several months using machine learning techniques.

“This research will help us predict these extreme events with more certainty and provide a warning to our marine industry and important coastal communities.”

This can guide efforts such as early harvesting, or, at a coastal cultivation facility, even moving stocks.

Heat waves can occur relatively quickly, and are triggered by a variety of factors.

“On a local scale, these factors include ocean currents that build up areas of very warm water, warming through the ocean surface from the atmosphere and reduced wind speeds that prevent the mixing of the oceans,” he said.

“The likelihood of ocean heat waves is also influenced by weather and large-scale climate patterns, such as El Niño and La Niña.”

Research has shown that global climate change is also having a big impact, with heat waves becoming 34 percent more frequent, and 17 percent longer, since the mid-20th century.

Even more concerning, Smith said, is that the number of heatwave days has increased by more than 50 percent each year.

“The recent ocean heat wave has had a devastating effect on marine ecosystems around the world,” he said.

“For example, they have triggered widespread mortality of marine species, shifts in the abundance and distribution of commercial and recreational fish stocks and the need to limit or shut down fisheries due to disease outbreaks, or the growth of harmful algae.”

Over time, he said the increased exposure of marine ecosystems to extreme temperatures could lead to “irreversible loss of important species or habitats”, such as seaweed forests and seagrass meadows.

“Ocean heat waves are therefore of serious concern to our marine life around New Zealand, which has been thriving on cooler seas,” he said.

“The impacts associated with ocean heatwaves are also a threat to aquaculture and fisheries, New Zealand’s industry worth over $ 4 billion per year.”

Scientists solve the mystery of shells

Meanwhile, scientists working on another Moana Project study combined Mātauranga Māori – or Māori lore – with other strands of science to solve the shellfish mystery.

Green-lipped mussels are an important cultivated species in New Zealand, and resources are valued at more than $ 300 million a year.

Although the aquaculture industry relies heavily on wild-caught baby mussels, or saliva, it is unclear which wild mussel beds supply them.

“Knowing the source of the splash enables the protection of spawning stocks and thus helps the future-resilient New Zealand shellfish aquaculture industry,” said Moana project and science director João de Souza.

In their new study, University of Victoria marine biologist Professor Jonathan Gardner and his team will put together what he calls a “unique combination” of science to reveal where shellfish larvae come from, how they travel, and where they end up.

“By doing that we will be able to predict the movement of larvae now and under different climate change scenarios.”

With population genetics, samples from the collected shells are genotyped – a process that effectively provides DNA fingerprinting linking different populations.

Microochemical analysis, which involves using a laser to take small samples of the shells of shells, can also provide a chemical record of the age of the shells, and where they traveled.

Mātauranga Māori offers local ecological knowledge that can help establish the location of the splash-producing clam reefs.

Finally, a physical model of the flow will be combined with biological data to predict and see the movement of green-lipped shellfish larvae in the Bay of Plenty, where the first samples were taken.

A further trip is planned to collect mussel larvae using a “splash line” – a vertical line of saliva-catching ropes for the walking spit to settle down.

After collection, any saliva that settles will undergo micro-chemical analysis.

“While the Bay of Plenty is the focus right now, we will also be sampling from several other areas, including Ninety Mile Beach which is where most of the shellfish spit is caught free,” said Gardner.

It is also hoped that the study, which is expected to take up to two years, will help combat invasive species, inform marine spatial planning efforts, and assist coastal restoration efforts.