Are New Zealand's marine heatwaves a warning to the world?

In the shallows of Aramoana, fish roiled the surface of the bay, flickering through the water, reflecting the winter sunlight. Some floated belly up, stunned and dying. Others spun in tight circles. In shallow pools created by the eddying tide, they lay piled on their sides. Occasionally a fish would raise a single fin, worrying the water’s edge.

Peter Langlands waded in, grabbing live fish one by one. He had long been an active fisher on the coastlines of New Zealand’s South Island, and knew they were ray’s bream: good eating. Later, he would fillet them, and cook the firm white flesh with spices for a curry. Even as he thought ahead to the evening meal, though, Langlands felt a pang of worry.

The fish had been beaching for months now – masses of them, through April, May, June, July. Entire schools died flapping in the Otago bays, their scales a dark, briney silver – an offshore fish, meant for deeper, colder waters. Locals posted videos of them surging over the sandbanks or laid out in their hundreds on the sand; of toddlers striding through the shallows to yank one out by the tail. In 30 years of fishing, and writing for the local fishing magazine, Langlands had never seen anything like it.

Fish strandings are by no means unheard of – schools get chased in by predators, carried by storms, caught by the shallow sandbars of a bay. “Generally when that happens, though, you’re talking about individual fish – not thousands and thousands over a six month period,” says Langlands. “I’ve never heard of anything happening on this scale before in New Zealand.”

Langlands was not alone in his alarm. New Zealand’s waters are changing, and those who watch them closely – anglers and divers, fish farmers and scientists – have reported unusual signs: mass deaths, disappearing species, changing animal behaviour. The ray’s bream strandings – along with mass deaths of salmon, penguins, kelp and sea sponges – have coincided with a series of unprecedented heatwaves hitting waters across New Zealand, raising the temperature of some coastlines more than four degrees.

New figures provided to the Guardian by scientists studying ocean temperature shifts show that on average, over the year to April 2023, New Zealand’s coastal waters sat stewing in marine heatwave conditions for 208 days. Some southern regions experienced marine heatwave conditions for more than 270 days during the period. In the north island’s Bay of Plenty, the waters remained in heatwave for an entire year.

With little respite for species to recover between the waves of heat, scientists warn that some ecosystems are reaching tipping points under the surface, with effects that will be felt years into the future. No one yet knows what it will mean for the fish, seabirds, whales, dolphins, and New Zealand’s multi-billion dollar fishing industry.

As scientists and communities begin to reckon with the impact, the conditions hitting Aotearoa provide a preview of the future of the world’s oceans under climate change: waters around the world are projected to rise by about 4C on average by 2100, if the world maintains its course on global heating. Heatwaves around New Zealand are already seeing spikes that high, giving a glimpse of what it can do to species under the surface.

Warning signs

Dr Daniel Thomas lifted the little kororā by its torso, and leaned in to take a closer look. The world’s smallest penguin species, the fairy penguin, is beautiful: their blue feathers shine. The tubby birds are often spotted waddling together over the dunes of New Zealand’s coasts.

Taking a scalpel, Thomas cut the dead bird open, unzipping its feathers from the chin down. “The very first thing we are looking for is – do they have muscle?” he says. “The biggest muscles in these birds’ bodies should be their flight muscles, sitting on their chest. Invariably, we don’t see it.”

These birds came to Thomas’s dissection table after being gathered on the beaches in 2018 – another period of extreme heatwaves, the “hot blob year” in New Zealand when a huge warm expanse of water moved across the Pacific, raising water temperatures by about 1.5 degrees. Studies over the last decade found that large hot blob events have killed more than a million seabirds in a year.

Carefully inspecting their tiny frames, he saw all the signs that they were starving when they died: no fat, no muscle mass, and thick dark blood in the gut.

When marine heatwaves began hitting New Zealand again in 2022, dead kororā began washing up on beaches in their hundreds. Their small bodies would lie part-buried in the sand of the Bay of Plenty – 183 one day, 109 another. The department of conservation concluded that the birds had died starving – as warm waters redistributed fish deeper and further, the penguins could not reach them.

Marine heatwaves can be caused by a number of factors, including ocean currents, low pressure weather, and La Niña weather systems. But as greenhouse gases heat the climate overall, oceans become hotter too.

“In the background of all of this you’ve got anthropogenic climate change. As the oceans are warming it’s much easier to tip the ocean over into a marine heatwave, because you’re starting from a warmer baseline,” says Dr Robert Smith, an oceanographer who studies marine temperatures with the Moana project and University of Otago.

‘The ocean has been doing the heavy lifting’

The first academic studies are only just emerging on New Zealand’s last two years of heatwaves. A joint paper newly published in the journal Weather and Climate called the temperatures of summer 2021/22 season “unparalleled”. In the months since then, however, conditions have grown even hotter. “It was a really big marine heatwave last summer and autumn – but this year it’s been worse,” says Smith, a co-author of the study.

The latest research can also more conclusively map rises in temperatures against the die-offs of some species living in warmer waters. Off the coast of Fiordland, sea sponges bleached en masse as temperatures rose. On the West Coast, the thick golden fronds of giant bull kelp that usually line the coast suddenly died off – with 90% of the species disappearing from the local marine reserve that year. One of the species they tracked was kororā, mapping how penguin deaths and starving chicks coincided with the heatwaves.

“The impacts on marine fauna of the 2021/22 heatwave were much larger than any that have been reported during previous heatwaves,” says Prof James Bell, a marine biologist at Victoria University. “For most marine species in our waters, we don’t know their thermal thresholds – the temperatures they’re able to tolerate – so it’s possible that future, more intense marine heatwaves will have even bigger impacts than we’ve seen to date.”

Dr Rochelle Constantine, one of New Zealand’s top marine scientists, spends much of her time tracing the journeys and migrations of New Zealand’s larger marine mammals. Some, like the Tohorā, or southern right whale, are regarded as “climate sentinels” as their changes in behaviour can signal shifts beneath the surface.

“The ocean has been doing most of the heavy lifting with absorbing global heat,” says Constantine. Seas are currently taking up more than 90% of the excess heat generated by greenhouse gas emissions. “That absorption of heat – that’s affecting not just surface waters, but even in the depths.”

The bay in front of her curves out into the expanse of the Hauraki Gulf – a global hotspot for ocean life, home to sharks and orca, whales and common dolphins, shellfish and manta rays. About 25% of the world’s known seabird species and 20% of the known whale and dolphin species have been seen in the Gulf at some point. As the water heats, many marine species – Bryde’s whales and common dolphins – are on the move.

“People who are watching the systems … are struggling to describe it as fast as it needs to be. But we do know it’s changing, [and] it’s the pace of change that’s most worrying,” says Constantine.

“These are spaces that have been relatively normal for … hundreds of thousands or millions of years, and are now actually changing so rapidly that animals and ecosystems can’t quite cope.”

‘‘We thought we had more time’’

The changes in the ocean are so stark they have been noticed outside scientific circles.

In the hills above Blenheim, between the wineries and pine plantations, trucks rumbled through January along the narrow road. They would make the journey 160 times over, through the hot summer months, winding from the coast to the hill and back again. Their cargo was tonnes upon tonnes of fish: king or “chinook” salmon, the most expensive variety of the salmon family, prized enough that a single large fish can sell for up to $1,700.

Usually, it would be sliced into sashimi, or smoked and laid atop hors d’oeuvres. Instead, it lay rotting in the truckbeds, more than 1,300 tonnes of it, carried to be dumped in a pit in the hills.

In Marlborough’s fish farms last year, the fish had died in their thousands, unable to survive the rising temperatures around them. In warmer areas, about 42% of total fish stock died. The country’s largest salmon producer, NZ King Salmon, announced it would have to shut down some of its farms as the climate heated waters around the sounds.

“When I joined this company, I never heard of the term ‘marine heatwave’,” said CEO Grant Rosewarne, as the company reckoned with the losses. “Recently, there’s been three of them.

“We thought we had more time,” he said. “Climate change is a slow process. But faster than many people think.”

New Zealand’s seafood industry plays a key role in the economy, contributing around $2bn in export earnings and employing more than 13,000 people. As sea temperatures warm, they are wreaking havoc with some of the most profitable sections of that industry.

“There’s been definitely changes with marine fisheries – with a lot more warmer water fish being caught further south,” Langlands says. “I really do feel fear. And feel for the price of seafood in New Zealand.”

“It’s a terrifying thing, if they keep coming,” says Rachel Brooking, New Zealand’s minister for oceans and fisheries. “We do need to take it very seriously.”

As the climate continues to heat, Niwa projects that the average number of days of marine heatwave a year could double by the end of the century.

Some sense of what changes lie ahead for New Zealand can be found in its recent past. Five years earlier, Constantine remembers, a group of scientists had sat bobbing in a boat off the wild coast of the South Taranaki bight. They were there to see a group of pygmy blue whales, which had been visiting these waters for centuries. In the summer months, pods liked to linger at upwellings where cool water from the deep flows up to mix with warm water near the surface, creating a rich zone of zooplankton. The researchers scanned the horizon for the broad, dark shapes of their backs as they cut through the water, the long, high channels of mist that come when they exhale. That year, though, the waters were still. Looking out at the wide, empty ocean, the scientists realised the whales were gone.

“It was like, where are they?” says Constantine. But as the hot blob of 2018 had moved down New Zealand’s coast, the changes had sent the whales hundreds of kilometres south, in search of food and cooler waters, where the scientists eventually found them.

“In 2018, that really hit home for us,” she says: the heat was changing the way animals behaved and lived and hunted for food. Those who could move, like the whales, did. But those that were tied to a place could find themselves in trouble.

“This had been a place where blue whales had been coming to feed long before humans were even here,” Constantine says.

Before her, the Hauraki Gulf shimmers, the sun reflecting off the sea’s surface like a foil, obscuring the waters below.