Weather: What looming marine heatwave means for our ocean species

The summer forecast will warrant close monitoring of Northland's penguin population. Photo / Tourism Waitaki

Another summer marine heatwave will have widespread impacts on the myriad species living in our oceans, experts say, from damaging bleaching events to strange changes to fish distribution.

New Zealand's aquaculture industry is also preparing for the effects of unusually warmer seas, with one Niwa forecast already picking a below-average summer mussel yield in Pelorus Sound.

Niwa's latest guidance predicts sea surface temperatures around the country rising to more than 1C above average toward the end of the year – along with anomalies of 1.5C or greater in coastal waters.

Under mid-range forecasts, January could bring anomalies of nearly 1C near Coromandel, along with 1.7C in Golden Bay, 0.6C in Opotiki and 1.4C in Pelorus Sound.

🌊 The @niwa_nz Sea Surface Temperature Update for Sep 2022-Feb 2023 suggests that Aotearoa New Zealand's coastal waters have an elevated risk for marine heatwave conditions this summer.

This pattern is expected to lead to above normal glacial melt 🏔️https://t.co/HirHdFuYXH pic.twitter.com/CNLNI92Dkv

— NIWA Weather (@NiwaWeather) September 15, 2022

Forecaster Ben Noll said the coming event may prove as strong as last year's - and potentially comparable to a severe event in 2017-18 that coincided with our hottest-ever summer and killed off swathes of kelp.

Noll is part of a team that uses a range of climate models and data sources to produce specific growing forecasts for the Pelorus Sound greenshell mussel industry.

Current modelling indicated a 10 per cent reduction in meat yield over the summer months, relative to the 1997-2005 average, he said.

It comes after the country's largest salmon exporter, New Zealand King Salmon, was recently forced to close farms in Pelorus Sound after a heatwave led to 1300 tonnes of dead salmon being sent to landfill.

"As the predictive capacity of models of our coastal environment improve, we are changing how we farm within and between regions," said Aquaculture NZ's technical director, Dr Dave Taylor.

"And, to ensure we remain resilient, we are continuing to invest in breeding programmes, new feed formulations, and innovative farming technologies."

The prospect of more marine heatwave conditions had worrying implications for a range of other species.

This year, a cascade of effects caused by warmer seas likely contributed to the deaths of scores of little blue penguins in Northland.

Massey University's Professor John Cockrem said the summer forecast warranted close monitoring of the region's penguin population.

Cockrem said higher sea temperatures at the surface translated to less mixing and movement of nutrients that fed phytoplanktons and zooplanktons – and hence disrupted the entire food chain.

"At least in the medium term, warming water is unlikely to be a good thing for penguins," he said, adding that increasingly intense storms would bring their own impacts.

Last month, a Massey University-led study found warming oceans could gradually push blue and sperm whales to lower latitudes this century.

A schematic showing change in habitat suitability for sperm and blue whales in New Zealand for the most severe climate change scenario. Image / Katharina J. Peters.

However, with a lack of existing data, it was difficult to say what effect heatwaves were already having on marine mammals like whales and dolphins, said the study's leader, Professor Karen Stockin.

"In terms of climate change monitoring for marine mammals in New Zealand, this is the first study we're aware of that's been done."

In ongoing work that involved analysing historic tissue samples, Stockin and colleagues were trying to tease out climate change-linked physiological impacts from other factors like age, sex and diet.

Meanwhile, recent marine heatwaves had brought plenty of anecdotal evidence to show what effects these changes had on fish species.

Fishers have reported snapper spawning weeks earlier and turning up in typically-colder waters where they're rarely seen, including Southland, where kingfish are also increasingly being spotted.

"The species that typically show the biggest shifts during environmental events tend to be offshore, oceanic ones whose prey distribution are affected by things like currents, hydrology and nutrients."

The inshore fishing industry is carrying out a three-year study into heatwave impacts, alongside the monitoring it carries out as part of the multi-agency Moana Project.

Otago University physical oceanographer Dr Robert Smith said the shifts being observed in fisheries highlighted why it was important not just to consider the surface temperatures, but also those deeper in the water column.

"At the moment, waters off the northeast coast of New Zealand are between about 1C and 2C warmer than normal, down to a few hundred metres," he said.

"These can be really deep-reaching events that can have impacts at the shelf edge around New Zealand."

When temperatures reached several degrees above normal 100m below the surface – as had been recently observed on the West Coast – it carried implications for species like crayfish or benthic organisms that couldn't easily escape the heat.

"So, one of the interesting things to watch for this coming summer will be how deep this heat extends into the subsurface."

A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University

In January, Victoria University marine biologist Professor James Bell and colleagues will head to Fiordland, where recent sea surface temperature anomalies of 5C were found to be the most impactful ever seen in New Zealand.

Earlier this year, scientists were alarmed to discover millions of sea sponges there had turned from velvet-brown to bone-white – making for one of the worst bleaching events ever documented among sponge species anywhere.

"We have had some anecdotal reports that some of these sponges are showing colour again, which means they might be recovering – but we don't yet have any scientific evidence for this," Bell said.

Extremely high sea temperatures over last summer and autumn had similarly caused die-off among hundreds of thousands of affected sponges to the north of New Zealand, he said.

"We're not sure why a lot of these sponges are generally susceptible, but with temperatures increasing again over summer, we'd be worried about what this will do to them."

Smith said one of the biggest questions for marine ecosystems was what impact heatwaves would have as they grew not just stronger, but longer.

Scientists have warned that average sea temperatures around New Zealand could rise by 1.4C within four decades – and almost 3C by the century's end.

That would mean that, by mid-century, we could be facing 260 days of marine heatwaves per year – and 350 days by 2100 – compared with the 40-odd days we see now.

For some regions such as the southern tip of the South Island, recent Niwa-led research found, there was a high chance that marine heatwaves could start to last more than a year.