It is official, the seas around New Zealand in 2018 were the hottest on record.

While it might sound nice for your summer dip, scientists are warning the warming waters - fueled by climate change - could have "catastrophic" impacts on our marine life.

A record-breaking marine heatwave last summer was the key driver of 2018's hot temperatures, with parts of the Tasman Sea as much as 6C higher than average at times.

Those scorching seas saw anomalies such as snapper in Fiordland, and Queensland groper around Northland. It also saw high mortality rates for both the salmon and mussel farming industries.

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It comes after Niwa announced this week 2018 was the country's second-hottest year on record.

Scientists say this year the sea was on track for another hot one, with the Tasman Sea and areas just off the east coast currently about 3C above average, and some a whopping 6C warmer than normal.

Just north of Napier the sea was currently 24C - 6C above the average of 18C for this time of year, and putting it on par with Sydney and New Caledonia, which were both experiencing slightly cooler seas than usual.

According to data provided by Niwa the average sea surface temperature for 2018 was 15.6C - 0.87C above the 1982-2010 long-term average.

These figures have been backed up by the United Kingdom's Met Office, whose figures show 2018 was 1.8C warmer since their records began in 1854.

Niwa said 2016 was the second warmest year on 15.4C, and 1999 in third on 15.3C.

Since 2013 four of the six warmest sea surface temperatures on record have occurred.

New Zealand's seafood sector - worth $4.18b annually to New Zealand's economy - faced some stark threats from these rising ocean temperatures and marine heatwaves.

Sea temperatures around New Zealand from January 1 to 7 show parts of the Tasman Sea and off the east coast are 2-3C above average. Image / Niwa
Sea temperatures around New Zealand from January 1 to 7 show parts of the Tasman Sea and off the east coast are 2-3C above average. Image / Niwa

University of Auckland marine scientist Dr Andrew Jeffs said the changes in temperature could have "catastrophic" impacts on our marine ecosystem.

Last summer's marine heatwave saw a high mortality rate for salmon in the Marlborough Sounds, causing millions of dollars in losses. In the Hauraki Gulf temperatures as high as 25C caused issues for the green-lip mussel farming industry.

"They were simply falling off the lines," Jeffs said.

"Sea temperatures do naturally fluctuate, but very gradually and only a small amount. Most species only have a narrow range they can survive in."

With temperature changes species generally moved within their range, whether further south or into deeper water.

"But when they can't move, eventually we start to lose species."

When under stress marine organisms are also more susceptible to disease.

"It basically puts the whole ecosystem under stress."

Warmer seas could also attract new predators and competition for local species.

In Japan, a herbivorous fish wiped out an area of seaweed that had been a nursery for baby lobster.

In 2018 the the Tasman Sea experienced its thrid warmest year on record, with an average temperature of 18.7C. Image / Niwa
In 2018 the the Tasman Sea experienced its thrid warmest year on record, with an average temperature of 18.7C. Image / Niwa

Another factor was the changing of currents. New Zealand is being affected by changes in the flow of the East Australian Current, which typically brought more tropical species here, and even baby packhorse lobster.

"We are starting to see disturbance to the natural marine environment, and there is more to come," Jeffs said.

"We have triggered the system and we are not slowing down, still putting more C02 in the atmosphere. The ecosystem changes could be catastrophic."

It was not all bad news however, with some species thriving in the warmer waters.

"We know snapper breed really well in the Hauraki Gulf in warm summers, so, after last year's marine heat wave, we will probably have a bumper crop in about four to five years."

The Moana Project, led by MetOcean Solutions' chief scientist Professor Moninya Roughan, was awarded an $11.5m Government grant last year to research New Zealand's marine environment in a time of rapid ocean warming.

Niwa meteorologist Ben Noll said last year's record sea temperature was down to three factors.

The first was persistent high pressure systems, meaning plenty of sunlight to warm the top of the water, and light winds to prevent mixing with the cooler, deeper water.

The second were prevailing northeast winds, driven by La Nina, bringing warm air down from the tropics, and a corresponding lack of southerlies.

Sitting on top of it all was climate change, which Noll said was providing a "long tail wind" to water temperatures.

Figures from the UK Met Office show 2018 was the warmest year in our seas on record, and 1.8C warmer since their records began in 1854. Image / Jim Salinger, UK Met Office
Figures from the UK Met Office show 2018 was the warmest year in our seas on record, and 1.8C warmer since their records began in 1854. Image / Jim Salinger, UK Met Office

Last year's marine heat wave was likely still causing the above-average sea temperatures at the moment, but scientists were holding off calling it another heat wave, Noll said.

Likewise while the marine heat wave contributed to a record-breaking hot January on land, it was not clear yet if the same would occur this year.

"But as an island nation, what happens in the sea tends to happen on land," Noll said.

Climate scientist Jim Salinger said warmer seas also provided more fuel for storms, and meant tropical cyclones would maintain their form longer as they approached New Zealand.

"The transition from tropical cyclone to ex-tropical cyclone occurs further south than normal, meaning they maintain their intensity for longer."

Last year three ex-tropical cyclones came near land, two made landfall. In an average year one might come close to New Zealand.

"Even a small increase in ocean temperature can lead to dramatic effects on land," Salinger said.

"Every 1C increase in temperature means storms can hold 7 per cent more moisture."

Last year's ex-tropical cyclone Gita brought huge deluges to Motueka, Tākaka and south of Kaikōura.