How worsening marine heatwaves are disturbing NZ’s largest animal

A blue whale raises its fluke for a deep dive in the South Taranaki Bight, off the North Island's West Coast. Photo / Leigh Torres, Oregon State University

They’ve melted glaciers, energised storms, driven tropical fish into colder climes and helped fuel a run of record-warm temperatures.

Now, the marine heatwaves which have engulfed New Zealand over recent years have been shown to disrupt local feeding and breeding of our own population of the planet’s largest animal: the blue whale.

The big mammals have long captured imaginations for their huge size - adults are known to grow to well over 20 metres.

It wasn’t until 2018 that scientists revealed the South Taranaki Bight was home to a group genetically distinct from others in the Pacific and Southern oceans.

Soon after, researchers found how links between the whales, their prey and ocean conditions were gradually changing on a warming planet.

Those concerns have only deepened with a new study revealing the dramatic ramifications of abnormally warm seas for the ocean giants, the sounds of which were captured by seafloor-mounted hydrophones over two years.

“For blue whales, like many marine organisms, sound is critically important,” said the study’s lead author, Dr Dawn Barlow of Oregon State University’s Marine Mammal Institute.

The species are known to produce two distinct types of vocalisations: song, which is used by males and thought to play a role in breeding behaviour, and “D calls”, which are used by both sexes and linked with feeding.

Their recordings captured these sounds every day of the year – if not every hour – and showed them following seasonal patterns.

D calls were associated with upwelling conditions, when cold, nutrient-rich waters drove ocean productivity and led to aggregations of krill – the blue whales’ main food source.

Barlow said that added to evidence this call was related to feeding.

A pair of blue whales come up for air in the South Taranaki Bight. Photo / Leigh Torres, Oregon State University

“In contrast, blue whale song showed a very clear seasonal peak in autumn, and was less obviously correlated with environmental conditions.”

To explore the hypothesised function of song as a breeding call, the researchers turned to a perhaps more obscure source of information: historical whaling records.

“Whenever a pregnant whale was killed during commercial whaling operations, the length of the foetus was measured,” Barlow explained.

“By looking at the seasonal pattern in these foetal lengths in the Southern Hemisphere, we can presume that births occur around the time of year when foetal lengths are at their longest.”

The records indicated this was from April to May.

“By back-calculating the 11-month gestation time for a blue whale, we can presume that mating occurs generally in May to June, which is the exact time of the peak in song intensity from our recordings.”

While the researchers had set out to investigate habitat use patterns in the region, they hadn’t anticipated that something else would shift their focus.

“When marine heatwaves swept through the South Taranaki Bight, with cascading effects across the ecosystem, we had no choice.”

The impact on blue whales could be clearly observed.

“We found that during marine heatwaves, when water temperatures were particularly warm, D calls were dramatically reduced compared to during productive upwelling conditions,” Barlow said.

“During the fall breeding peak, song intensity was likewise dramatically reduced following the marine heatwave.”

A blue whale mother and calf come up for air in the South Taranaki Bight, with the South Island in view. Photo / Dawn Barlow, Oregon State University

This suggested that, following poor feeding conditions, blue whales might put less effort into reproduction.

“Our finding that D calls were reduced during marine heatwaves fits with what we would expect: anomalously warm conditions seemed to reduce the food resources available to the blue whales,” Barlow said.

“But what really surprised us was the relationship between marine heatwaves and lower song intensity during the breeding season.

“Essentially, we were seeing evidence for reduced reproductive effort following marine heatwave events.”

Study senior author Dr Leigh Torres said the research team was now interested in taking a deeper look at the health consequences for the species.

“We want to know if and how changing ocean conditions, such as warmer temperatures, decrease the availability and quality of prey for blue whales,” she said.

“And then what the consequences of these changes in prey may mean for blue whale foraging behaviour, body condition and reproductivity rates.”

Given marine heatwaves are predicted to become longer, stronger and more frequent around New Zealand, there was already enough cause for concern.

“Blue whales are long-lived animals and seem to be able to adapt by perhaps putting less effort into reproduction during a particularly poor year for feeding, as we showed in our study,” Barlow said.

“However, while they may have the flexibility and capacity to adapt for a year or two at a time, what will happen if and when these conditions become more frequent, as they are projected to do?”

“The potential impacts are concerning, and how these blue whales and the ecosystem they rely on will fare in the future remains to be seen.”