A changing climate could have big consequences for a recently discovered New Zealand population of the largest animal on Earth, the blue whale, a new study finds.
The big mammals have long captured imaginations for their huge size – adults are known to grow to well over 20m.
But it wasn't until 2018 that scientists revealed the South Taranaki Bight was home to a group genetically distinct from those of the Pacific and Southern oceans.
In a just-published study, the same team have learned more about how the links between the whales, their prey and ocean conditions are changing in a warming planet – lessons that could prove critical to their conservation.
Leigh Torres, an assistant professor at Oregon State University in the US, said one of those insights was that the whales didn't roam about New Zealand's coasts at random.
"We found that the same ocean patterns that determine where whales are also determine where their prey are, under both typical and warm ocean conditions," she said.
"The more we learn about what drives these whales' movement, the more we can help protect them from whatever threats they face."
The study's lead author, Oregon doctoral student Dawn Barlow, said her team wanted to specifically understand why the whales chose the habitats they had – and precisely where these were.
"We know this area is important to this population of whales, and we want to understand what it is about this spot that is desirable to them."
The region was often rich in prey - blue whales feast on patches of krill - but that prey was also patchy and influenced by changing ocean conditions, including warmer temperatures and changes in ocean properties.
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The area also had frequent shipping traffic and activity from oil and gas exploration – and the sound of local seismic blasting had been found to be loud enough to drown out the whales' calls.
Using data collected during typical summer conditions in 2014 and 2017, and warmer-than-average conditions in 2016, the researchers analysed how changing ocean conditions affected the whales' distribution in the region, along with how much prey was available.
They found that, during a regional marine heat wave in 2016, there were fewer aggregations of krill for the whales to dine on.
With fewer options, the whales pursued the densest aggregations of krill they could find, Barlow said.
The researchers also found that, during both warm and more typical ocean conditions, the whales were more likely to feed in areas where the water was cooler.
During the marine heatwave, when even the coolest water temperatures were higher than normal conditions, the whales still sought the coolest waters available for feeding.
In this region, cooler water temperatures represented deeper water that was pushed toward the surface in a process called upwelling and tended to be nutrient-rich, Torres said.
The nutrient-rich water supported aggregations of krill, which in turn provided sustenance for the blue whales.
In their study, the researchers were able to bring all of the pieces of this trophic pathway together to describe the relationships between oceanography, krill and whales.
As warmer ocean conditions become more frequent, this new knowledge could be used to inform and adjust spatial management of human activities in the region in an effort to reduce impacts on New Zealand blue whales, Torres said.
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"Documenting information like this can really help us understand how to reduce threats to these animals," Torres said.
"We need continued monitoring to understand how these whales will respond to both the changing climate and human impacts."
The blue whales found off New Zealand, Australia and Chile are not quite as large as Antarctic blue whales, which scientists believe to be the largest animals to have ever lived on Earth.
Antarctic blues, when they reach adulthood, can range from 28 to 30 metres in length, while other blue whales, though slightly smaller, are still formidable at about 22m.
